An overview of methodologies for the determination of volatile organic compounds in indoor air

Abstract

Volatile organic compounds are a broad and important class of pollutants affecting the indoor air quality. They are emitted from commercial products, building materials, furniture, occupant activities and even occupants, etc., and can participate in the indoor chemistry reacting with oxidants or being formed from secondary reactions. Some VOCs are classified as carcinogens and are associated with a variety of health effects. Characterizing and quantifying the VOCs in the indoor environments is of paramount importance in order to implement preventive measures to minimize the human exposure. A correct assessment of human exposure or characterization of emission sources and indoor activities requires appropriate and efficient methods for sampling and analysis. Therefore, this review focuses on the different methodologies for monitoring VOC that must be selected when a sampling plan is designed considering the objective of the measure. Selecting the most suitable procedures for assessing VOCs requires proper knowledge on the existing standards and off-line (including the selection of the sorbent media) and online instrumentation. Knowing the advantages and drawbacks of the different techniques available can help to plan future studies.

Keywords:

• VOCs
• indoor air
• real-time measurements
• formaldehyde
• passive sampling

1. Introduction

Indoor air pollution has become in the last decades an issue of major concern for the public health in conjunction to outdoor air pollution.^[1,2] In general terms, people spend most of their time indoor: at home, office, school, shopping centers… It is proven in many scientific studies that indoor air concentrations for many pollutants use to be higher than outdoors due mainly to the presence of indoor sources (building materials, furniture, air fresheners, cleaning products, cooking, etc.).^[3–10]

The indoor air pollution comprises inorganic gases (such as nitrogen oxides, carbon dioxide and monoxide, ozone, etc.), Volatile Organic Compounds (VOC, such as benzene, toluene, xylenes, formaldehyde, etc.), particulate matter (PM10, PM2.5 and UFP-ultrafine particles), Semivolatile Organic compounds (SVOC such as Polycyclic Aromatic Hydrocarbons-PAHs, phathalates, musks, etc.) and last airborne microorganisms and microbial flora.^[11–14] These pollutants have been assessed by Health organizations which shown adverse health effects on humans such as sick building syndrome, respiratory problems, allergies, lung malfunction or even cancer.^[15–22] In recent years several indoor air quality (IAQ) guidelines have been published not only by international agencies and organizations such as WHO but also by different countries.^[2,23–25] However, it is important to highlight that only some countries (e.g., South Korea and Japan) provide their Indoor Air Quality (IAQ) values as regulations while other countries suggest their IAQ values as guidelines.^[26] Recently, the EU calls to regulate the IAQ within the implementation of Ambient Air Quality Directives.^[27] It is noteworthy the effort of the European Commission in order to introduce in the European legislation the IAQ.

Chemical reaction mechanisms are still a wide field of research with more unknowns than certainty. Master Chemical Mechanism (MCM) describes in detail gas-phase tropospheric degradation of VOCs with hundreds of reactions to explain only some of the atmospheric processes among others.^[28–32] In the last decades, some interesting works have been developed for modeling the indoor reaction mechanisms.^[33,34] Even though, more studies are needed since the number of unknowns are still huge. Waring and Wells^[35] made a modeling study focused on the indoor reactivity due to the presence of oxidants such as O₃ OH·and NO₃· radicals. On the other hand, Kruza et al.^[36] recently developed an indoor air chemical model for studying the impact of O₃ in indoor pollution.

From this point of view there is a need to measure with high accuracy and precision the indoor pollutants that might have an effect over the human health. On the other hand, general public need to know, as indoor air quality problems gain more awareness to many groups of people, the levels of harmful substances, ideally in a quick and low-cost method. For the first issue, scientific community has established methods for measuring the concentration of pollutants and species found in closed atmospheres. Furthermore, international organizations (such as ISO, ASTM, EPA etc) have published standardized methods for such measurements. The ISO series of 16000 and 16017 standards are the base not only for determining the concentration of various organic gas pollutants in indoor air but also the sampling strategy to be adopted.^[37–45] Recently, WHO has published methods for sampling and analysis of chemical pollutants in indoor air summarizing the literature for measuring the most common chemical pollutants found in public setting for children.^[12] Concerning the second issue, different scientific groups are working with the aim of developing low-cost systems to measure indoor air pollutants.^[46]

In general terms, VOCs are a broad class of pollutants, some of which are carcinogens and are associated with a variety of other health effects such as irritation, allergic effects and respiratory symptoms.^[47–49] Tables 1–7 show examples of the different chemical families of VOCs that have been measured in indoor air together with their sources and typical concentrations found in different indoor environments. The sources of volatile organic compounds indoors are diverse. Almost all the indoors activities can generate VOCs. Besides, some building materials, cleaning products, floor surface material, among others, are the main sources of VOCs in indoor air.^[50–52] Additionally, some VOCs can enter into the buildings from outdoors infiltrating the building envelope o by means of ventilation, introducing different pollutants and oxidants such as OH radicals, O₃ or NOx that can promote degradation reactions indoors.

Table 1. Examples of aliphatic hydrocarbons detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point (°C)	Indoor sources	Range of Indoor Concentrations (µg/m^3)
Aliphatic hydrocarbons
Alkanes
n- Pentane	109-66-0	36	Adhesives, lubricants, personal care products, fuels, plastics, home building and construction materials [49]	0.3-5.1 (schools, Spain) [5] 4 (median, homes, Canada) [153]
n-Hexane	110-54-3	69	Sealants, paint, craft supplies [49] Adhesives and solvents applied to internal surfaces (e.g., glues, wallpapers, chipboards, insulation foams) [152]	0.02-146 (homes, Germany) [154] 1.2-8 (homes, Spain) [53] <LOD-8 (schools, Spain) [5] 0.1-7 (offices, Europe) [152]
n-Heptane	142-82-5	98	Consumer and commercial products, paint, adhesives, building materials [50]	0.2-8.9 (homes, Europe) [155] 0.4-168 (homes, USA) [156] 0.2-69 (offices, Finland) [157] 0.2-58.5 (schools, Spain) [5] 0.2-9.7 (public buildings/schools, Europe) [155]
n-Octane	111-65-9	125		0.04-73 (homes, Germany) [154] 0.6-4.4 (homes, Spain) [53] 0.2-5 (offices, Finland) [157] 0.1-7.3 (schools, Spain) [5]
n-Nonane	111-84-2	151	Paint, coatings, solvent, cleaning products [159]	0.02-199 (homes, Germany) [154] 0.7-37 (homes, Spain) [53] 0.2-4 (offices, Finland) [157] <LOD-15.9 (schools, Spain) [5]
n-Decane	124-18-5	174	Cigarettes, solvent, fuel [49]	0.01-527 (homes, Germany) [154] 1-141 (homes, Spain) [53] 0.2-14 (offices, Finland) [157] <0.2-21.8 (schools, Spain) [5]
n-Undecane	1120-21-4	196	Petroleum-based indoor coatings: wood stain, polyurethane wood finish, floor wax, outside traffic [50]	0.01-342 (homes, Germany) [154] 0.04-56 (homes, USA) [156] 0.2-10 (offices, Finland) [157] < LOD-22.6 (schools, Spain) [5] Max: 4.3 (schools, USA) [158]
n-Dodecane	112-40-3	216		0.02-132 (homes, Germany) [154] 0.04-17 (homes, USA) [156] 0.1-8 (offices, Finland) [157] 3.3-24.8 (schools, Spain) [5] Max. 3 (schools, USA) [158]
n-Tridecane	629-50-5	235	Adhesives and sealants, Fuels and related products Lubricants and greases [159]	0.02-165 (homes, Germany) [154] 0.06-20.2 (homes, USA) [156] 0.1-6 (offices, Finland) [157] n.d-9.6 (schools, Spain) [5] Max. 1.5 (schools, USA) [158]
n-Tetradecane	629-59-4	254	Automotive care products, Fuels and related products. Ink, toner, and colorant products. Lubricants and greases. Pesticides [159]	0.15-467 (homes, Germany) [154] 0.13-108 (homes, USA) [156] 0.2-7 (offices, Finland) [157] Max. 8.2 (schools, USA) [158]
n-Pentadecane	629-62-9	271	construction and building materials (e.g., flooring, tile, sinks, bathtubs, mirrors, wall materials/drywall, wall-to-wall carpets, insulation, playground surfaces). Solvent. Pesticides [159]	0.05-356 (homes, Germany) [154] 0.09-25.5 (homes, USA) [156] 0.2-9 (offices, Finland) [157] Max. 5.8 (schools, USA) [153]
n-Hexadecane	544-76-3	287	Construction and building materials, Fabric, textile, and leather products, solvent, diesel and gasolines fuels. Pesticides [159]	0.02-195 (homes, Germany) [149] 0.2-5.3 (homes, USA) [151] 0.1-5 (offices, Finland) [152] Max. 2.6 (schools, USA) [158]
Cycloalkanes
Methylcyclopentane	96-37-7	72	Solvent, component of the naphthene fraction of petroleum [49]	0.03-130 (homes, Germany) [154]
Cyclohexane	110-82-7	81	Cleaning products and household care (carpet, floor, metal polish), arts and crafts adhesive, construction and building materials, paint, personal care products [159]	0.01-343 (homes, Germany) [154] 0.2-16.4 (homes, USA) [156]
Methylcyclohexane	108-87-2	101	Arts and crafts/office supplies, cleaning products and household care, Fragrances, colognes, and perfumes [159]	0.01-32 (homes, USA) [156] 0-6.2 (homes, Europe) [155] 0.1-36.6 (private buildings/schools, Europe) [155] 0.1-10.4 (homes, Spain) [53] Max. 53.1 (schools, USA) [158]

<LOD: below detection limit; n.d: not detected

Table 2. Examples of aromatic hydrocarbons and heterocyclic compounds detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point (°C)	Indoor sources	Range of Indoor Concentrations ( µg/m^3)
Aromatic hydrocarbons
Benzene	71-43-2	80	Cigarettes, gasoline, solvent, adhesive remover, motor oil [49]	1-109 (different indoor environment, European Union) [162] 0.3-28 (homes, USA) [156] 0.008-31.6 (homes, Germany) [154] 0.7-5.1 (homes, Spain) [53] 0.1-6 (Oficces, Europe) [152] 0.2-11.1 (offices, Kuwait) [163] 1-21 (schools, Europe) [164] 0.2-2.1 (schools, Spain) [5]
Toluene	108-88-3	111	Cigarettes, gasoline, solvents, adhesives, paint, aerosols, pest control [49]	5-358^a (different indoor environment including hospitals, European Union) [162] Nd-160 (schools, world) [55] 1.1-179.8 (homes, USA) [156] 1.1-31.4 (schools, Spain) [5] 0.2-21.5 (Offices, Europe) [152]
o-Xylene	95-47-6	144	Cigarettes, gasoline, paint, paint thinner [49]	0.2-20.5 (homes, Europe) [155] 0.008-47.5 (homes, Germany) [154] 0.2-29 (offices, Finland) [157] Max: 2.2 (schools, USA) [158]
m,p-Xylene	108-38-3/106-42-3	139/138	Cigarettes, degreasers, solvents, spray lubricants [49]	2-88.2^a (different indoor environment, European Union) [162] 1.4-98.5 (homes, USA) [156] 1.6-20.4 (homes, Spain) [53] 0.4-190 (offices, Finland) [157] Max: 3.6 (schools, USA) [158]
Styrene	100-42-5	145	Cigarettes, auto exhaust, rubber, plastic, disposable containers [49]	nd-30.3^a (different indoor environment, European Union) [162] 0.01-39.7 (homes, Germany) [151] n.d-36.3 (schools, Spain) [5] 0.3-22.7 (offices, Kuwait) [163]
Ethylbenzene	100-41-4	136	Cigarettes, paint, sealants, automotive products, insecticide [49]	0.07-47.8 (homes, Germany) [154] 0.4-22.5 (homes, USA) [156] 0.2-26.9 (public buildings/school, Europe) [155] Max: 0.9 (schools, USA) [158]
1,2,3-trimethylbenzene	526-73-8	176	Fuel injector cleaner, fuel and additives, solvent [49]	0.008-31.7 (homes, Germany) [154] 0.11-61 (schools, Italy) [165]
1,2,4-trimethylbenzene	95-63-6	169	Fuel, additives, solvent, paint, coatings, adhesive, herbicides [49]	0.2-58.9 (homes, Europe) [155] 0.2-44.3 (public buildings/school, Europe) [155] 0.008-126 (homes, Germany) [154] 0.2-24(offices, Finland) [157] 0.2-11.7 (schools, Spain) [5]
1,3,5-trimethylbenzene	108-67-8	165	Paint, paint thinner, solvent, fuel additive, coating [49]	0.008-35.6 (homes, Germany) [154] 0.9-9.6 (homes, USA) [156] 0.1-6 (offices, Finland) [157]
n-Propylbenzene	103-65-1	159	Paints and coatings, solvents (PubMed)	0.01-20.3 (homes, Germany) [154] 0.01-7.9 5 (homes, USA) [156]
n-Butylbenzene	104-51-8	183	Pesticide manufacturing; solvent for coating compositions; plasticizer; surface active agents; asphalt component; naphtha constituent [159]	0.02-2.3 (homes, USA) [156]
2-Ethyltoluene	611-14-3	165	Household product (Paint thinner) [159]	0.01-12.5 (homes, USA) [156]
Naphthalene	91-20-3	218	Cigarettes, moth balls, deodorizers, burning wood or fuels [49]	0.6-83.5 (homes, European Union) [162] 0.2-200.6 (homes, USA) [156] <LOD-15 (schools, Europe) [164] Max: 2.9 (schools, USA) [158]
1-methylnaphthalene	90-12-0	244	The active ingredient is no longer contained in any registered products (PubMed) burning of wood, tobacco, or fossil fuels, industrial discharges (ATSDR, 2005)	0.02-0.75 (homes, USA) [156]
2-methylnaphthalene	91-57-6	242		0.05-1.6 (homes, USA) [156]
Heterocyclic compounds
Tetrahydrofuran	109-99-9	66	Adhesives, sealant, paints, coatings, solvents [159]	0.007-18.7 (homes, Germany) [154] 0.3-6.7 (homes, USA) [156]
2,5-dimethylfuran	625-86-5	63	Flavouring [159], cigarettes [160], biomass burning [161]	0.13-0.82 (homes, USA) [156]

a Range of mean concentration

Table 3. Examples of halogenated compounds and terpenes detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point/°C	Indoor sources	Range of Indoor Concentrations (µg/m^3)
Halogenated
Chlorobenzene	108-90-7	131	Ambient air - soil gases (building located close to contaminated lands i.e., landfill sites, lands affected by contaminated soil or groundwater plumes [50]. Glues, pesticides [159]	0-22.7 (homes, Germany) [154] 0.02-0.35 (homes, USA) [156]
1,2-Dichlorobenzene	95-50-1	180	Cleaning products, adhesives, degreaser, Baby cream or lotion, engine lubricants. Odor masking products [159]	0.01-0.51(homes, USA) [156]
1,4-Dichlorobenzene	106-46-7	173	Cleaning products, Deodorants and antiperspirants, pesticides, engine lubricants, Odor masking products, moth balls [159]	0-01-2126 (homes, USA) [156] 0.01-1.87 (schools, Italy) [165] 0.2-24.4 (offices, Kuwait) [163]
Methylene chloride	75-09-2	40	Adhesives, solvent, cleaning products, degreaser, wood finish, insulation, pesticide, paint remover, Fragrances, colognes, and perfumes [159]	0.35-7.85 (homes, USA) [156] Max: 47.7 (schools, USA) [158] 0.2-96.9 (offices, Kuwait) [163]
Chloroform	67-66-3	61	Laundry and fabric Treatment, disinfection by-products, solvent (PubMed, 2022). Byproduct of chlorination of water (showering, washing clothes, dishes) [49]	0.08-7.1 (homes, USA) [156] Max: 15.3 (schools, USA) [158]
Tetrachloromethane (Carbon tetrachloride)	56-23-5	76	adhesives and adhesive removers, caulk/sealant, flame retardants, solvent [159]	0.02-45.4 (homes, Germany) [154]
1,2-Dichloroethane	107-06-2	84	Lubricants, adhesives, sealants, oils, paints, coatings), engine maintenance [159]. Tap water (water disinfectant byproduct) [50]	0.18-3.93 (homes, USA) [156]
1,1,1-Trichloroethane	71-55-6	74	Arts and crafts cleaner, pens and markers, Fluids used to cover inked, Home air fresheners, including candles with a fragrance, cleaning products, Ink for inkjet printers, toners for laser printer, adhesive, degreaser, sealant, hair conditioner, paints, pesticides, laundry and fabric treatment [159]. Tap water (water disinfectant byproduct) [50]	0.19-4.47 (homes, USA) [161] n.d ∼ LOD (offices, Kuwait) [163]
1,1,1,2-Tetrachloroethane	630-20-6	130	Solvent; insecticides, herbicides, soil fumigants, bleaches, paints and varnishes [159]	0.1-1.0 (homes, USA) [156]
Trichloroethylene	79-01-6	87	Degreaser, adhesives, sealants, paint, coatings [49]	0.02-16.2 (homes, Germany) [154] 0.04-1.5 (homes, USA) [161] LOD-126 (schools, Europe) [164]
Tetrachloroethylene	127-18-4	121	Wearing or storing dry-cleaned clothes [166] Auto care products, household cleaners, lubricants, solvent [49]	0.05-13.7 (homes, USA) [156] LOD-81 (schools, Europe) [164]
Terpenes
α-Pinene	80-56-8	156	Food favoring, pine scented cleaners, odor masking products [49]	0.012-854 (homes, Germany) [154] 0.01-44.5 (homes, USA) [156] 0.2-214.1 ((homes, Europe) [155] 0.3-6 (schools, Spain) [5] Max: 55.7 (schools, USA) [156] 0.2-84 (offices, Finland) [157] <0.41-36 (Offices, Europe) [152]
β-Pinene	18172-67-3	164	Food favoring, pine scented cleaners, odor masking products, laundry and dishwashing products [49]	0.012- 575.6 (homes, Germany) [154]
d-Limonene			Personal care products, fragrance, perfume, solvent, insecticide, cleaners, food favoring [49]	0.073-642 (homes, Germany) [154] 0.4-173 (homes, USA) [156] 0-493(homes, Europe) [155] LOD-672 (schools, Europe) [164] Max: 158(schools, USA) [158] 0.2-240 (offices, Finland) [157] 0.2-80.5(Offices, Europe) [152]
Camphene	79-92-5	158	Air freshener, bathroom cleaner, insect repellent, laundry and dishwashing products, personal care products [159]	0.54- 11.03 (schools, Italy) [165]
Cumene (Isopropylbenzene)	98-82-8	152	Crude oil, paints, lacquers, and enamels, adhesives, wood finish, surface sealers, paints, herbicide, pesticides [159]	0.006- 18.7 (homes, Germany) [154] 1.23-2.9 (homes, USA) [156]
p-Cymene (p-Isopropyltoluene)	99-87-6	177	Air fresheners, including candles with a fragrance. Bathtub, tile, and toilet surface cleaners. Cleaning and furnishing care products. Personal care products [159]	0.8 17.9 (homes, USA) [156]
δ-3-Carene	13466-78-9	167	Air freshener, bathroom cleaner, disinfectant, fragrance component, dishwashing products [159]	0.019- 303.5 (homes, Germany) [154] 0.2-45 (offices, Finland) [157] n.d-0.24 (schools, Lisbon) [167]
Longifolene	475-20-7	254	Odor agents, leaning and furnishing care products Laundry and dishwashing products [159]	0.057 11.4 (homes, Germany) [154]

Table 4. Examples of alcohols, esters and glycol/glycolethers detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point/°C	Indoor sources	Range of indoor concentrations (µg/m^3)
Alcohols
Methanol	67-56-1	65	Industrial solvent for inks, resins, adhesives, and dyes. Ingredient in paint, varnish removers. Air fresheners, candles with a fragrance, cleaning products and household care. Building materials	1.6-289 (offices, Kuwait) [163]
Ethanol	64-17-5	78	Adhesives and sealants Agricultural products (non-pesticidal). Air care products. Anti-freeze and deicing products Automotive care products Beverage. Fuels and related products. Ink, toner, and colorant products Laundry and dishwashing products. Paints and coatings. Personal care products	1.6-289 (offices, Kuwait) [163]
1-Propanol	71-23-8	97	Air care products. Cleaning and furnishing care products. Food packaging. Fuels and related products. Ink, toner, and colorant products. Paints and coatings. Paper products .Personal care products	0.3-486 (offices, Kuwait) [163]
2-Propanol	67-63-0	82		0.5-55.4 (offices, Kuwait) [163]
1-Butanol	71-36-3	118	Adhesives and sealants. Cleaning and furnishing care products. Fuels and related products. Ink, toner, and colorant products Paints and coatings. Detergent alcohol	0.02-115.3 (homes, Germany) [154] 0.4-6.4 (offices, Kuwait) [163] 0.3-31 (offices, Finland) [157]
2-methyl-1-propanol	78-83-1	108	Agricultural products (non-pesticidal). Fuels and related products. Ink, toner, and colorant products. Paints and coatings. Cleaning products and household care. Building materials (e.g., flooring, tile, sinks, bathtubs, mirrors, wall materials/drywall, wall-to-wall carpets, insulation…)	0.022-57.5 (homes, Germany) [154]
2-Ethyl-1-Hexanol	104-76-7	182		0.305-153.6 (homes, Germany) [154] 0.35-34 (Offices, Europe) [152]
Phenol	108-95-2	182	Adhesives and sealants.Building/construction materials - wood and engineered wood products. Cleaning and furnishing care products. Floor coverings. Foam seating and bedding products. Paints and coatings. Repackaged.	0.4-3.3 (homes, USA) [156] 0.2-30 (offices, Finland) [157]
Esters
Ethyl acetate	141-78-6	77	Adhesives and sealants. Home air fresheners, including candles with a fragrance. Personal care products (e.g., body cleaners, washes, shower gels, fragrances, perfumes, make up, deodorants …). leaning and furnishing care products. Electrical and electronic products. Ink, toner, and colorant products	0.025-598 (homes, Germany) [154] 0.51-117.4 (homes, USA) [156] 0.4–334 (offices, Finland) [157] Max: 7.9 (schools, USA) [158]
n-Butyl acetate	123-86-4	126		0.0098-450 (homes, Germany) [154] 0.4-41 (offices, Finland) [157] 0.26-2.60 03 (schools, Italy) [165]
glycol/glycolether
1-Metoxy-2-propanol	107-98-2	118	Adhesives and sealants. Cleaning products and household care. Construction and building materials. Agricultural products (non-pesticidal). Arts, crafts, and hobby materials. Automotive care products. Chemical substance used as pesticide stabilizer, in swimming pool cleaners and surfactants.Electrical and electronic products. Ink, toner, and colorant products. Laundry and dishwashing products. Lubricants and greases. Paints and coatings	0.2-67 (offices, Finland) [157]
2-Butoxyethanol	111-76-2	171	Products placed on the skin for decorative purposes (body paints, markers, glitters, play cosmetics, Halloween cosmetics, and products such as henna). Arts and crafts/office supplies. Cleaning products and household care (e.g., Home air fresheners, including candles with a fragrance, for carpets, flooring). Home maintenance. Personal care products (e.g., Deodorants, antiperspirants, fragances, shampoos). Pesticides. Pet care. Electrical and electronic products	<0.01-80 (Offices, Europe) [152]
2-(2-Butoxyethoxy)ethanol	112-34-5	230	Cleaning products and household care (e.g., Home air fresheners, including candles with a fragrance). Materials used for construction (e.g., flooring, tile, sinks, bathtubs, mirrors. Ink for inkjet printers. Home maintenance. Personal care products. Pet care.	0.2–56 (offices, Finland) [157]
2-Phenoxyethanol	122-99-6	245	Arts and crafts/office supplies (body paint). Cleaning products and household care (e.g., air fresheners). Ink, toner, and colorant products. Home maintenance. Personal care products.	0.2-80 (offices, Finland) [157]

Table 5. Examples of carboxylic acids detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point/°C	Indoor sources	Range of Indoor Concentrations (µg/m^3)
Carboxylic acids
Formic acid (Methanoic)	64-18-6	101	Cleaning products and household care (e.g., air freshener) . Materials used for construction (e.g., flooring, tile, sinks, bathtubs, mirrors, wall materials/drywall, wall-to-wall carpets, insulation…). Adhesives. Personal care (body hygiene, fragance…). Building/construction materials - wood and engineered wood products. Degradation of cellulosic and lignin material. Latex paints. By-products of ozone reaction with skin oils and other unsaturated organic molecules. Incomplete combustion and/or high-temperature volatilization from fuels [165].	24 µg/m^3 (13 ppb)^a (Baroque Library Hall of the National Library, Prague) [168] 0.02-1.66 (libraries and archives, Italy) [85] 15-53 µg/m^3 (8-28 ppb) (Homes, USA) [150] 13.8 ppb (Home, USA)^b [169] 1.2 ppb (University classroom, USA)^a [138]
Acetic acid (Ethanoic)	64-19-7	118		215 µg/m^3 (87 ppb)^a (Baroque Library Hall of the National Library, Prague) [168] 0.40-24.7 (libraries and archives, Italy) [85] 30-130 µg/m^3 (12-53 ppb) (Homes, USA) [150] 51.5 ppb (Home, USA)^b [169]
Propionic acid (Propanoic)	79-09-4	141	Pet care products. Linoleum flooring [165]	38 ppt (University classroom, USA)^a [138] 3100 ppt (Home, USA) [169]
Butanoic Acid (Butyric)	107-92-6	163	Cleaning products and household care (home air fresheners including candles with a fragrance). Construction and building materials. Personal care (make up and related)	110 ppt (University classroom, USA)^a [138] 1430 ppt (Home, USA)^b [169]
Pentanoic acid (valeric)	109-52-4	186	Herbide. Lubricants and greases	54 ppt (University classroom, USA)^a [138] 570 ppt (Home, USA)^b [169]
Hexanoic acid (Caproic)	142-62-1	202	Cleaning products and household care (air fresheners including candles with a fragance). Adhesives and sealants. Paints and coatings	58 ppt (University classroom, USA)^a [138] 590 ppt (Home, USA)^b [169]
Heptanoic acid (Enanthic)	111-14-8	223	Floor coverings. Paints and coatings	15 ppt (University classroom, USA)^a [381] 130 ppt (Home, USA)^b [169]
Octanoic acid (Caprilic)	124-07-2	240	Cleaning and furnishing care products. Cleaning compound. Lubricants and greases. Paints and coatings. Personal care products	39 ppt (University classroom, USA)^a [138] 260 ppt (Home, USA)^b [169]
Nonanoic acid (Pelargonic)	112-05-0	254	Herbicide	17 ppt (University classroom, USA)^a [138] 90 ppt (Home, USA)^b [169]
Decanoic acid (Capric)	334-48-5	269	Cleaning products and household care (air fresheners including candles with a fragance). Personal care products. Floor coverings. Paints and coatings.	9.7 ppt (University classroom, USA)^a [138] 32 ppt (Home, USA)^b [169]
Undecanoic acid (Undecylic)	112-37-8	284	Cleaning agent, humectant	0.58 ppb (University classroom, USA)^a [138] 16 ppt (Home, USA)^b [169]

a : ranged of or average indoor concentrations

b average concentration in summer

Table 6. Examples of aldehydes and ketones detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point/°C	Indoor sources	Range of Indoor Concentrations (µg/m^3)
Aldehydes
Formaldehyde	50-00-0	−19	Concrete and plaster additives, cosmetics, disinfectants, fumigants, photography, and wood preservation. Manufacturing urea-formaldehyde resins, used in particleboard products. Cleaning products and household care. Construction and building materials. Furniture and furnishings. Home maintenance (e.g., paint/stain and related products, etc). Personal care products (body hygiene, hair, make up, nails polish…). Ink for inkjet printers. Reaction producto of VOCs with oxidants. Cigarettes [50]. Electronic cigarettes [170]	1-66 (schools, Europe) [164]. 11.2-65.5 (schools, Spain) [5] Max. 32 (schools, USA) [158] 6.5-48.5 (public buildings, European Union)^a [162] 17.1-91.4 (homes, Spain) [53] 3.9-57.2 (homes, Europe) [155] 2.5-88 (homes, Canada) [172] 1.7-48.5 (offices, Europe) [152]
Acetaldehyde	75-07-0	20	Adhesives and sealants. Cleaning products and household care. Construction and building materials. Furniture and furnishings. Personal care products (body washes, shampoos, lotion, creams…). Cigarettes [50]	2.7-13.3 (schools, Spain) [5] 11.2-45.5 (homes, Spain) [53] 3.7-41.3 (homes, Europe) [155] 9.81-20.61 (homes, European Union)^a [162] 0.7-12 (offices, Europe) [152] 10.5 to 48.7 (homes, Canada)^a [172]
Acrolein	107-02-8	52.5	Ink for inkjet printers. Biocide Wood-based emissions (e.g., particleboards) and latex paints [171]. Cigarettes [50]	0.81-4.39 (university students homes, Spain) [54] 0.5-7.5 (offices, Europe) [152]
Propanal	123-38-6	49	Used in the manufacture of plastics, in the synthesis of rubber chemicals, and as a disinfectant and preservative	0.2-3 (schools, Spain) [5] 2.3-13.1 (homes, Spain) [53] 0.4-12.7 (homes, Europe) [155] 0.5-26 (public buildins/schools, Europe) [155] 0.1-10.5 (offices, Europe) [142]
crotonaldehyde	4170-30-3	104	Cleaning products and household care (Home air fresheners, including candles with a fragrance ). Degreaser. Cosmetics. Ink, toner, and colorant products	n.d-0.8 (schools, Spain) [5] n.d-3.6 (university students homes, Spain) [54]
Benzaldehyde	100-52-7	178,1	Home air fresheners, including candles with a fragrance. Degraser. Cosmetics. Cigarettes [50]	<LOD-4.3 (schools, Spain) [5] 0.7-3.3 (homes, Spain) [53] 0.02- 28.5 (homes, Germany) [154] 0.3-5 (offices, Europe) [152]
Butanal	123-72-8	74,8	–	14.7–88.7 (homes, Spain) [53] 0.28-64 (university students homes, Spain) [54] 0.5-4.5 (offices, Europe) [152]
pentanal	110-62-3	103	Cigarettes [50]	<LOD-4.3 (schools, Spain) [5] 4.4–45.5 (homes, Spain) [53] 0.018 33.9 (homes, Germany) [154]
(o,m,p-) Tolualdehydes	529-20-4/ 104-87-0/ 104-87-0	199-200 (o-/m-) 204-205 (p-)	–	n.d-1.8 (schools, Spain) [5] <LOD–3 (homes, Spain) [53]
hexanal	66-25-1	129	Cigarettes [50] Air fresheners, including candles with a fragrance	3.9-34.6 (schools, Spain) [5] 15.1–174.9 (homes, Spain) [53] 0.037-110.1 (homes, Germany) [154] 6.2-198.1 (homes, Europe) [155] 1.2-159.6 (public buildins/schools, Europe) [155] 2.5-27.5 (offices, Europe) [142]
heptanal	111-71-7	153	Cigarettes [50] Air fresheners, including candles with a fragrance. Bathtub, tile, and toilet surface cleaners:	Max. 2.7 (schools, USA) [173]
Octanal	124-13-0	171	Cigarettes [50] Air fresheners, including candles with a fragrance. Cleaning products and household care. Personal care products	Max. 5.7 (schools, USA) [173] 0.021-50.7 (homes, Germany) [154]
Nonanal	124-19-6	195	Air fresheners, including candles with a fragrance. Bathtub, tile, and toilet surface cleaners. Sunscreams.	Max. 16 (schools, USA) [173] 0.039- 598 (homes, Germany) [154]
Ketones
Acetone	67-64-1	56	Lubricants, adhesives, sealants, oils, paints, coatings, glues. Air fresheners, including candles with a fragrance. Cleaning products. Materials used for construction (e.g., flooring, tile, sinks, bathtubs, mirrors…). Furniture and furnishings. Home maintenance. Personal care products. Yard herbicide. Pesticides.	<LOD-43-7 (schools, Spain) [5] 8-196 (homes, Spain) [53] 10.4-165.1 (homes, Europe) [155] 1.8-336.8 (public buildins/schools, Europe) [155]
2-butanone	78-93-3	79.64	Cigarettes [50]. Arts and crafts. Air fresheners, including candles with a fragrance. Cleaning products. Toners used in laser printers. Furniture and furnishings. Adhesives, sealant, wood finish, insulation, paints. Personal care products. Pet care.	0.4-25 (offices, Finland) [157]

a ranged of or average indoor concentrations

Table 7. Examples of Volatile methyl siloxanes (VMS) detected in indoor air: CAS number, boiling point, indoor sources and some typical indoor concentrations.

Chemical compound	CAS number	Boiling point/°C	Indoor Sources	Range of Indoor Concentrations (ng/m^3)
Volatile methyl siloxanes
Hexamethyldisiloxane (L2)	107-46-0	99	Adhesives and sealants. Paints and coatings Personal care products (hair care products, Cheek blushes, bronzers, and rouges and products for priming face for make-up)	n.d-4300 (offices, UK and Italy) [169] n.d.-13000 (bathrooms, UK and Italy) [90] n.d. 93000 (living rooms, UK and Italy) [90]
Octamethyltrisiloxane (L3)	107-51-7	175	Personal care products (deodorants, antiperspirants, Body cleaners, washes, shower gels and Eyelash mascaras)	n.d-5.62 (homes, USA) [174] n.d-1.29 (offices, USA) [174] n.d (schools, USA) [174] n.d-2.65 (salons, USA) [174] n.d-710 (offices, UK and Italy) [90] n.d-5400 (bathrooms, UK and Italy) [90] n.d. 7100 (living rooms, UK and Italy) [90]
Decamethyltetrasiloxane (L4)	141-62-8	194	Personal care products (deodorants, antiperspirants, hand/body lotion, shampoos/conditioner products, cosmetics, susncreams…)	n.d-8.39 (homes, USA) [174] n.d (offices, USA) [174] n.d-2.54 (schools, USA) [174] n.d-8.87 (salons, USA) [174] n.d-5900 (offices, UK and Italy) [90] n.d-8500 (bathrooms, UK and Italy) [90] n.d- 2500(living rooms, UK and Italy) [90]
Dodecamethylpentasiloxane (L5)	141-63-9	232	Personal care products (hand/body lotion, hair care products, Cheek blushes, bronzers, and rouges, Foundation make-up and concealers, chemical products for tanning, staining, or coloring the skin, sunscreams)	n.d-106 (homes, USA) [174] n.d-13.7 (offices, USA) [174] n.d. −3.55 (schools, USA) [174] 1.68-28.3 (salons, USA) [174] n.d-1400 (offices, UK and Italy) [90] n.d.-9800 (bathrooms, UK and Italy) [90] n.d.-2200 (living rooms, UK and Italy) [90]
Tetradecamethylhexasiloxane (L6)				n.d-191(homes, USA) [174] n.d-49.2 (offices, USA) [174] n.d.-40.3 (schools, USA) [174] 7.12-150 (salons, USA) [174]
Hexamethylcyclotrisiloxane (D3)	541-05-9	134	Adhesives and sealants, cleaning and furnishing care products, personal care products	3.46-68.6 (homes, USA) [174] 1.96-5.99 (offices, USA) [174] 6.25-20.2 (schools, USA) [174] 6.34-16.1 (salons, USA) [174] 850-16000 (offices, UK and Italy) [90] Max. 9300 (childcare facilities, USA) [175] 1300-350000(bathrooms, UK and Italy) [90] 510-270000 (living rooms, UK and Italy) [90]
Octamethylcyclotetrasiloxane (D4)	556-67-2	175.8	Construction and building materials, home maintenance (caulk/sealant, paint/stain and related products, personal care products (deodorants, antiperspirants, eye products, hand/body lotion, hair coloring, hair care, bronzer, make-up and related, sexual wellness products, including personal lubricants, shaving cream, sunscreen)	4.37-210 (homes, USA) [174] 0.06-7.8 (offices, USA) [174] 12.8-245 (schools, USA) [174] 193-722 (salons, USA) [174] 300-3900 (university classroom, USA) [176] 940-20000 (offices, UK and Italy) [90] Max. >78500 (childcare facilities, USA) [175] 1900-270000 (bathrooms, UK and Italy) [90] 2100-80000 (living rooms, UK and Italy) [90]
Decamethylcyclopentasiloxane (D5)	541-02-6	210	Cleaning products and household care (home air fresheners, including candles with a fragrance), home manteinance (paint/stain and related products), personal care products (body hygiene, baby lotion, baby oil, fragances, hair care products, make up and related, self-tunner, Sexual wellness products, including personal lubricants, aftershave, shaving cream, sunscream …). Adhesives and sealants. Cleaning and furnishing care products	18-812 (homes, USA) [174] 6.36-92.5 (offices, USA) [174] 111-1020 (schools, USA) [174] 375-3710 (salons, USA) [174] 1.7-2.4 (offices, Spain)^a [93] 41000-99000 (university classroom, USA) [176] 2400-170000 (offices, UK and Italy) [90] Max. >88200 (childcare facilities, USA) [175] 3800-300000(bathrooms, UK and Italy) [90] 8400-160000(living rooms, UK and Italy) [90]
Dodecamethylcyclohexasiloxane (D6).	540-97-6	245	Adhesives and sealants. Cleaning and furnishing care products. Ink, toner, and colorant products. Personal care products (body hygiene, hair care products, make up and related, eye care, bronzer, aftershave, sunscream)	7.91-240 (homes, USA) [174] 3.09-30.7 (offices, USA) [174] 10.5-136 (schools, USA) [174] 121-885 (salons, USA) [174] 1100-2000 (university classroom, USA) [176] n.d.-15000 (offices, UK and Italy) [90] n.d.-170000(bathrooms, UK and Italy) [90] n.d-180000 (living rooms, UK and Italy) [90]

a Average or range of average concentration

The most studied VOCs in the indoor air are: oxygenated VOCs (e.g., aldehydes: formaldehyde, acetaldehyde…, see Table 6); aromatic hydrocarbons (e.g., benzene, toluene, ethylbenzene, xylenes, 1,2,3-trimehtylbenzene…, see Table 2); esters (e.g., ethyl acetate, butyl acetate …see Table 4); terpenes (α-pinene, limonene…, see Table 3); chlorinated hydrocarbons (trichloroethylente, tetrachloroethylene…, see Table 3); linear and cyclic methylsiloxanes (e.g., octamethyltrisiloxane-L3, decamethylcyclopentasiloxane-D5…, see Table 7) and the polycyclic aromatic hydrocarbon, naphthalene, among the main chemical families. Most of these pollutants are included in the WHO priority list of chemicals as the most common chemicals to be found in indoor air in public settings for children (schools, kindergartens, and daycare centres).^[12]

Formaldehyde and acetaldehyde are ubiquitous compounds generated in the combustion processes such as cooking operations, oxidation reactions or can be emitted by consumer products and building materials, as shown in Table 6. Formaldehyde is usually the main carbonyl compound found in indoor air.^[53–56] The primary sources include furniture, wooden products with formaldehyde based-resins, paints glues, textiles, and insulating materials.^[57] In addition, cleaning products, electronic equipment, cosmetics, insecticides and paper products could contain and release formaldehyde.^[58] Formaldehyde is included in the list of pollutants that have to be controlled indoors by WHO.^[2]

Another significant group of pollutants is known as BTEX. This group comprises benzene, toluene, ethyl benzene and xylenes (m-, o-, p-) that can originate from the building materials, cleaning products, house-hold products, adhesives, or smoking to mention some, see Table 2. Synthetic materials are widely used in the buildings, then a large number of VOCs are emitted indoors. Exposure to benzene in air has been associated with reduced birth weight, preterm birth and neural tube defects.^[59,60] No safe level of expore can be recommended for benzene.^[2]

During the HOMEchem campaign^[61,62] researchers studied the different VOCs profiles generated during the most common usual home activities like cleaning, cooking, use of personal-care products etc. The chemical measurements were carried out with an array of real-time instruments with inlets measuring both indoor and outdoor air, passive samplers and low-cost sensors.

The methods employed for assessing the VOCs concentrations indoors depend on the objective and the materials and instruments available. Recently, Villanueva et al.^[13] reported a guidance for best practice when planning a monitoring campaign aiming at assessment of IAQ with general considerations depending on the sampling objective and considering the general procedures for a quality assurance and quality control program. Although the review is focused on inorganic pollutants, the sampling plan and recommendations can be applied to VOCs.

The laboratory techniques used for measuring volatile organic air pollutants (both indoors and outdoors) imply expensive instruments and time-consuming data analysis. Nevertheless, low-cost sensors except from obvious advantages have several drawbacks such as calibration issues, drifting, accuracy over time, not as promise real time values.^[46]

A long-term study carried out in Australia,^[63] evaluated 25 years (from 1991 to 2016) of research focused on VOCs measured indoors. Due to the long-term period considered, sampling methods varied widely: passive sampling and active sampling studies were considered, with different sampling times, analysis methods, etc. The majority of the VOCs studies considered, 82%, used active sampling with the 18% used passive sampling. Regarding the sampling matrix and material, there were stainless steel sorbent tubes, glass tubes, single and multi-sorbent tubes. Carbonyl compounds, including formaldehyde, were sampled mainly in passive mode, 62% of the studies, while the other 38% used active sampling through silica DNPH coated cartridges. Variability in the passive sampling was wide, with studies that used monitors, others radial cartridges and others passive disks. Authors mentioned that the diversity of sampling procedures and techniques made difficult to get consistent conclusions. Therefore, they remark the importance of standard guidelines for sampling and for the data analysis.

This review is the result of part of the work done within the INDAIRPOLLNET (INDoor AIR POLLution NETwork, Cost Action: CA17136). This action is focused on the identification of appropriate mitigation strategies to optimize IAQ. Different workgroups (WG) are focused on different questions regarding IAQ. One of them, WG-3 has identified a list of indoor pollutants, including VOCs, inorganic compounds, and particulate matter relevant for assessing indoor pollution. In general terms, the pollutants selected in that list and represented in Tables 1-7, in case of VOCs, have been considered due to their occurrence and their significance in terms of human health.

In this review we summarize the different available methods to determine the concentration of organic gas pollutants that can be found at indoor air from conventional to novel and promising techniques. The aim is not an exhaustive revision of the methods but an identification of well-stablished and useful methods that can be employed in indoor campaigns. The majority of the instruments and methodologies described in this paper can be used both for indoor and outdoor measurements. Nevertheless, there are issues that need to be considered in each case, as if electricity is required for the measurements, or the noise produced. Besides, this review can be used as a guidance for designing indoor air quality (IAQ) campaigns since it shows the advantages and drawbacks of the different measuring techniques and instrumentation that must be selected according to the final objective of the IAQ campaign.

2. Definition of VOCs and its classification

Volatile organic compounds are defined by the European Environment Agency as the “Organic chemical compounds that under normal conditions are gaseous or can vaporize and enter the atmosphere”. The Environmental Protection Agency (EPA) defines VOCs as any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates and ammonium carbonate which participates in atmospheric photochemical reactions.”^[64]

Volatile Organic compounds can be classified in a variety of ways such as by physical characteristics, that is, molecular weight and boiling point, by chemical species and chemical reactivity; and by their sources, depending on the purpose. The first, traditional classification, based on simple physicochemical characteristics allows organic gases found at indoor air to be categorized to three classes:

• VVOC (Very Volatile Organic Compounds: compounds whose boiling point is in the range from <0 °C to (50 °C to 100 °C))

• VOC (Volatile Organic Compounds: compounds whose boiling point is in the range from (50 °C to 100 °C) to (240 °C to 260 °C)

• SVOC (Semi-Volatile Organic Compounds: compounds whose boiling point is in the range from (240 °C to 260 °C) to (380 °C to 400 °C)

as World Health Organization define and adopted by ISO 16000-6:2011.^[42]

The lower the boiling point the higher the volatility and the greater the vapor pressure of the pollutant. This classification, based mainly on the boiling point, takes into account aspects for the analysis, especially of gas chromatography and is essential for the selection of the appropriate off-line sampler. Pollutants with greater vapor pressure must be collected on sampling media with a greater specific surface.^[65] Another classification of VOCs is considering their chemical families independent of their molecular weight and size as shown in Tables 1-7. This classification is useful especially when certain chemical families present a different sampling and analysis technique with regard to the rest of VOCs, such as aldehydes.

3. Methodologies for measuring volatile organic compounds in indoor air

In indoor air, building materials, house activities as cooking or cleaning, consumer products or smoking can release a huge variety of VOCs that can affect the human health.^[66–69] In order to measure VOCs concentration, usually very low, there are different techniques and methodologies available. Analytical techniques for measuring air can be divided into off-line techniques and on-line or real-time techniques.

Before describing the methods for sampling and analysis of VOCs in indoor air, it is important to highlight that the ISO 16000 series of standard are of particular interest for preparing an indoor campaign. The most important ISO standards for measuring VOC in indoor are the following:

• ISO 16000-1:2004. Indoor air-Part 1: General aspects of sampling strategy^[37]

• ISO 16000-2: 2004. Indoor air- Part 2: Sampling strategy for formaldehyde^[38]

• ISO 16000-3: 2011. Indoor air- Part 3: Determination of formaldehyde and other carbonyl compounds in indoor air and test chamber air — Active sampling method^[39]

• ISO 16000-4: 2011. Indoor air-Part 4. Determination of formaldehyde — Diffusive sampling method.^[40]

• ISO 16000-5: 2007. Indoor air-Part 5. Sampling strategy for volatile organic compounds.^[41]

• ISO 16000-6:2011. Indoor air-Part 6: Determination of volatile organic compounds in indoor and test chamber air by active sampling on Tenax TA® sorbent, thermal desorption and gas chromatography using MS or MS-FID.^[42]

• ISO 16000-33:2017. Indoor air-Part 33. Determination of phthalates with gas chromaography/mass spectrometry (GC/MS).^[43]

• ISO 16017-1:2000. Indoor, ambient and workplace air — Sampling and analysis of volatile organic compounds by sorbent tube/thermal desorption/capillary gas chromatography — Part 1: Pumped sampling.^[44]

• ISO 16017-2: 2003. Indoor, ambient and workplace air - Sampling and analysis of volatile organic compounds by sorbent tube/thermal desorption/capillary gas chromatography - Part 2: Diffusive sampling.^[45]

3.1. Off-line techniques

Off-line techniques are the methodologies that need to sample the air in a matrix before the sample analysis in the laboratory. Besides, off-line techniques can be classified as active or passive. The active air sampling involves the use of a pump or vacuum to force the air compounds into the sampler. The sampler can be a cartridge filled with a sorbent, a tube with a selected sorbent specific for a family of compounds, a Teflon bag or a filter among others. Analyte concentrations are calculated afterwards considering the total sampling volume. A scheme of a sorbent tube can be seen in Figure SI1 (supplementary material). Passive sampling implies the free flow of the chemical compounds from the air to the sampler as a result of a physical process such as diffusion or permeation. The sampler is filled with a suitable trapping media that can be the same as that used in active sampling. Passive samplers are exposed for a defined time period that ranges from hours to days and provide data on average concentration during the sampling period. Concentrations are calculated considering the exposure time and the sampling rate known as diffusive uptake rate. The scheme of a diffusive or passive sampler from Radiello® is shown in Figure SI2.

Off-line technique with active sampling could give time profiles for the pollutants although it is a tedious task while passive sampling offers a long-term average concentration.^[5] On the other hand, active sampling needs pumps electric power and can be noisy. While passive sampling is cheap, noiseless, does not require power supply, is easy to install and does not require professional staff. In both cases, once the samples are collected, they are transported to the laboratory and refrigerated until analysis.

The concentration of the VOCs is calculated according to the following equation: Eq. 1 $$C(\mu g{m}^{-3})=\frac{m(\mu g)}{Q(mlmi{n}^{-1})xt(min)}$$Eq. 1

Where m is the mass of the target pollutant, t is the sampling time and Q is the sampling rate that in the case of passive sampling is also called diffusive uptake rate.

Procedures for quality assurance and quality control should be performed regardless of the method selected. Several analytical parameters such as precision, accuracy, linearity, limits of detection (LOD) and limits of quantification (LOQ) must be calculated to ensure the quality of the analysis of samples. Blank emissions and artifacts formations must be evaluated with laboratory and field blanks that are analyzed as the sampled tubes or cartridges. More information about analytical quality control is available in the WHO report^[12] and Villanueva et al.^[13]

3.1.1. Active sampling

One of the most important ways for sampling actively VOCs listed in Tables 1–7 except aldehydes are using different type of cartridges or solid sorbents.^[70] The analysis of the samples implies an extraction step followed by the analysis in the analytical instrument. The extraction can be carried out using thermal desorption (TD) or solvent desorption (SD) depending on the kind of sampler used. The analysis is carried out by means of different analytical systems depending on the target pollutants such as liquid chromatography (LC) or gas chromatography (GC) coupled to different detectors such as mass spectrometer (MS), ultraviolet (UV), diode array (DAD) or flame ionization detector (FID). On the other hand, it is important to have an estimation of the expected concentrations to prevent breakthrough. For samplers which contain backup sampling media, only the front section of the sampler should be used. If the mass of analyte found on a backup sampler totals 5% of the mass found on the front sampler, breakthrough has occurred and the capacity of the sampler has been exceeded.^[71]

Sorbent tubes are an important group of samplers used to capture the VOCs in air. A wide range of sorbent materials can be used to fill the tubes, as can be seen in Table 8. In general terms, sorbent tubes are not suitable for highly volatile compounds even they can be used for polar and nonpolar compounds.^[71–73]

Table 8. List of some of the most used adsorbents for sampling VOCs.

Sample tube sorbent	Analytes carbon atoms range	Max. Temp. (°C)	Hydrophobic	Temp. and Gas flow for conditioning	Temp. and mind. Gas Flow for description
Carbotrap C	n-C8-n-C20/Alkyl benzenes and aliphatics	>400	Yes	350 °C and 100 ml/min.	325 °C and 30 ml/min.
Carbopack C	n-C8-n-C20/Alkyl benzenes and aliphatics
Anasorb GCB2	Alkyl benzenes and aliphatics
Tenax TA	n-C6-n-C26/Aromatics except benzene, apolar compounds	350	Yes	330 °C and 100 ml/min.	300 °C and 30 ml/min.
Tenax GR	n-C7-n-C30/Alkyl benzenes, some PAHs and PCBs	350	Yes	330 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbotrap	(n-C8) n-C, to n-C14/oxygenated VOCs and apolar compounds, wide range of VOCs	>400	Yes	350 °C and 100 ml/min.	325 °C and 30 ml/min.
Carbopack B	(n-C8) n-C, to n-C14/oxygenated VOCs and apolar compounds, wide range of VOCs
Anasorb GCB1	(n-C8) n-C, to n-C14/oxygenated VOCs and apolar compounds, wide range of VOCs
Chromosorb 102	n-C5-n-C12 included oxygenated VOC /halogenated VOCs	250	Yes	250 °C and 100 ml/min.	225 °C and 30 ml/min.
Chromosorb 106	n-C5-n-C12 included oxygenated VOC	250	Yes	250 °C and 100 ml/min.	250 °C and 30 ml/min.
Porapak Q	n-C5-n-C12/oxygenated VOCs	250	Yes	250 °C and 100 ml/min.	225 °C and 30 ml/min.
Porapak N	n-C5-n-C8/volatile nitriles and alcohols	180	Yes	180 °C and 100 ml/min.	180 °C and 30 ml/min.
Spherocarb	C3-n-C8 includes CH2Cl, ethylene oxide, CS2 and volatilepolars MeOH, EtOH an acetone	>400	No	400 °C and 100 ml/min.	390 °C and 30 ml/min.
Carbosieve SIII	C3-C16/for VVOCs	400	No	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carboxen 1000	C3-C16/for VVOCs			350 °C and 100 ml/min.	300 °C and 30 ml/min.
Anasorb CMS	anesthetic gases			350 °C and 100 ml/min.	300 °C and 30 ml/min.
Zeolite	1,3-butadiene and nitrous oxide	350	NO	330 °C and 100 ml/min.	300 °C and 30 ml/min.
Molecular sieve 13X	mercaptanes, aromatics, branched-chain hydrocarbons, and organosulfur compounds
Coconut Charcoal (Coconut Charcoal is rarely used)	VOCs in general	>400	–	–	–
Tenax/ CB:comb. Tube type 1	C3-C16/for VVOCs	350	Yes	330 °C and 100 ml/min.	300 °C and 30 ml/min.
Carb B/CMS comb. Tube type 2	C3-C16/for VVOCs	400	No	350 °C and 100 ml/min.	325 °C and 30 ml/min.
Carb 300 type comb, tube type 3	C3-C16/for VVOCs	400	No	350 °C and 100 ml/min.	325 °C and 30 ml/min.
Carbotrap X	aromatic VOCs	400	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbopack Z	aromatic VOCs	400	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbopack X	C5/6-C9	400	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbograph 5	C5-C8	400	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbograph TD-1	n-C4/5-n-C14 including ketones alcoholsa and aldehydes	350	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Cabosieve S-III	C2-C5	400	No	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carboxen 569	C2-C5	300	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carboxen 1000	C3-C4 including haloforms and freons	300	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.
Carbotrap B	n-C4/5-n-C14 including ketones alcoholsa and aldehydes	400	Yes	350 °C and 100 ml/min.	300 °C and 30 ml/min.

There is a large selection of commercial sorbents that can match the VOCs group of interest compounds. Nevertheless, if none of the available sorbent material fits the target compounds, it is easy to make home-made combinations.

The number of VOCs retained on a sorbent is determined to a large extent by the sorbent bed length and sorbent mass. Typically, a sorbent tube has a length of 90 mm and an outer diameter of 6 mm, containing 0.1-1 g of the sorbent (see Figure SI1). Some of the most used sorbent materials are summarized in the EPA Method TO-17 and ISO Standard 16017-1 that present the guidelines for sorbent selection.^[44,71] A list with different sorbent material (Tenax TA, Tenax TG, Carbotrap B and C, Carbopack B and C, Chromosorb 102, Carbosieve S-III, etc.), analyte volatility range (considering number of C or the boiling point), analysis temperature, specific surface area and examples of analytes that can be quantified are presented in abovementioned methods. Several reviews focused on the VOCs monitoring can be found in the literature.^[74–78]

ISO 16000-6 specifies a method based on Tenax TA as adsorbent to sample indoor air VOCs.^[42] A great number of published studies use Tenax TA and Tenax GR to collect VOCs (Figure SI1).^[79–82] The analysis of the trapped VOCs is performed with directly insert the sampling tube in a Thermal Desorption System coupled to a Gas Chromatography/Mass Spectrometer or Flame Ionization Detector instrument (TDS-GC/MS or FID). For example, Sekar et al.^[83] reviewed 480 references for benzene analysis and found 60% MS in contrast to 40% of FID while the Thermal desorption injection was at the top of the methods used. This protocol considers VVOC the compounds eluted before C6 i.e., C1 to C6 and SVOC those compounds eluted after C16 so that SVOC can be organic molecules with more than 16 carbon atoms.

Fortenberry et al.^[84] studied the VOCs (and the particles) present indoors and their evolution with natural ventilation using a thermal desorption aerosol gas chromatograph (TAG) for hourly in situ measurements. On the other hand, off-line samples were taken through adsorbent tubes and analyzed by GC-MS and HPLC. The researchers could observe the oxidation of nonanal and 2-decanone once outdoor oxidants entered indoor. Cincinelli et al.^[85] measured VOCs in libraries and archives in Florence (Italy) in order to characterize the indoor air quality for determining the presence of harmful pollutants for the cultural heritage institutions. In this study, samples were taken in sorbent tubes filled with Tenax GR and graphitized carbon black at a flow of 200 mLmin⁻¹. Acetic and formic acids (compounds widely generated in archives and libraries as a result of the degradation of lignin and cellulose) were measured with sorbent tubes filled with Tenax TA, Carbograph and Carboxen. After the sampling, the tubes were analyzed in a thermo-desorber GCMS instrument. They detected BTEX as the main pollutants together with volatile methylsiloxanes (VMS), some aldehydes (hexanal, benzaldehyde and furfural), terpenes and organic acids. As outlined by the researchers, the indoor air quality monitoring is important for warning about the damages that valuable/historical objects can suffer in order to preserve them. Smedemark et al.^[86] utilized silica gel tubes and 20 ml of 0.1 M solution of sodium hydroxide to trap formic acid and acetic acid from air. The quantification of the two acids from the both sampling media was done by Ion Chromatography (IC) and allows a much more precise determination of C₁-C₂ carboxylic acids in comparison to the use of Tenax TA® and subsequent analysis by TD-GC/MS.

Recently, Gonzalez-Martin et al.^[87] have reviewed the strategies for indoor pollution comparing the use of conventional techniques, such as adsorbent tubes analyzed by GCMS, with newer ones such as biological-based methodologies.

Prior to air sampling, sorbent tubes must be conditioned at high temperature with an inert carrier gas at a determine flow rate. The quantification can be performed with external standards spiked on pre-cleaned tubes or using the internal standard method.^[42,71]

Another possibility to extract the VOCs from the adsorbent material (activated charcoal) is using solvent desorption. This method is based on the European Standard EN 14662-2 that although it is for ambient air quality, the active samplers and the analytical methodology using solvent desorption and analysis by capillary gas chromatography can also be used for indoor air as an alternative to TD. It is a less environmentally sustainable option due to the use of solvents such as CS₂. This European Standard is for benzene but the solvent desorption methodology can also be used for the rest of VOCs of interest.

Volatile methylsiloxanes are a group of chemicals widely used in a great variety of industrial products and consumer goods, including personal care products, household products, cleaning agents, sealants, etc (see Table 7). This kind of compounds have been collected using tubes filled with Tenax TA or with a combination of sorbents such as silica gel, carbon sieve and charcoal,^[88] Tenax TA/carbon-sieve^[89] or Tenax GR/graphitized carbon black^[90]. In all these cases the analysis was carried out with two-stage TD coupled to GC-MS. Isolute ENV + commercial SPE cartridges has also been used to sample VMS.^[91,92] Companioni-Damas et al.^[93] compared the retention efficiency of five sampling sorbents (activated coconut charcoal, Carbopack B, Cromosorb 102, Cromosorb 106 and Isolute ENV+) and Isolute ENV + was found the most effective. The sampling volume was optimized to 2700 L. The analytes were extracted by means of solvent extraction and analyzed by GC-MS. To increase sensitivity of the method, concurrent solvent recondensation-large volume injection (CSR-LIV) was applied. Homem and Ratola,^[94] recently published a review about the analytical methods of VMS in the environment including air.

Another kind of cartridges that can be used for VOCs sampling are C18 coated cartridges. C18 is an octadecylsilane bonded silica sorbent with the surface passivated with non-polar paraffinic groups, non-polar, hydrophobic and relatively inert. After treatment, the samples use to be analyzed by GC-MS.^[95–97] Figure SI3 shows a picture of this C18 cartridge.

For sampling carbonyl compounds (ketones and aldehydes), the most frequent used sorbent material is silica gel impregnated with 2,4-dinitrophenylhydrazine (DNPH). DNPH-Silica cartridges trap aldehydes and ketones in air by reacting them with the 2,4-dinitrophenylhydrazine (DNPH) in the cartridge to form stable hydrazone derivatives. The derivatization reaction (Figure 1) takes place during sample collection. The derivatives are later eluted and analyzed.

Figure 1. Derivatization reaction between 2,4-dinitrophenylhydrazine (DNPH) with carbonyl compounds to form stable hydrazones (DNPH-derivatives).

A detailed sampling protocol for formaldehyde in indoor air is described in ISO 16000-2^[38] standard but it could be applicable to the rest of carbonyl compounds listed in Table 6. Methodology for sampling and analysis is based on EPA Method-TO11A^[98] and Method-TO5,^[99] updated in order to analyze samples by LC-MS.^[98,100,101]

The ISO 16000-3 standard^[39] is based on a method with active sampling on silica cartridges coated with acidified 2,4-DNPH and UV detection. In addition,ISO 16000-3 standard specifies a method for the purification of 2,4-DNPH by multiple recrystallizations in UV-grade acetonitrile in order to prepare the DNPH-Silica cartridges. Moreover, C18 cartridges, mentioned above, coated with acidic DNPH solution by the user can be utilized. However, there are very few references using this sampling methodology.^[102] Figure SI4 shows some examples of commercially available DNPH cartridges for collecting carbonyl compounds. This kind of sorbent material needs a liquid solvent extraction for the analysis and the extract is analyzed by HPLC with the appropriate detector (usually DAD, MS or Ultraviolet detector-UV). Carbonyl compounds can be found both to VVOC and VOC category, so the use of DNPH can cover a wide range of boiling point of the compounds in contrast to other sorbent materials.

Special care should be taken if high concentrations of ozone are suspected in the sampling site (e.g., photocopiers in offices). It has been proven that ozone interferes negatively, by reaction with DNPH and their hydrazone derivatives in the cartridge. To avoid this interference a denuder or ozone scrubber can be placed previous to the DNPH cartridge.

A complete review about the different approaches and devices that are applied in airborne formaldehyde monitoring has been recently published by Dugheri et al.^[103] Tubes and cartridges filled with solid sorbents, gas sampling bags, impingers and denuders for active sampling of formaldehyde and the analytical instruments (LC-MS/MS, Electrospray Ionization (ESI), atmospheric pressure chemical ionization (APCI), HPLC, GC coupled to different detectors) are described to determine the concentrations of formaldehyde in air.

For all the different samplers, the tube filled with the appropriate sorbent media is attached to a pump and certain volume of air is passed through the tube/sorbent material as shown in figure SI1. The flow rate and the total sampling volume of the indoor air are essential for the active sampling method. Usually for typical VOC the collected air is in the order of 5-10 Liters while for carbonyl compounds are much higher in the order of around 100 Liters.^[10] The sampling flow rate is in the range of 50-200 ml/min for VOCs^[39] while for aldehydes in the range of 0.5–1.2 L/min. The sample size should be less than 75% of the mass of DNPH added to the cartridge.^[39]

Finally, ISO 16000-33^[43] describes a detailed sampling protocol for gaseous phthalates by means of active samplers either using an adsorbent tube with Tenax® TA according to ISO 16000-6^[42] or using an adsorbent tube filled with Florisil®.^[43] In the laboratory, sorbent tubes filled with Tenax® TA are extracted by using thermal desorption and tubes with Florisil® are extracted with a solvent, later the extract is analyzed by means of GC-MS.

As a conclusion, if the sampling strategy include the collection of target pollutants with active sampling, it is necessary to know and select the appropriate adsorbent media(s) to use, in order to trap the target compounds and analyze their concentrations. On the other hand, the selection of the analytical procedure on the instrumentation is also the key role for determining as much as possible organic gasses in one shot.

3.1.2. Passive sampling

Passive samplers are based on the mass transport described by Fick’s first law of diffusion. Depending on the type of diffusion barrier used they can be diffusive passive samplers or permeation passive samplers. In the first case, the free diffusion of the analytes through the gas layer happens while in the second one, the analytes are transported by way of permeation through a semipermeable membrane.

In indoor air quality campaigns, passive samplers are usually selected since they present several advantages already described above such as are lightweight, small and do not need electrical power.^[104] In addition, passive sampling present spatial resolution because it allows to deploy several passive samplers simultaneously in several places.

The diffusive sampler due to their function usually are typical for long-term mean concentrations of pollutants in indoor air covering periods of basically of one day to four weeks.^[105] The critical value of a diffuse/passive sampler is the sampling/diffusive uptake rate (volume of the analyte/time) which is a function of diffusion coefficient, a thermodynamic property of each chemical substance. Also, the sampling rate is related to the geometric parameters of the sampler.^[105,106] The uptake rate is usually determined by the manufacturer by means of a calibration in a standard atmosphere and is specific for each sampler model and for each pollutant being a function of temperature during sampling time. For that, temperature must be registered during the sampling period in order to re-calculate the uptake rate and then the concentration of the pollutant.^[12]

The sorbent materials used for passive sampling are the same as for active sampling.^[45] There are two types of diffusive samplers commercially available: one for direct use on a TDS-GC/MS or FID of the sampler cartridge after the sampling period without any treatment (like active sampling with sorbent tubes suitable for thermal desorption analysis) and the second one to extract the trapped organic gases with the appropriate solvent and then inject the extract to a proper analytical instrument.^[105] The aforementioned samplers are widely used for the monitoring of volatile organic gases. As for active sampling, DNPH sorbent is also used as the appropriate sorbent for carbonyl compounds (ketones and aldehydes) collection.^[106,107] Standards EN 14412 (specific for IAQ) and EN 13528-3 provide a guide to selection, use and maintenance of passive samplers.

As commented above, ISO 16000-5^[41] standard provides the sampling strategy for VOCs in indoor air and the sampling and analytical procedure can be found in ISO 16017-2^[45] using thermal desorption of passive samplers and GC-MS, GC-FID or GC-PID (PID, photoionization detector). In addition, this Standard provides also recommendations to prepare passive samplers in the laboratory.

On the other hand, the European Standard EN 14662-5 describes the sampling and an analytical procedure for measuring benzene, using CS₂ for desorption and analysis by means of GC-MS or GC-FID. This method although is for ambient air quality, the diffusive samplers and the analytical procedure can also be used for indoor air and it is applicable to a wide variety of VOCs.^[53]

There are several diffusive samplers based on carbon available in the market for collecting VOCs (Radiello, Analyst, Dräger ORSA-5, 3 M 3500/3520, SKC, etc.). The Radiello® system uses a cylindrical outer surface that acts as diffusive membrane in which gaseous molecules move both axially and parallel toward the adsorbent bed (cylindrical collection cartridge) and coaxial to the diffusive surface, as can be seen in Figure SI2(a). The radial design allows for a very large diffusive surface relative to the adsorbing surface while maintaining a small diffusive distance between the diffusive and adsorbing surfaces. At present, there are over 11 different cartridge adsorbents and four different diffusive bodies to sample hundreds of different gaseous compounds (inorganics and organics) under a variety of conditions. For each compound, the Fondazione Salvatore Maugeri (FSM) has developed detailed desorption and analytical protocols using the most common analytical instruments. Among them, it is possible to analyze aldehydes by chemisorption on 2,4-dinitrophenylhydrazine and analysis by HPLC coupled to UV detector or DAD and VOCs/BTEX by chemical desorption after collection over activated charcoal or thermal desorption after collection over graphitized carbon. It is also possible the collection of anesthetic gases adsorbed on activated charcoal, phenols collected on Tenax TA and analyzed by thermal desorption and 1,3-butadiene and isoprene, also analyzed using thermal desorption after collection on Carbopack X. As mentioned, detailed protocols for the sampling and analysis procedures can be find in the Radiello ® official reports.

As aforementioned, Dugheri et al.^[103] published a complete review about the different passive sampler devices (badged-type, cylindrical type, radial-type and SPME) for collecting formaldehyde in air.

In Greece, Kalimeri et al.^[108] measured some VOCs, radon, CO₂, O₃ and PM2.5 in 3 schools. The different VOCs, from benzene to formaldehyde and terpenes, and O₃ were measured using different kind of Radiello ® passive samplers with the specific analysis methodologies. The study measured formaldehyde concentrations in the range 7-29 µg/m³ for summer period and 2-7 µ g/m³ for winter season. The more likely source of formaldehyde were the furniture and the wooden products treated with formaldehyde-based resins. Benzene, the other VOC together with formaldehyde, with known carcinogenic effect, presented concentrations in the ranges 1.5-6 µg/m³ for summer and 4.1-9.4 µ g/m³ for winter. From the indoor/outdoor ratio it was concluded that both, benzene and formaldehyde, had indoor sources. Depending on the specific environment, those indoor sources were more or less significant. In Spain, Villanueva et al.^[5,53] studied the indoor pollution at homes and schools. Radiello passive samplers were used for VOCs (cartridges filled with activated charcoal) and carbonyl compounds (cartridges filled with Florisil coated with 2,4-DNPH). The VOCs cartridges were exposed for 2 weeks and the carbonyl ones for 1 week for 2 months. Analysis of the samples were carried out by means of GC-FID for the VOCs and HPLC-DAD for carbonyl compounds. The authors found that formaldehyde and hexanal were the most abundant pollutants at both homes and schools. Villanueva et al.^[54] also used Radiello ® passive samplers for measuring carbonyl compounds in dwelling of university students but using empty and clean Radiello cartridges that were refilled in the laboratory with silica gel impregnated with 2,4-DNPH that was purified and acidified with phosphoric acid according to the method described in ISO 16000-3 Standard.^[39] The most abundant carbonyls in the living rooms and bedrooms were formaldehyde, acetone, acetaldehyde, hexaldehyde and butyraldehyde being indoor sources prevailing in all flats.

Another sampling technique widely used currently is Solid Phase Microextraction (SPME) developed by Arthur and Pawliszyn.^[109] With SPME, the analytes are absorbed from the liquid or gaseous sample on to an absorbent coated fused silica fiber, which is part of the syringe needle, for a fixed time.^[110] SPME is a solvent-free technique which is sensitive because of the concentration factor achieved by the fiber, and selective because of the different coating materials which can be used. One of the advantages of SPME is the possibility to sample directly the vapor phase in equilibrium with the matrix (headspace (HS)-SPME), or the matrix extract or solution (liquid sampling-SPME) directly provided that suitable fibers are used. The solid-phase microextraction followed by gas chromatography-mass spectrometry method (SPME-GC-MS) is widely used to identify and quantify the volatile compounds in environmental samples. Suppliers (as Merck) have brochures with information about the different kind of fibers and GC methods to analyze the different VOCs. Kearney at al.^[111] placed solid-phase microextraction fibers comprised of di-vinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) around an object in a museum in order to examine in-situ the deterioration of it and manage to identify up to 15 organic compounds and among others acetic acid, phenol, dimethyl phthalate and diethyl phthalate. The fibers were analyzed by manual injection into the port of a GC/MS instrument.

Recently Gonçalves et al.^[112] proposed a semipermeable membrane device (SPMD) filled with 3 ml acetonitrile or mineral oil for passive sampling of toluene and benzene in gas phase. This kind of passive sampler was employed in five nail salons and research laboratories with no need of further extraction allowing their direct injection into the chromatographic system (HPLC-DAD).

On the other hand, O’Connell et al.^[113] used silicone wristbands as a personal passive sampler and managed to identify 49 different compounds including PAHs, consumer and personal care products, pesticides, phthalates and other industrial compounds of 22 participants in the study. In France, the MERMAID study measured indoor and outdoor pollutants in low-energy schools.^[114] The authors measured organic and inorganic compounds, combining periods with occupancy and periods unoccupied. Organic compounds were measured through Radiello samplers. The main pollutants, among the BTEX, carbonyls, glycols, terpenes, etc., found were carbonyl compounds, representing 40-82% of all VOCs.

Other study, performed in Japan, measured the indoor and outdoor concentration of VOCs in summer and winter.^[104] This study used different passive samplers depending on the target compounds: DSD-BPE/DNPH for sampling carbonyl compounds and ozone simultaneously developed by Uchiyama et al.^[115]; DSD-TEA for acid gases (silica gel impregnated with triethanolamine (TEA) is packed in the sampler as the absorbent. The acid gases present in air react with TEA in the absorbent to form corresponding anions); DSD-NH₃ for basic gases, acid and OCs-SD for collection of VOCs (Carboxen 564 as absorbent). Carbonyl compounds were analyzed in a HPLC with UV detection instrument, acid and basic gases were analized in an Ion Chromatrography (IC) instrument and VOCs were analyzed in a GC-MS system. Authors found that the majority of the compounds detected showed higher concentrations in summer than in winter. In that sense, results showed a strong temperature dependence for formaldehyde, toluene and ammonia.

One of the problems that must be avoiding when using passive samplers is the back diffusion that happens when adsorbed mass of pollutant is higher than the maximum amount allowed by the adsorbing material capacity. The extent of back-diffusion depends on concentration and exposure time. For that, it is important to follow the manufacturer recommendations with regard to the sampling time and the concentrations of pollutants.

Different research groups have performed indoor studies using passive samplers. WHO recommends this kind of samplers to collect sixteen gaseous pollutants with the aim of assessing health risks from combined exposure to multiple chemicals in indoor air in public settings for children.^[12] During the European OFFICAIR project, more than 140 office rooms were selected for collecting VOCs using passive sampling in 8 European countries. Authors found that laser printers were a significant formaldehyde source as well as photocopiers were a source of acetaldehyde. Authors found interesting relations between aldehyde and VOCs in office buildings involving furnishing and building materials.^[116]

3.2. On-line techniques

There is an increased awareness of the issues that polluted air can generate to the human health. Besides, the legislation worldwide is more restrictive each time. Therefore, there is a need for real time measurement of pollutants, implying that the instruments have to be running in a continuous basis in order to obtain results within a short time -few hours at maximum. In some instruments, hard mathematical treatments are needed so, the time required for the results is higher.

To monitor VOCs with high time resolution is essential to understand the evolution of the concentrations and patterns in indoor air especially during certain daily activities such as cooking, cleaning or determine the effects on indoor air of human occupancy. Likewise, characterizing sources and emissions of VOCs in indoor air would allow to understand the indoor chemistry and to know the exact human exposure. In this regard, the House Observations of Microbial and Environmental Chemistry (HOMEChem) study^[61,62] is the most recent example of a large, collaborative indoor air campaign performed in the USA where everyday activities carried out indoors, such as cooking, cleaning, window-opening, etc… were followed by a number of analytical instrumentations for measuring VOCs, particles, and inorganic compounds. On-line measurements were done using PTR-MS-ToF, Iodide-CIMS, Acetate-CIMS, GC with thermodesorption among others instruments. Passive VOCs samplers of different types were used and analyzed by GC-MS and LC-MS.

The use of on-line instruments for determining the concentration of pollutants indoors has the non-negligible advantage of showing time profiles in real time. The time resolution is a key factor in order to determine indoor VOCs sources, since the air composition can change quickly. However, almost all those instruments required large spaces for their installation, they are weight, noisy and need power supply.

In many studies, combination of on-line and off-line measurements are used.^[61,117,118] Rizk et al.^[119] assessed the VOCs emission rates and sorption coefficients for several surfaces in a classroom of a low energy school building in France. Chemical compounds were measured using both off-line and on-line techniques. Carbonyl compounds were measured using silica DNPH coated cartridges followed by HPLC-DAD analysis, hydrocarbons from 2 to 12 carbons were quantified by using an on-line GC-FID each 90 min and also a PTR-MS-ToF was used for quantifying VOCs emitted by the surfaces.

Among the instrumentation available for measuring on-line, there are some that cannot be moved easily and are difficult to be used indoors, while others can be transported with just some cautions. In the first group are FTIR (Fourier transform infrared spectroscopy) and DOAS (Differential Optical Absorption spectroscopy) instruments widely used in smog chamber experiments or even in ambient air monitoring campaigns, but not used in indoor real- environments. Some other instruments, for example, the GC have been adapted for on-line measurements with sensors that can monitor BTEX. During the last decades, proton transfer reaction (PTR) and chemical ionization (CI) mass spectrometry (MS) has been used for in situ characterization of indoor VOCs.^{[120 and reference there in]} Farmer et al.^[61] for an intensive indoor air study also use an in-situ four-channel GC with FID and Electron Capture Detector (ECD) for more complicate molecular analysis. Claflin et al.^[120] for an athletic center indoor study developed an automated field deployed GC equipped Thermal desorption (TD) and automated detector that changes between two high-resolution time-of-flight mass spectrometers: PTR-TOF-MS and EI-TOF-MS

Table 9 summarizes the instrumentation deployed for direct and rapid analysis of organic gases found indoors. This table also includes instrumentation used in monitoring stations of VOC as well as spectroscopic methods for individual compounds such as formaldehyde. All these instruments are enough bulky for using them in indoor measurements. It is needed to install them in specific rooms from where the sampling lines arrive to the sampling locations as done during the HOMEchem campaign.^{[61,62,121–124]} Manufactures try to improve the volume and the necessarily apparatus in order to be portable, but again these instruments are quite big and heavy compared to the available room for an indoor air study.^[125]

Table 9. List of indicative on-line instruments used for analysis of organic volatile gases.

Instrument	Compounds analyzed	Reference
On line gas analyzers for monitoring station (e;g; GC/FID)	BTEX (benzene, Toluene, Ethylbenzene, Xylenes)	[177]
GC-PID, GCxGC-PID (Photoionzation Detector)	BTEX and various VOC	[178]
Commercial portable GC-MS	Various VOC	[178, 179]
Micro GC micro-machined thermal conductivity detector (GC/μTCD)	Various VOC	[178]
PTR-MS (Proton Transfer Reaction Mass Spectrometer)	Various VOC	[180]
PTR-TOF-MS (Time-of-Flight)	Various VOC	[176]
SRI-TOF-MS (Selective Reagent Ionization)	Various VOC	[181]
SIFT-MS (Selected Ion Flow Tube)	Various VOC	[182]
GC with Thermal desorption preconcentration and automatic switch between PTR-TOF-MS and EI-TOF-MS (Electron Ionization)	Various VOC	[120]
Commercial Spectrophotometer at 630 mm wavelength	HCHO	[183]
Commercial on line chemical analyzer or customized instrument, based on the Hantzsch (acetyl- acetone) reaction	HCHO	[184]
CIMS-Iodide/Acetate	Various VOC	[134, 135]

The following sub-sections describes the real time instruments for VOCs most commonly used by researchers, some of them have already been mentioned above.

3.2.1. Proton transfer reaction mass spectrometry (PTR-MS)

Proton transfer reaction mass spectrometry (PTR-MS) is a versatile analytical technique that has been applied worldwide for the measurement of VOCs. This technique is based on chemical ionization by means of proton-transfer reactions.^[126,127] The PTR-MS (with Time-of-Flight (TOF) or single quadrupole) uses hydroxonium (H₃O⁺) ions to ionize the analyte molecules. The relatively soft chemical ionization technique results in only weak fragmentation of the analyte. The hydronium ion can effectively protonate a great number of VOCs with proton affinities greater than that of water, such as alkenes, aromatics, carbonyls etc. then proton transfer happens in every collision. Nevertheless, there are compounds that do not react in that way, as happen with the alkanes.^[126,128]

The protonation reaction (equation 2(2) $${\text{H}}_{\text{3}}{\text{O}}^{+}+\text{ VOC }–->\text{ VOC}-{\text{H}}^{+}+{\text{ H}}_{\text{2}}\text{O}$$(2) ) occurs when the VOC has a proton affinity higher than that of H₂O (691 kJ/mol): (2) $${\text{H}}_{\text{3}}{\text{O}}^{+}+\text{ VOC }–->\text{ VOC}-{\text{H}}^{+}+{\text{ H}}_{\text{2}}\text{O}$$(2)

After protonation of the analyte, the resultant ions are focused through a lens system and are directed to the flight tube, where the ions are separated according to their flight time. The travel time before the ions reach the detector, a multi-channel plate, depends on the mass to charge ratio. To be protonated, the proton affinity of a molecule needs to be greater than the proton affinity of H₂O. The mass resolution (m/Δm) of 4000 (depending on the instrument) for the PTR-TOF-MS is sufficient to separate two compounds with the same nominal mass, while this is not possible with a quadrupole PTR-MS. A picture of a commercial instrument can be seen in Figure SI5.

Tang et al.^[119] measured occupant-related VOC emissions from engineering students in a classroom using a PTR-TOF-MS (PTR-TOF 8000, IONICON Analytik GmbH), and reported that VMS were the most abundant VOCs emitted. In the HOMEchem study a PTR-TOF-MS was also used to monitor the VOCs.^[62] Schripp et al.^[130] studied five exemplary applications of PTR-MS (release of paint additives during drying process, emission of VOCs from active hardcopy devices, reference material evaluation, diffusion studies and emission testing of building products) with the aim of illustrating possibilities and limitations of the PTR technique in the field of IAQ research. They concluded that if identification of VOCs is necessary, the measurements need to be accompanied by GC/MS analytics or a PTR instrument with higher mass-resolution (e.g., PTR-TOF-MS).

Other instrumentation used for direct analysis techniques are selective reagent ionization time-of-flight mass spectrometry (SRI-TOF-MS) and selective ion flow tube mass spectrometry (SIFT-MS). This technique is based on the PTR-ToF-MS with the additional ability to ionize analyte species using alternative reagent ions such as NO⁺ as well as the more usual H₃O⁺ providing more structural information than traditional PTR-ToF-MS. Other ions such as NO⁺ and O₂⁺, can be useful for the identification of VOCs and for the detection of VOCs with proton affinities below that of H₂O.^[131]

3.2.2. Chemical ionization mass spectrometer (CIMS)

The Chemical Ionization Mass Spectrometers (CIMS) are instruments than can selectively ionize compounds in gas and particulate phase.^[132–135]

The CIMS-ToF instrument is a system of different pumped stages, where the ambient air is introduced by a critical orifice into the ion-molecule reaction chamber (IMR). An scheme of the process is shown in Figure SI6. Different reagent ions can be used, depending on the target compounds of interest. The introduction of the reagent ions into the IMR is made orthogonally from the entrance orifice. A Polonium commercial source formed the negative ions. A portion of the ionized gas is introduced into the collisional dissociation chamber (CDC) with a short, segmented RF quadrupole with a frequency and amplitude that can be tunable. An electric field is created in the CDC to promote a collisional dissociation.

In general, CIMS-based instruments (see figure SI7) show several interesting advantages such as linearity and reproducibility, minimal sample preparation, selectivity and sensitivity for a wide number of compounds, organic and inorganic and also in gas and particulate phases, and a high time resolution. Among the drawbacks, one of the most significant is the fragmentation of the ions and the difficulties to distinguish between isobaric compounds at high time resolutions. Selecting an adequate chemical for the adduct ionization, the group of compounds that can be determined is wide: iodide and bromide ions are used for the detection of molecular halogens and halogen oxide radicals or VOCs, chloride ions for the measurement of carboxylic acids or CF₃O⁻ for detecting hydroperoxides and hydroxynitrates.^[136,137]

Recently, online chemical ionization mass spectrometry (CIMS) has begun to be used in field measurements of VOCs indoors.^[62,138,139] Liu et al.^[138] measured gas-phase carboxylic acids in real-time inside and outside of an university classroom using a high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) equipped with an acetate ion source. The average indoor concentration of carboxilic acids was 6.8 ppb, of which 87% was contributed by formic and lactic acid. On the other hand, Ye et al.^[140] checked the efficacy of different “air cleaners” devices that remove VOCs by sorption and/or oxidative degradation. For that, VOCs were monitored using a Vocus proton-transfer-reaction mass spectrometer (PTR, Tofwerk AG/Aerodyne Research, Inc.) for hydrocarbons and relatively unoxidized species, and an ammonium chemical ionization mass spectrometer (NH₄ ⁺ -CIMS, Ionicon Analytik) for more oxygenated species. However, formaldehyde was measured by fiber laser-induced fluorescence (FLIF). Historical buildings, libraries and museums are also targets for indoor pollution studies since the understanding of how the outdoors -and the emitted indoor pollutants- can react indoors affect the valuable materials and inheritances. For example, Pagonis et al.^[141] studied emission rates from painting and occupant activities at a university art museum. Authors measured VOCs online using a single quadrupole PTR-MS and highly oxygenated organic compounds were determined with a Iodide-CIMS and a Nitrate-CIMS instrument. In general terms, measured concentrations of acetic and formic acid and total VOCs were below the guidelines for museums.

3.2.3. Trace organic gas analyzer (TOGA)

One of the most widely used techniques for the VOCs analysis is GC-MS. In that line, Trace Organic Gas Analyzer (TOGA) is a combination of fast online GC-MS. The system has a cryogenic preconcentrator, a gas chromatograph (GC), a mass spectrometer (MS), and a zero air/calibration system. The columns used varies depending on the VOCs of interest. TOGA measures a wide range of VOCs with high sensitivity (ppt or lower), frequency (2.0 min.), accuracy (often 15% or better), and precision (<3%) On the other hand, the system should be calibrated periodically with mixtures of interest.^[139,142]

3.2.4. Real-time measurements of formaldehyde

Formaldehyde is a ubiquitous compound, with carcinogenic effects and one of the most concern pollutants. Formaldehyde indoors can be formed from photochemical reactions or can be emitted from the furniture, paintings, resins, etc… there the importance of its determinationwith high time resolution is essential.

Several instruments are commercially available on the market to monitor formaldehyde in real-time based on different anaytical systems such as infrared, photometric, differential optical absorption, cavity ring-down spectroscopy, fluorimetric, and MS techniques. A list with on-line instruments and their characteristics can be found in Dugheri et al.^[103]. Some of these instruments are described below:

The Hantzchs Monitor (see figure SI8) is a continuous monitor specific for gaseous formaldehyde (HCHO). This type of instrument is currently commercially available from Aero-Laser GmbH (AL4021). It is based on the Hantzsch reaction, the cyclization of a β-diketone, an amine, and formaldehyde, used to produce a fluorescent derivative from HCHO. The fluorescence sensitivity leads to a gaseous detection limit of about 0.2 ppbv with a glass coil scrubber as the collection device for gaseous HCHO. Among the advantages of this monitor are the high selectivity, high reagent stability, and low reagent cost, with a sampling system that give high sample efficiency, stability of behavior, and simplicity of design. Response time can be set up to a second. It is a compact device that can be used both in outdoor and indoor campaigns.

Another instrument for measuring formaldehyde indoors is the Fiber Laser-Induced Fluorescence (FILIF) instrument.^[143,144] Briefly, the beam from a 20 mW, 353 nm tunable, pulsed, narrow-bandwidth laser is directed into a 32-pass White-type cell. The resulting fluorescence from HCHO from 390 to 500 nm is focused into a photomultiplier tube. The output beam from the White cell is directed into a glass cell containing high concentrations of gas phase HCHO for wavelength reference. The HCHO mixing ratio is proportional to the difference between the fluorescence signal observed when the laser is on and off the absorption feature, as well as the laser power. FILIF calibrations is performed with a HCHO permeation tube heated to 85°C in a portable calibration gas generator. An intercomparison between the FILIF and the Hantzchs Monitor showed that under all conditions the two techniques were well correlated (R² ≥ 0.997), and linear regression statistics showes measurements agree with within stated uncertainty (15% FILIF + 5% Hantzsch).^[143]

A new commercial instrument for measuring HCHO use cavity ring-down spectroscopy (CRDS) (G2307, Picarro, Inc.). Its limit of detection is specified as 0.3 ppbv (3σ) for an integration time of 300 s, and its accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 ppbv) . Glowania et al.^[145] carried out a comparison of formaldehyde measurements using three instruments, Hantzsch, CRDS and DOAS, in an atmospheric simulation chamber. They reported that the three instruments provide reliable and accurate formaldehyde measurements in ambient air when the instrument zeros of the CRDS and Hantzsch monitors are adequately taken into account., The CRDS instrument has the advantage of being small.

Proton-transfer mass spectrometry can also detect formaldehyde, however, due to the low proton affinity of formaldehyde, the sensitivity of PTR-MS is too low and exhibits a strong dependence on the water vapor mixing ratio.^[146–148] Therefore, PTR-MS measurements of formaldehyde concentrations have not become standard.^[145]

3.3. Small sensors

Over the last decades, an enormous interest is given to the build-up of small low-cost sensors and their wireless communication technologies for mapping and control the indoor air quality.^[36] An universal definition of low cost is not given yet but according to the United States Environmental Protection Agency (USEPA) low-cost sensors are devices that cost less than 2500$(USD).^[149] The main sensor technology used for low-cost small sensors specialized for organic gases are: electrochemical cell sensors, photoionization detectors, metal oxide semiconductor sensors, piezoelectric-based gas sensors i. e. surface acoustic wave, quartz crystal microbalances and tuning fork and optical sensors such as colorimetric gas sensors.^[149,150] The interferometry technique has also been used for VOC detection in the last decade using thin sensitive film (poly(dimethylsiloxane-PDMS film or zeolitesensitive thin film) which change the optical properties and volume when it is exposed to organic gas molecules.^[150,151] The determination of the VOC concentration at indoor air is a big challenge of the low-cost sensors field, because the issues of high sensitivity and selectivity, good precision and the wide operation must be solved. Even if low-cost sensors give good results when tested in a control environment they must be performed well at the real world when the matrix of the pollutants is “unknown” with lot of interferences. In addition, long-term stability and drifting issues, long-life lasting issues, energy consumption and power supply, data acquisition and data treatment are among others topics for continuing the research in order to improve the performance of the low-cost sensor devices.

4. Conclusion

The accurate determination of the concentration of organic gas pollutants at indoor places is a vital parameter for human health. Several methods for such concentration measurements have been developed and standardized to have precise results. In order to measure indoor pollution, active or passive sampling techniques followed by the appropriate chromatographic analytical method can be used. As shown in the previous sections, the selection of the sorbent media is directly connected with the targeted compounds. The definition of the sampling air volume in combination with the air flow rate and the sampling time must be consider for an accurate calculation of organic gas concentrations concerning all the available methods (on-line and off-line). The analytical techniques and instrumentation characteristics (type and length of column used, thermal program, gases used, solvents etc.) must be suitable and compatible for the targeting compounds for analysis.

From a perspective point of view, to explore the opportunities of utilizing a general and comprehensive sorbent media suitable for as many organic gases as possible, together with the relevant chromatographic analytical method is strongly suggested. Concerning the later, the use of a complementary on-line instrumentation to support the determination of organic gases concentration that conventional techniques cannot be able to do with one shot, is an open issue for the upcoming research. The future development of low cost or not, but definitely small, sensors for monitoring the concentration of organic gases with simplification, multiplexing, high sensitivity and selectively with wide operating range and good precision at almost real time measurements might be the focusing of novel approaches on indoor air quality studies. Nevertheless, the later instrumentation developments (PTR-MS-ToF, CIMS instruments…) allow the real time measurement of an, each day, wider group of chemical compounds with a high time resolution and sensitivity. Besides, the increasing interest and concern about the indoor air has lead to the construction of study-homes- as the used in the HOMEChem experiment or the Net-Zero Energy Residential Test Facility (NZERTF) both in the USA-, where the use of bulky, usually noisy, but very powerful on-line instrumentation can be used. Those instrumentation is also widely used for specific studies in Museums, Art Galleries, Public buildings (as schools or hospitals) as is shown in this review. At the same time, several Research institution and Universities have their own room-laboratories for studying the indoor processes and chemistry. Anyway, depending on the aim of the study, the funding, location etc the options are diverse and always useful.

Concerning all the above, the holistic methodology of determination the organic gas concentrations at indoor air is a great challenge for future research activities.

Acknowledgements

Thanks are due to the European Cooperation in Science and Technology (COST) through the financial support to COST Action INDoor AIR POLLution NETwork (INDAIRPOLLNET), CA17136.

Disclosure statement

No potential conflict of interest was reported by the authors