ABSTRACT
The aim of this study was to measure the air concentrations of carbon dioxide (CO2) and formaldehyde (HCHO) in daycare centers to determine relevant influencing factors, including temperature, relative humidity (RH), type of facility, number of children, type of ventilation system, ventilation time, and air cleaning system. The authors measured HCHO, CO2, temperature, and RH in the center of classrooms in 289 daycare centers. Spearman’s correlation and Mann–Whitney analyses were used to examine the relationships and differences in HCHO and CO2 for varying temperatures, RH values, and categorical indoor environmental factors. There were no significant differences in the HCHO and CO2 air concentrations with varying numbers of children, ventilation times, or ventilation and air cleaning system types. However, both the HCHO and CO2 air concentrations were significantly different for varying RH values, which were divided into five categories (p < 0.001). Only the HCHO air concentrations were significantly different for varying temperatures, which were divided into five categories (p < 0.001). Significant correlations were found between HCHO air concentrations and the temperature (r = 0.35, p < 0.0001), RH (r = 0.51, p < 0.0001), and CO2 (r = 0.36, p < 0.0001). The study results support maintaining an appropriate temperature and RH range for reducing airborne HCHO in daycare centers. Further research is needed to elucidate the precise mechanisms responsible for the relationships observed in this study.
Implications: Data from 289 daycare centers in Seoul, South Korea, indicate that HCHO concentrations show a positive correlation with indoor temperature and relative humidity. This indicates that keeping temperatures low will help keep HCHO concentrations low, by both a direct and an indirect effect, since low temperatures also cause low relative humidity.
Introduction
High-quality air is essential for the health and welfare of occupants in various indoor environments, where most people spend a large portion of their time (Branco et al., Citation2014). Indoor air pollutants in South Korea include particulate matter (≤10 μm; PM10), carbon dioxide, formaldehyde, airborne bacteria, carbon monoxide, nitrogen dioxide, radon, various volatile organic compounds, asbestos, and ozone (Ministry of Environment of Korea [MEK], Citation2014). High carbon dioxide (CO2) concentrations in indoor environments are considered an indicator of poor ventilation or air exchange rates. The poor ventilation in turn indicates a possible accumulation of other pollutants in the indoor air, which would negatively affect the learning ability of pupils (Griffiths and Eftekhari, Citation2008).
Among various indoor air pollutants, formaldehyde (HCHO) is one of the most frequently detected volatile organic compounds (VOCs) in indoor environments, as well as being the most abundant aldehyde. This compound is classified as carcinogenic by the International Agency for Research on Cancer (IARC, Citation2012). The range of potential sources of formaldehyde is extensive due to its wide use as a chemical additive in numerous industrial processes. Many detrimental health effects have been observed from exposure to HCHO, including upper respiratory tract irritation, eye irritation, and inflammatory and hyperplastic changes in the nasal mucosa (Edling et al., Citation1988). Sensory (eye and nasal) irritations in humans have been reported at 1 ppm of HCHO (Arts et al., Citation2006). In addition, various case-control and cross-sectional studies have suggested a possible association between low-level HCHO exposure and asthma exacerbation (Gilbert, Citation2005).
Since May 2004, the Ministry of Environment of South Korea has been maintaining and controlling the indoor air quality of facilities for public use, including daycare centers, to protect occupants from indoor environmental pollutants. The children (including infants and toddlers) in these facilities spend most of their time indoors and therefore experience significant exposure to any air pollutants (Almeida et al., Citation2011). There is a growing concern about the exposure of children to air pollutants, and several methods for assessing indoor air quality in daycare centers have been investigated. These assessment studies have been completed in primary, secondary, and nursery schools. A number of studies on nursery school indoor air quality have focused on aspects such as ventilation (Gładyszewska-Fiedoruk, Citation2011), particulate matter assessment (Sousa et al., Citation2012; Branco et al., Citation2014), associations with building characteristics (St-Jean et al., Citation2012), and allergens (Salo et al., Citation2009).
Exposure to HCHO and CO2 is significantly influenced by local conditions, such as pollutant sources, emission concentrations, and microenvironments (Yoon et al., Citation2011; St-Jean et al., Citation2012). Therefore, it is important to identify the sources of HCHO and CO2 by researching factors that could influence their concentrations in indoor environments. There is currently no research focusing on how indoor temperature and relative humidity (RH) affect exposure to HCHO and CO2 in daycare centers. This study therefore aimed to comprehensively evaluate the levels of HCHO and CO2 in daycare centers in Seoul, in order to determine the relationships and differences in HCHO and CO2 for various temperatures, RH values, numbers of children present, ventilation times, and types of ventilation and air cleaning systems.
Materials and methods
Basic study design
This study was conducted in 186 residential and 103 private daycare centers (289 daycare centers total) in southern Seoul, Korea, using a cross-sectional study design. Samples were taken during the winter period from November 27, 2012, to February 4, 2013. Instruments were placed in the center of the room where the most children were staying at each daycare center. The instruments were located 1 m away from the building wall and 1–1.5 m above the floor, which is the approximate breathing zone of the children. Sampling was performed once over a 60-min period for each location.
All of the daycare centers studied are subject to the Korean Act on Indoor Air Quality for multiple-use facilities. The daycare center managers and teachers were given a survey checklist to complete, whereas field technicians gathered basic information on the facilities (number of children present, ventilation timing, and types of ventilation and air cleaning systems).
Sampling and analysis of HCHO and CO2
The sampling of HCHO was conducted with a 2,4-dinitrophenylhydrazine (DNPH) cartridge and a BUCK VSS-5 pump (A.P. Buck Inc., Orlando, FL, USA). Sampling was performed at a flow rate of 0.5–1.2 L/min. Samples were collected twice during the 60-min sampling period, at 30 and 60 min, and then two samples were merged to present as a single sample by using an average value. An ozone scrubber (Waters, Milford, MA, USA) was used at the inlet of the cartridges to remove the effects of ozone. In addition, the cartridge was wrapped in aluminum foil to block direct sunlight. After sampling, HCHO was extracted from the cartridges using 5 mL of acetonitrile. The extracts were analyzed by high-performance liquid chromatography (HPLC), equipped with a ultraviolet-visible (UV-VIS) detector.
The CO2 was measured continuously using nondispersive infrared (NDIR) using an indoor air quality (IAQ) monitor (IQ-610xtra; GrayWolf Sensing Solutions, Hartford, CT, USA; range of 0–10,000 ppm; accuracy ±3% of reading, ±50 ppm) with high-sensitivity spectroscopic sensors to detect CO2 in the gaseous environment by its characteristic absorption wavelengths. Measurements were logged every minute, and the 30-min means were calculated for each indoor daycare center.
Measurement of temperature and RH
Indoor temperature and RH were recorded using an IAQ monitor (IQ-610xtra; GrayWolf Sensing Solutions) for temperature (range of −10 to +70 °C; accuracy ±0.3 °C) and RH (range of 0.0–100.0% RH, accuracy ±2% RH for values <80% RH), with an internal data logger. The mean for each 30-min sampling time was calculated for use in statistical comparisons.
Statistical analyses
Nonparametric statistics were used to analyze the relationships of HCHO and CO2 air concentrations, because these values were not distributed normally or log-normally. Spearman’s correlation analyses were used to examine the relationship of the HCHO and CO2 concentrations to the indoor temperature and RH. Mann–Whitney analyses were conducted to determine whether there were differences in HCHO and CO2 levels for varying numbers of children, ventilation times, or types of ventilation and air cleaning systems. In addition, the Kruskal–Wallis test was used to determine differences in the HCHO and CO2 levels relevant to the temperature and RH, each divided into five categories. All of the statistical analyses were performed using SAS version 9.3 software (SAS Institute Inc., Cary, NC, USA).
Results
shows the levels and distributions of HCHO, CO2, temperature, and RH in the daycare centers. The HCHO levels ranged from 7.5 to 139.5 μg/m3, with an average of 40.6 μg/m3 for all of the daycare centers. The CO2 levels ranged from 447.7 to 2955.3 ppm, with an average of 1286.2 ppm. The temperatures ranged from 10.3 to 23.7 °C, with an average of 18.6 °C, and the RH ranged from 15.2% to 60%, with an average of 34.6%.
Table 1. Distribution levels of formaldehyde, CO2, temperature, and relative humidity in daycare centers.
shows a comparison of the HCHO and CO2 concentrations by type of facility, number of children, ventilation time, and types of ventilation and air cleaning systems. There were no significant differences for HCHO or CO2 between groups for any of these categories.
Table 2. Comparison of formaldehyde and CO2 levels by categorized groups for indoor factors in the daycare centers.
Significant correlations were found for the HCHO concentrations with CO2 (r = 0.36, p < 0.0001), temperature (r = 0.35, p < 0.0001), and RH (r = 0.51, p < 0.0001) ().
Figure 1. Correlations of formaldehyde levels to temperature (left), relative humidity (center), and CO2 (right).
There were significant differences for HCHO and CO2 concentrations for varying temperatures and RH values (p < 0.001) (). The mean levels of HCHO were the lowest when the temperature and RH were less than 16 °C and 20%, respectively, whereas the mean HCHO levels were highest when the temperature and RH fell between 22 and 24 °C and 50% and 60%, respectively. The CO2 levels were significantly higher when the RH increased (p < 0.001) (), but there was no significant difference between the CO2 concentrations and temperature.
Table 3. Comparison of formaldehyde and CO2 levels by the range of temperatures and relative humidity in the daycare centers.
Discussion
This study indicated that the indoor air concentrations of HCHO (40.6 ± 19.6 μg/m3) in the daycare centers did not exceed the Korean IAQ standard of 100 μg/m3 or the World Health Organization (WHO) standard (WHO, Citation2000). However, these concentrations did exceed standards in Hong Kong and Finland (both 30 μg/m3) (Hong Kong Environmental Protection Department [HKEPD], Citation1999; Finnish Society of Indoor Air Quality and Climate [FiSIAQ], Citation2001). According to Wolkoff and Nielsen (Citation2010), a maximum HCHO concentration of 1000 μg/m3 is considered protective against both acute and chronic sensory irritation in the general population. Nonetheless, awareness of the IAQ risks and the availability of appropriate regulation are lagging behind the technologies (Kumar et al., Citation2016). In a comparison of similar indoor facilities for children, the HCHO mean level measured in preschools was 45.3 μg/m3 (Yoon et al., Citation2011), and in various urban nursery schools, the mean was 35 μg/m3 (Branco et al., Citation2015). Reported concentrations were similar to those found in our study, although one study found much higher concentrations in kindergartens (162.7 μg/m3) (Yang et al., Citation2009).
Previous studies indicated that HCHO levels (~60 μg/m3) measured in the bedrooms of 224 healthy children (ages 6–13) were not associated with detrimental effects on the lung function (forced vital capacity [FVC] and forced expiratory volume in 1 sec [FEV1]) (Franklin et al., Citation2011). Another study (Garrett et al., Citation1999) was conducted in 80 homes with 148 children (ages 7–14), of which 53 were asthmatics. An association (odds ratio = 1.40, 95% confidence interval: 0.98–2.00) between the HCHO levels and atopy was found with an increase of 10 μg/m3. It should be noted that no association was identified between the HCHO concentrations in the bedrooms and asthma incidents or lung effects for that study (Garrett et al., Citation1999). In another study, HCHO concentrations were measured in the homes (bedrooms and living rooms) of 88 asthmatic children (ages <3), as well as a nonasthmatic control group of 104 children (Rumchev et al., 2002). A HCHO concentration of >60 μg/m3 in the bedroom was associated with a 39% increase in the risk of asthma compared with HCHO concentrations of <10 μg/m3. However, it should be noted how other factors may contribute in private homes with daycare centers, such as gas heating and new materials, low air exchange rate, and smoking (Gilbert et al., Citation2005).
The average concentrations of CO2 in the daycare centers exceeded the Korean IAQ threshold of 900 ppm, the standard recommended by the Ministry of the Environment in Korea (MEK, Citation2014), with 204 of the 289 locations (70.6%) exceeding 1286.2 ± 510.6 ppm. These levels also exceeded the recommended thresholds from the U.S. Environmental Protection Agency (EPA) (800 ppm) and Finland (700 ppm) (Environmental Policy Working Group, 1999; FiSLAQ, 2001). In addition, the American Conference of Governmental Industrial Hygienists (ACGIH) recognizes a threshold value of 600 ppm for a comfortable environment (Air Duct Cleaners, Citation2013). Concentrations of CO2 have been sampled in child daycare centers for various studies. Values presented ranged from 360 to 3300 ppm, with a mean of 710 ppm (Zuraimi et al., Citation2008); from 447 to 2037 ppm (Roda et al., Citation2011); and from 723 to 2252 ppm, with a mean of 1333 ppm (St-Jean et al., Citation2012).
These findings, along with the findings of the present study for daycare centers, show levels that are relatively higher than those of other indoor environments. These results could be ascribed to the activities of the children, including running indoors, which increases breathing and subsequently the CO2 concentrations. CO2 is harmless and even essential to life at low concentrations; however, higher concentrations increase the risk of headaches, depression, and illness (National Institute for Occupational Safety and Health/Occupational Safety and Health Administration [NIOSH/OSHA], Citation1981).
One possible reason for no significant differences in the HCHO and CO2 concentrations for varying numbers of children could be a discrepancy between the reported and actual numbers of children present at the sampling time. In the case of ventilation system types and ventilation timing, the respondents most likely indicated general conditions, rather than specifically during the sampling times, and no significant difference was found between the levels of HCHO and CO2 for varying ventilation system types and timing. Previous studies have found that the number of children in the classroom was a main determinant of CO2 levels (Branco et al., Citation2015). High levels of CO2 have also been found in other environments where children were present (Mumovic et al., Citation2009; Pegas et al., Citation2011). Furthermore, a significant association between higher CO2 levels and the absence of a mechanical ventilation system has been indicated (Pegas et al., Citation2011). The presence of each additional child per cubic meter in a room was found to increase the level of CO2 by nearly 70 ppm, whereas the presence of a ventilation system was associated with a decrease of 562 ppm in the average CO2 level (Daneault et al., Citation1992). In addition, the presence of a mechanical ventilation system has been previously correlated significantly with lower HCHO levels (St-Jean et al., Citation2012). Therefore, because the mean levels of CO2 found in the current research exceeded the standards, and because CO2 is often considered a surrogate for the presence of pollutants in the indoor air, the mechanical ventilation systems were possibly not functioning properly at the time of sampling (Daisey et al., Citation2003; Shendell et al., Citation2004; Mendell and Heath, Citation2005; Ana et al., Citation2015). Additionally, although an air cleaning system (air purification, dehumidifier, or vegetation) existed and was used often for many of the daycares, the system was possibly not in use during the sampling periods for HCHO and CO2, which would explain the lack of significant correlation between the type of air cleaning system and the pollutant concentrations.
The HCHO levels showed a significant correlation with temperature, RH, and CO2 (). Our results are consistent with those of other studies, indicating that HCHO concentrations depend on temperature and RH (Parthasarathy et al., Citation2011). This is also true for the CO2 level, which has been associated with HCHO (Clarisse et al., Citation2003). The association between HCHO and temperature is understandable considering that a heating system is often used to remove indoor TVOCs (total volatile organic compounds) (Follin, Citation1997). A comparative study on HCHO, TVOCs, and dampness was conducted between 193 homes where the children (ages 9–11) suffered from persistent wheezing and 223 control spaces. The results indicated that a low HCHO concentration was associated with an increase in wheezing (Venn et al., Citation2003). However, these authors have suggested that this result could have been influenced or even dominated by the effects of dampness. A possible explanation for the RH effect on HCHO emissions is a likely competition between the HCHO and water molecules for adsorption sites. Therefore, a rise in humidity was unfavorable to formaldehyde adsorption, which would lead to an increase in the atmospheric formaldehyde concentrations (Hamami et al., Citation2012; Yu et al., Citation2015).
The present study found mean concentrations of CO2 that exceeded the Korean IAQ standard of 1000 ppm. A lack of proper natural ventilation was being used in many indoor spaces, as the sampling was conducted in winter. The positive association between CO2 and RH is a cause for concern, as the older children who exhale CO2 could be exposed to high levels of RH, which could then lead to negative health effects. A previous study reported that there was sufficient evidence for an association between moisture-related factors and health effects, such as asthma symptoms and hoarseness (Kallvik et al., Citation2015)
In the present study, it was shown that maintaining the lowest possible indoor temperature in a daycare center was preferable; moreover, a lower temperature also lowers the relative humidity. However, it is difficult to maintain a low temperature in a facility where children are cared for because they are more sensitive to the cold, especially during winter. The suggested ranges of winter indoor temperatures and RH in the daycare centers, with a view to minimizing the level of HCHO, are temperatures of 18–22 °C and RH of 20–40%. In addition, it is important to consider the main sources of HCHO emissions in the building and furnishing materials, such as wood and wood-based products, fiberboard, air fresheners, and the like (Atkinson and Arey, Citation2003; Salthammer et al., Citation2010).
The present study had various limitations that could have affected the results. The first such limitation was the short 60-min sampling period, which could lead to significant uncertainty in the measurements, resulting in poor reproducibility and weak consistency in comparisons. The second limitation was having the assessment period only during one winter season. The influence of seasonal and interannual climate differences should be further explored. Nevertheless, this study carried out 289 microenvironmental measurements in daycare centers, using a standard method. The results we have obtained suggest the importance of appropriately controlling the indoor temperature and RH to minimize the levels of airborne HCHO in daycare centers.
Conclusion
This study evaluated the levels of HCHO and CO2 associated with the indoor environmental factors of number of children, ventilation time, types of ventilation and air cleaning systems, temperature, and RH for daycare centers in Seoul. The results showed that there were significant correlations of HCHO concentrations with CO2, temperature, and RH. Concentrations of CO2 were significantly correlated only with RH (a positive correlation), however. An increase in temperature was associated with significantly higher HCHO and RH levels, whereas significantly higher levels of CO2 were associated only with increased RH. This study indicates that controlling temperature and RH is an important consideration in minimizing the levels of HCHO in daycare centers.
Funding
This research was supported by Basic Science Research Program through the National Research Foundation of Korean (NRF) funded by the Ministry of Science, ICT and Future Planning (2015R1C1A1A02037363) and was supported by the Korean government (2016R1C1B2016366).
Additional information
Funding
Notes on contributors
Sung Ho Hwang
Sung Ho Hwang is an associate scientist at Cancer Risk Appraisal & Prevention Branch, National Cancer Control Institute, National Cancer Center, Ilsan, South of Korea.
Gil Bong Lee
Gil Bong Lee is a researcher with Seegene Medical Foundation.
Im Soon Kim
Im Soon Kim is a professor with Kwangwoon University.
Wha Me Park
Wha Me Park is an executive vice chairman at Korea Environmental Safety & Health Association, Seoul, South Korea.
References
- Air Duct Cleaners. 2013. Fact Sheet: Indoor air quality standards. http://www.airductcleanersusa.com/indoor air quality standards/(accessed July 2014).
- Almeida, S.M., N. Canha, A. Silva, M.D. Freitas, P. Pegas, C. Alves, M. Evtyugina, and C.A. Pio. 2011. Children exposure to atmospheric particles in indoor of Lisbon primary schools. Atmos. Environ. 45:7594–7599. doi: 10.1016/j.atmosenv.2010.11.052
- Ana, G.R., P. Ojelabi, and D.G. Shendell. 2015. Spatial-temporal variations in carbon dioxide levels in lbadan, Nigeria. Int. J. Environ. Health Res. 25:229–240. doi: 10.1080/09603123.2014.938024
- Arts, J.H.E., M.A.J. Rennen, and C. Deheer. 2006. Inhaled formaldehyde: Evaluation of sensory irritation in relation to carcinogenicity. Regul. Toxicol. Pharmacol. 44:144–160. doi: 10.1016/j.yrtph.2005.11.006
- Atkinson, R., and J. Arey. 2003. Gas-phase troposheric chemistry of biogenic volatile organic compounds: A review. Atmos. Environ. 37(Suppl. 2):S197–S219. doi: 10.1016/S1352-2310(03)00391-1
- Branco, P.T.B.S., M.C.M. Alvim-Ferraz, F.G. Martins, and S.I.V. Sousa. 2014. Indoor air quality in urban nurseries at Porto city: Particulate matter assessment. Atmos. Environ. 84:133–143. doi: 10.1016/j.atmosenv.2013.11.035
- Branco, P.T.B.S., M.C.M. Alvim-Ferraz, F.G. Martins, and S.I.V. Sousa. 2015. Children’s exposure to indoor air in urban nurseries—Part I: CO2 and comfort assessment. Environ. Res. 140:1–9. doi: 10.1016/j.envres.2015.03.007
- Follin, T. 1997. Airing out pollutions In. Proc Healthy Buildings/IAQ 3:353–356. Washington, DC: Healthy Buildings/IAQ.
- Clarisse, B., A.M. Laurent, N. Seta, Y. Le Moullec, A. El Hasnaoui, and I. Momas. 2003. Indoor aldehydes: Measurement of contamination levels and identification of their determinants in Paris dwellings. Environ. Res. 92:245–253. doi: 10.1016/S0013-9351(03)00039-2
- Daisey, J.M., W.J. Angell, and M.G. Apte. 2003. Indoor air quality, ventilation and health symptoms in schools: An analysis of existing information. Indoor Air 13:53–64. doi: 10.1034/j.1600-0668.2003.00153.x
- Daneault, S., M. Beausoleil, and K. Messing. 1992. Air quality during the winter in Quebec day-care centres. Am. J. Public Health 82:432–434. doi: 10.2105/AJPH.82.3.432
- Edling, C., H. Hellquist, and L. Odkvist. 1988. Occupational exposure to formaldehyde and histopathological changes in the nasal mucosa. Br. J. Ind. Med. 45:761–765. doi: 10.1136/oem.45.11.761
- Finnish Society of Indoor Air Quality and Climate (FiSIAQ). 2001. Classification of Indoor Climate 2000: Target Values, Design Guidance and Product Requirements. Espoo, Finland: FiSIAQ.
- Franklin, P., P. Dingle, and S. Stick. 2011. Raised exhaled nitric oxide in healthy children is associated with domestic formaldehyde levels. Am. J. Respir. Crit. Care. Med. 161:1757–1759. doi: 10.1164/ajrccm.161.5.9905061
- Garrett, M.H., M.A. Hooper, B.M. Hooper, P.R. Rayment, and M.J. Abramson. 1999. Increased risk of allergy in children due to formaldehyde exposure. Allergy 54:330–337. doi: 10.1034/j.1398-9995.1999.00763.x
- Gilbert, N.L., M. Guay, J.D. Miller, S. Judek, C.C. Chan, and R.E. Dales. 2005. Levels and determinants of formaldehyde, acetaldehyde and acrolein in residential indoor air in Prince Edward Island, Canada. Environ. Res. 99:11–17. doi: 10.1016/j.envres.2004.09.009
- Gładyszewska-Fiedoruk, K., 2011. Analysis of stack ventilation system effectiveness in an average kindergarten in north-eastern Poland. Energy Build. 43:2488–2493. doi: 10.1016/j.enbuild.2011.06.001
- Griffiths, M., and M. Eftekhari. 2008. Control of CO2 in a naturally ventilated classroom. Energy Build. 40:556–560. doi: 10.1016/j.enbuild.2007.04.013
- Hamami, A.A., P. Turcry, and A. Aït-Mokhtar. 2012. Influence of mix proportions on microstructure and gas permeability of cement pastes and mortars. Cem. Concr. Res. 42:490–498. doi: 10.1016/j.cemconres.2011.11.019
- Hong Kong Environmental Protection Department (HKEPD). 1999. Hong Kong Air Quality Objectives, IAQ Objectives for Offices and Public Spaces. Hong Kong: HKEPD.
- International Agency for Research on Cancer (IARC). 2012. A Review of Human Carcinogens: Chemical Agents and Related Occupations. Monographs on the Evaluation of Carcinogenic Risks to Humans. No. 100F. Lyon, France: IARC.
- Kallvik, E., T. Putus, and S. Simberg. 2015. Indoor air problems and hoarseness in children. J. Voice 30:109–113. doi: 10.1016/j.jvoice.2015.02.012
- Kumar, P., A.N. Skouloudis, M. Bell, M. Viana, M.C. Carotta, G. Biskos, and L. Morawska. 2016. Real-time sensor for indoor air monitoring and challenges ahead in deploying them to urban buildings. Sci. Total Environ. 560–561: 150–159. doi: 10.1016/j.scitotenv.2016.04.032
- Mendell, M.J., and G.A. Health. 2005. Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature. Indoor Air 15:27–52. doi: 10.1111/ina.2005.15.issue-1
- Ministry of Environment of Korea (MEK). 2014. Indoor Air Quality Management in Public Facilities. Indoor Air Quality Management Act Amendment. http://www.eiskorea.org/04_Policy/02_Ordinance.asp?schMenuCode=MC200&schTabCode=MC210&strIdx=779&schCom=&schSearch=&intPage=1 (accessed November 15, 2016).
- Mumovic, D., J. Palmer, M. Davies, M. Orme, I. Ridley, T. Oreszczyn, C. Judd, R. Critchlow, H.A. Medina, G. Pilmoor, C. Peasron, and P. Way. 2009. Winter indoor air quality, thermal comfort and acoustic performance of newly built secondary schools in England. Build. Environ. 44:1466–1477. doi: 10.1016/j.buildenv.2008.06.014
- National Institute for Occupational Safety and Health/Occupational Safety and Health Administration (NIOSH/OSHA). 1981. Occupational Health Guidelines for Chemical Hazards. DHHS (NIOSH) Publication No. 81-123. Washington, DC: U.S. Government Printing Office.
- Parthasarathy, S., R.L. Maddalena, M.L. Russell, and M.G. Apte. 2011. Effect of temperature and humidity on formaldehyde emissions in temporary housing units. J. Air Waste Manage. Assoc. 61:689–695. doi: 10.3155/1047-3289.61.6.689
- Pegas, P.N., C.A. Alves, M.G. Evtyugina, T. Nunes, M. Cerqueira, M. Franchi, C.A. Pio, S.M. Almeida, and M.C. Freitas. 2011. Indoor air quality in elementary schools in Lisbon in spring. Environ. Geochem. Health 33:455–468. doi: 10.1007/s10653-010-9345-3
- Roda, C., C. Barral, H. Ravelomanantsoa, M. Dusséaux, M. Tribout, Y. LeMoullec, and I. Momas. 2011. Assessment of indoor environment in Paris child day care centers. Environ. Res. 111:1010–1017. doi: 10.1016/j.envres.2011.06.009
- Rumchev, K.B., J.T. Spickett, M.K. Bulsara, M.R. Phillips, and S.M. Stick. 2012. Domestic exposure to formaldehyde significantly increases the risk of asthma in young children. Eur. Respir. J. 20:403–408. doi: 10.1183/09031936.02.00245002
- Salo, P.M., M.L. Sever, and D.C. Zeldin. 2009. Indoor allergens in school and day care environments. J. Allergy Clin. Immunol. 124:185–192. doi: 10.1016/j.jaci.2009.05.012
- Salthammer, T., S. Mentese, and R. Marutzky. 2010. Formaldehyde in the indoor environment. Chem. Rev. 110:2536–2572. doi: 10.1021/cr800399g
- Shendell, D.G., C. Barnett, and S. Boese. 2004. Science-based recommendations to prevent or reduce potential exposure to biological, chemical, and physical agents in schools. J. Sch. Health 74:390–396. doi: 10.1111/josh.2004.74.issue-10
- Sousa, S.I., C. Ferraz, M.C. Alvim-Ferraz, L.G. Vaz, A.J. Marques, and F.G. Martins. 2012. Indoor air pollution on nurseries and primary schools: Impact on childhood asthma—Study protocol. BMC Public Health 12:435. doi: 10.1186/1471-2458-12-435
- St-Jean, M., A. St-Amand, N.L. Gilbert, J.C. Soto, M. Guay, K. Davis, and T.W. Gyorkos. 2012. Indoor air quality in Montréal area day-care centres. Can. Environ. Res. 118:1–7. doi: 10.1016/j.envres.2012.07.001
- Venn, A.J., M. Cooper, M. Antoniak, C. Laughlin, J. Britton, and S.A. Lewis. 2003. Effects of volatile organic compounds, damp, and other environmental exposures in the home on wheezing illness in children. Thorax 58:955–960. doi: 10.1136/thorax.58.11.955
- Wolkoff, P., and G.D. Nielsen. 2010. Non-cancer effects of formaldehyde and relevance for setting an indoor air guideline. Environ. Int. 36:788–799. doi: 10.1016/j.envint.2010.05.012
- World Health Organization (WHO). 2000. Air Quality Guidelines for Europe 2000, 2nd ed., 288. WHO Regional Publications, European Series, no. 91. Copenhagen, Denmark: WHO Regional Office for Europe.
- Yang, W., J. Sohn, J. Kim, B. Son, and J. Park. 2009. Indoor air quality investigation according to age of the school buildings in Korea. J. Environ. Manage. 90:348–354. doi: 10.1016/j.jenvman.2008.12.003
- Yoon, C., K. Lee, and D. Park. 2011. Indoor air quality differences between urban and rural preschools in Korea. Environ. Sci. Pollut. Res. 18:333–334. doi: 10.1007/s11356-010-0377-0
- Yu, K.P., W.M.G. Lee, and G.Y. Lin. 2015. Removal of low-concentration formaldehyde by fiber optic illuminated honeycomb monolith photocatalytic reactor. Aerosol Air Qual. Res. 15:1008–1026. doi: 10.4209/aaqr.2014.09.0202
- Zuraimi, M.S., and K.W. Tham. 2008. Indoor air quality and its determinants in tropical child care centers. Atmos. Environ. 42:2225–2239. doi: 10.1016/j.atmosenv.2007.11.041