Experimental study on the effect of the split-type air-conditioner on the transmission of smoking pollutants in a room

ABSTRACT

Environmental tobacco smoke (ETS) has become one of the most important sources of indoor air pollution. The study aimed to obtain the variation characteristics of typical air pollutant concentrations when people smoke in a closed room and explore the effect of the air-conditioner. A closed and air-conditioned room of 21 m² was taken as the research object. Fine particulate matter (PM_2.5) and total volatile organic compound (TVOC) were measured while 10 cigarettes were burnt in smoldering or smoking mode, with the air-conditioner on or off. The contents of nicotine in condensate samples were obtained by liquid chromatography. The impact of ETS on indoor air quality lasted for hours, causing typical pollutant concentrations to far exceed the Chinese standard. The PM_2.5 produced by smoking was 11 times higher than by smoldering, but the TVOC produced by smoldering was more than by smoking. After one hour of the cigarette burning off, the PM_2.5 concentration would be decreased by 96.1% with the air-conditioner on, in contrast to 67.9% with the air-conditioner off. Nicotine was detected in all samples of condensate from the air-conditioner. It is concluded that smoking cigarettes cannot be replaced by smoldering to evaluate the pollution of ETS. The air-conditioner has a positive effect on reducing the concentration of air pollutants produced by cigarette burning. More than 10% of the indoor nicotine may be taken away by condensate discharge, and its possible pollution should be paid attention to.

Implications: This study provides new evidence of the effect of the split-type air-conditioner on ETS. The TVOC concentrations, which were less considered previously, were measured. PM_2.5 concentration in human breathing zone can be reduced more quickly with the air-conditioner on. This study shows that there is a big difference in the concentrations of typical pollutants between smoking and smoldering. And it could be a guide for the formulation of relevant research methods. This study also demonstrates that the air conditioning condensate from the smoking room may contain nicotine. Attention should be paid to the recovery and utilization of such condensate.

Introduction

China has become the largest cigarette producing and consuming country in the world, and Chinese tobacco consumption amounts to more than 30% of the world’s Huang et al. (2020). Environmental tobacco smoke (ETS) has become one of the most important sources of indoor air pollution in China Chen et al. (2021). ETS is a mixture of side-stream smoke and exhaled mainstream smoke. It mainly contains particulate matter (PM), CO, CO₂, nicotine, stimulatory aldehydes such as acrolein, formaldehyde, and acetaldehyde (Baker 2006; Ichitsubo and Kotaki 2018; Li et al. 2020). It has a negative impact on the physical health of people and causes or contributes to many diseases, including cancer, cardiovascular diseases, respiratory infections and so on (Obore et al. 2020; Sheng et al. 2018; Warren 2006). It has been estimated that three million tobacco-induced deaths would occur annually in China by 2050 if the current smoking rate is not substantially reduced (Chen et al. 2015; Huang et al. 2020).

The current research on tobacco smoke focuses mainly on public places. A series of questionnaire survey and experimental measurement were performed in 10 different workplaces in Finland Heloma (2000). The researchers assessed the exposure of workers to ETS and found it varied considerably, depending on the position of the employees and the type of the workplace. Wei et al. (2019) evaluated the effectiveness of comprehensive smoke-free legislation in public places, such as restaurants, bars and hotels, through visual inspection and PM_2.5 monitoring. A monitoring plan was carried out in four English prisons, and the researchers found that the PM_2.5 concentrations in smoking areas were extremely high, posing a significant health threat to prisoners and staff members Jayes et al. (2016). Kim et al. (2017) measured PM_2.5 concentrations of different locations in a nonsmoking internet cafe in Korea, and found the evidence of ETS leakage from the smoking zone to other areas. Besides public places, tobacco smoke pollution in cars was also investigated. It was found that smoking a single cigarette in a car generates extremely high levels of PM_2.5 and the air pollution reaches unhealthy levels even under ventilation conditions Sendzik et al. (2009).

People usually spend a lot of time at home, where air quality also needs to be ensured. However, Global Youth Tobacco Surveys conducted in 132 countries indicate that 43.9% of students are exposed to secondhand smoke at home Warren (2006). According to a study of secondhand smoke exposure carried out in Shanghai, 61.6% of participants encountered smokers at home and 35.8% of nonsmokers reported being exposed to ETS at home during the past 7 days Zheng et al. (2014). Therefore, it is necessary to study the effects of ETS on indoor air quality (IAQ) in homes. Some researchers have assessed the effects of smoking behavior on IAQ and the effectiveness of interventions by installing sensors in volunteers’ homes to record PM_2.5 concentrations (Dobson et al. 2020; Ferdous et al. 2022; Renwick et al. 2018; Russo et al. 2015). They all used PM_2.5 concentration to represent the level of air pollution caused by smoking. ETS consists of 90% gaseous components (such as CO, VOCs and aldehydes), and many of them are hazardous substances Kimet al. (2020). None of these studies have assessed the levels of these gaseous components. In China, most of the smoking behavior at home takes place in a room with split-type air-conditioners, such as the living room, study room or bedroom. The enhancement of ventilation during smoking can help to reduce the concentration of indoor air pollutants Ma and Sun (2016). However, it is not always a welcomed approach, especially in the hot summer, when opening windows for ventilation is implemented at the expense of thermal comfort. Under such situations, most smokers choose to smoke with the air-conditioner on and the doors/windows closed. Up to now, the research on the effect of split-type air-conditioners on the transmission of smoking pollutants in a closed room are very few. Zhou et al. (2010) established that the two main sources of polycyclic aromatic hydrocarbons in dust from air conditioner filters were tobacco smoke and cooking. Yang et al. (2018) found that indoor concentrations of PM_2.5 were more than twice higher in smoking houses than in nonsmoking houses in the heating season. As to transmission of other pollutants, the properties related to radon have been surveyed in 94 offices, and the results showed that the radon properties are seriously affected by the running time of the air-conditioner at the time of measurements and by season Yu et al. (1992).

This study is aimed to obtain the variation characteristics of typical air pollutant concentrations when people smoke in a closed room, and to explore how the air-conditioner influences the pollutant concentrations in ETS. Since nicotine, a highly carcinogenic ingredient in ETS Schaal et al. (2018), is soluble in water, the condensate of the air-conditioner being discharged outdoors may also be a way to reduce indoor air pollution. Revealing nicotine contamination of the condensate is another aim of this study.

Methods

Research object description

The test room, located in Hangzhou city, covers an area of about 21 m² (7 m × 3 m) and has a height of 3.7 m. There is a window on the south outer wall and a split-type air-conditioner on the east inner wall. The air-conditioner (size: 0.8 m × 0.2 m × 0.25 m) is 2.4 m high from the ground. There are two tables in the room. The specific layout of the room is shown in Figure 1. According to the smoking situation in the room, point 1 and point 2 were set as cigarette burning positions, 0.8 m away from the wall. According to China Indoor Air Quality Standard (GBT 18883–2002), the measuring point was arranged in the middle of the room, with a height of 1.1 m (the breathing height of a sitting person).

Figure 1. Floor plan of the room.

Instrumental measurement

Five parameters including temperature (T), relative humidity (RH), total volatile organic compounds concentration (TVOC), and fine particulate matter (PM_2.5) concentrations were measured. T and RH are thermal parameters. TVOC concentrations were measured in the experiments because volatile organic compounds belong to a kind of typical gaseous pollutants in ETS Kim, Ph, and Jung et al. (2020). The cigarette smoke particles are in the size range of 100–700 nm, and have a mean diameter of about 300 nm (Li and Hopke 1993; Manikonda, Hopke, and Ferro 2016). Therefore, the PM_2.5 concentration is usually measured to represent the level of PM pollution caused by ETS. All parameters were measured at an interval of one minute. Table 1 presents the information about the instruments. All instruments were calibrated as per the manufacturers recommended calibration procedures and within the calibration interval.

Table 1. Instruments used for measurement.

Device	Parameters	Accuracy
Testo175-H1	T	±0.4°C
	RH	±2%RH (2%-98%RH)
RAE PGM-7320	TVOC	±3% (10–2000ppm)
CEM DT-9850 M	PM2.5	1 μg/m Li et al. 2020

Nicotine is the addictive component of ETS Selya et al. (2016). In this study, the content of nicotine in condensate samples was obtained by high-performance liquid chromatography (WATERS 2690) Page-Sharp et al. (2003). The analytical method has been validated. The linear range of nicotine standard solution was 0.5–100 μg/mL (R² = 0.9995).

Cases of study

The burning of a cigarette is a series of consecutive sequences of both smoldering (passive burning) and smoking (active burning), and cigarette pollutants yields are affected by the burning way Cahours and Verron (2017). Therefore, four cases were designed, as shown in Table 2. The detailed experimental procedure is as follows:

Table 2. Cases of a comparison study.

Case	Cigarette burning way	Air-conditioner
1	Smoldering	Off
2	Smoking	Off
3	Smoldering	On
4	Smoking	On

1) The room was left vacant for more than 4 hours before the experiment.

2) In case 3 and case 4, the air-conditioner was turned on two hours before the experiment started. The fan speed of the air-conditioner was set in high gear and the temperature was set at 26°C. During the entire experimental session, the air-conditioner blew horizontally in the same direction and the compressor cycled on and off to maintain the desired temperature set-point. The relative humidity was not set specifically.

3) In the prescribed area, two volunteers were asked to burn or smoke 5 cigarettes within half an hour at a uniform speed.

4) The measurements in each case lasted for two hours, including 0.5-hour background environment measurement (Part I), 0.5-hour cigarette burning process measurement (Part II), and 1-hour measurement after cigarettes burning-out (Part III).

In order to avoid the influences of human activities on the results of the experiment, the volunteers were asked to stay in the room during the whole experiment and remain sedentary, while the door and windows were closed and no one came in or went out.

Results

Indoor thermal environment

The indoor temperature and relative humidity measurement results are shown in Table 3. The temperature of each case did not fluctuate greatly. The indoor relative humidity showed a gradually increasing trend, which was caused by occupant moisture dispersion. The relative humidities of the cases with air-conditioning were significantly lower than those of the other cases, which proved the significant effect of the air-conditioner on indoor humidity control.

Table 3. Temperature and relative humidity results.

Parameters	Case	Max	Min	AVE	SD
T (°C)	1	29.9	29.2	29.8	0.15
	2	30.5	29.8	30.3	0.18
	3	26.8	26.3	26.5	0.11
	4	27.3	26.1	27.0	0.34
RH (%)	1	72.8	67.2	71.1	1.29
	2	71.8	63.5	67.7	1.94
	3	48.8	43.1	45.9	1.29
	4	44.8	39.0	41.5	1.40

Indoor air pollutants

For the two cases with air-conditioning, the measurement results of indoor pollution concentrations during the experiment are shown in Figure 2. The variation of the concentration of PM_2.5 is shown in Figure 2(a). In both cases, the PM_2.5 concentrations started to increase at the beginning of cigarettes burning and reached their peak values at the end of the burning, then decreased slowly. The initial indoor average concentration of PM_2.5 in case 1 was 16 μg/m³, and in case 2 was 12 μg/m³ . By the end of the experiment, the indoor PM_2.5 concentrations were 34 μg/m³ and 32 μg/m³.

Figure 2. Variation of indoor pollution concentrations with air-conditioner off. Case 1 is smoldering with the air-conditioner off, case 2 is smoking with the air-conditioner off, case 3 is smoldering with the air-conditioner on, and case 4 is smoking with the air-conditioner on.

The variation of the TVOC concentration was similar to that of PM_2.5 concentration, but declined more rapidly at the later stage. About 30 minutes after the cigarettes burning-off, the TVOC concentration would reduce to the initial value (< 0.1 mg/m³). The TVOC concentration for smoldering was slightly higher than that for smoking. The maximum TVOC concentration in case 1 was 1.3 mg/m³, while in case 2 it was less than 1 mg/m³.

Effect of air-conditioner

Split-type air-conditioner is a common air conditioning equipment for small rooms. This type of air-conditioner does not provide fresh air, so the dilution effect of fresh air on indoor ETS does not take place. In order to compare the four cases more clearly, the pollutant concentrations after cigarettes burning-out were fitted into the trend curves, as shown in Figure 3.

Figure 3. Effect of air-conditioner on indoor air pollutant concentrations. Case 1 is smoldering with the air-conditioner off, case 2 is smoking with the air-conditioner off, case 3 is smoldering with the air-conditioner on, and case 4 is smoking with the air-conditioner on.

According to Figure 3(a), the air-conditioner had a great impact on PM_2.5 concentration. When cigarettes were burning, turning on the air-conditioner would increase the PM_2.5 concentration in the breathing zone and shift its peak to 10 minutes earlier. The maximum concentration of PM_2.5 was above 4000 μg/m³, and more than five times the value for the case without air-conditioning. When the 30-minute cigarette burning was over, the PM_2.5 concentrations for the cases with air-conditioning dropped rapidly, and fell below 100 μg/m³ within 30 minutes. In the last hour of the experiment, the PM_2.5 concentration in the room was reduced by up to 96.1% with the air-conditioner on. Compared with 67.9% reduction for the air-conditioner off, it was indicated that the air-conditioner had a positive effect on reducing the PM_2.5 produced by cigarettes burning.

The effect of air-conditioner on TVOC concentration was similar to that on PM_2.5 concentration. According to Figure 3(b), the TVOC concentrations in the breathing zone were higher with air-conditioning when cigarettes were burning. The maximum concentrations of TVOC with air-conditioning were about three times the value for the case without air-conditioning. However, it can be observed that TVOC concentrations had decreased to less than 0.1 mg/m³ after 90 minutes of the experiment in all four cases. Therefore, the air-conditioner has no effect on reducing TVOC concentration. Both participants and volunteers reported significantly more eye irritation and nasal discomfort in case 1 and case 2.

The condensate from the air-conditioner was sampled in case 3 and case 4 at the end of Part II and 15 minutes later, respectively. The samples were named 3-A, 3-B, 4-A and 4-B, according to the order in which they were sampled. The amount of nicotine in the sample is shown in Figure 4. Because the authors did not detect any nicotine in the blank control of ordinary condensate, it is suggested that indoor nicotine can indeed be discharged outwards by dissolving in the condensate. The concentrations of nicotine in two samples from the same case were very similar, indicating that the discharge process will last for a period of time, at least more than 15 minutes.

Figure 4. Nicotine concentration of the condensate samples. The samples 3-A, 3-B, 4-A and 4-B are condensate from the air conditioner in case 3(smoldering with air-conditioner on) and case 4(smoking with air-conditioner on) at the end of cigarette burning and 15 minutes later, respectively.

Discussion

Pollutants from cigarette burning can cause serious air pollution in a closed room and continue to affect IAQ for hours. For the two cases without air-conditioning, the PM_2.5 concentrations were far above the relevant Chinese standard, 75 μg/m³ (GBT-18883-2020), during the period of cigarette burning. An hour after the cigarettes burning-off, the PM_2.5 concentrations for two burning ways (smoking and smoldering) were around 1.3 and 17.5 times the limit level, respectively. Although the TVOC concentration reduced to the initial state at the end of the test, both the maximum concentrations of case 1 and case 2 exceeded the standard limit of 0.6 mg/m³ (GBT-18883-2020). Therefore, cigarette burning can cause serious air pollution in a closed room, and its impact on IAQ will last for hours.

There are significant differences in concentrations between smoking and smoldering. Figure 2(a) shows the PM_2.5 concentration levels. The PM_2.5 concentration for smoking was 11 times higher than that for smoldering. Similar results were obtained by Guo, Shen, and Zhang (2017) they found that the PM emission by smoking in closed rooms could be up to 15 times as much as that by smoldering. This is because smoking makes cigarettes burn more fully. In addition, smokers re-exhale some of the smoke they inhale, which contributes to the diffusion of PM in the room. Figure 2(b) shows the TVOC concentrations for the two burning ways. The ETS in case 1 was side stream smoke emitted by cigarettes smoldering, and in case 2 was a mixture of side-stream smoke (approximately 50%) and exhaled mainstream smoke. The compositions of smokes for case 1 and case 2 were different, which may account for the difference in TVOC concentration Schick and Glantz (2005). Therefore, smoking cigarettes cannot be replaced by smoldering cigarettes to evaluate the pollution of ETS on IAQ.

The air-conditioner has a great impact on indoor PM_2.5 concentration, but it has little effect on indoor TVOC concentration. In this study, the air-conditioner increased the concentration of PM_2.5 in breathing zone during cigarettes burning, but it had a significant effect on reducing the concentration of PM_2.5 remaining after the cigarettes burning-off. This is due to the fan operation of the air conditioner which promoted the burning of cigarettes and the diffusion of smoke in the air. Because of the high concentration of particulate matter produced by cigarette smoking and the lack of ventilation measures, the PM_2.5 concentrations were still above the relevant standards, at the end of all four cases. Ott, Klepeis, and Switzer (2012) found that due to the small room volume and low air exchange, the PM_2.5 from smoking activity in a room can be very high and persist at measurable levels for many hours. This is consistent with the results obtained by this study. The air-conditioner increased the TVOC concentrations during smoking to a certain extent, but it did not prolong the time required TVOC attenuation. Some relevant studies have shown that indoor air flow can significantly affect the concentration and distribution of cigarette smoke (Faulkner and Fisk 1995; Ma and Sun 2016). In this study, the variation trends of indoor PM_2.5 and TVOC concentrations indicated that the air-conditioner had a significant effect on the pollution of ETS by promoting mixing within the room. And the mixing effect also can help the deposition of PM_2.5 on the walls and furniture, which potentially enhances PM_2.5 removal from air. In addition, the condensation effect of the air-conditioner also contributes to the reduction of pollutant concentrations. However, due to the lack of a control group, the mixing and condensation effects of the air-conditioner cannot be separately analyzed here. Although we observed significantly higher concentrations of air pollutants with the air-conditioner on, this was only for a short time during the burning. In terms of the change in air pollutant concentrations in the Part III (after cigarettes burning-out), we take the view that the air-conditioner has a positive effect on reducing the pollution of ETS. Nevertheless, further research is needed to investigate and distinguish the mixing and condensation effects of the air-conditioner.

During the experiments, the occupants still felt uncomfortable even when TVOC did not exceed the standard limit. This could be explained by the combination effect of low concentrations of pollutants, each of which is lower than the odor threshold. The air-conditioner can help to relieve the discomfort caused by air pollution. This may be due to the improved thermal sensation that affects the occupant’s perception of air quality Liu et al. (2019). With appropriate indoor temperature and humidity, the occupants will feel closer to neutral about the thermal environment, and their perception of indoor air quality will be better. Therefore, the air-conditioner not only helps to reduce the concentrations of indoor air pollutants, but also helps to improve the perceived air quality of occupants.

Although it is found that the air-conditioner can help to reduce indoor nicotine concentration, the inherent mechanism is a complex issue. Exposure of cigarette smoke to ozone or OH oxidation may lead to ultrafine particle formation Wang et al. (2018). For ordinary indoor environments, there is no evidence of significant chemical reactions about nicotine. Therefore, it is considered that the reduction of indoor nicotine concentration mainly depends on physical processes. Previous research showed that the nicotine of ETS is found mainly in the gas phase and decays rapidly. Probably the volatile non-protonated nicotine evaporates from the particulate matter within 5 minutes to leave behind only the protonated part, which behaves exactly like the particles Neurath et al. (1991). Therefore, the protonated part may be absorbed by furniture, walls or floors. The gas-phase nicotine also has the surface interactions with furniture, such as carpet Van Loy, Nazaroff, and Daisey (1997). Here all experiments were carried out in the same room with the same layout, the mixing effect of air conditioning can affect nicotine movement behavior and have a potential positive effect on the diffusion and deposition of nicotine, which is worthy of further experimental study.

In this study, one of the main objectives is to survey the effect the air-conditioner on nicotine excretion outside, and a new way of excretion has been evidenced. John et al. (2018) has demonstrated that the gas-phase nicotine fraction increases significantly with increasing temperature and decreasing relative humidity. During our experiments, when the indoor air entered the air-conditioner and exchanged heat with the evaporator, its temperature decreased and the relative humidity increased. At the same time, the gas-phase nicotine fraction was reduced significantly by dissolution in the condensate. The protonated nicotine would also dissolve in it. The condensate was much cooler than room temperature, thus the dissolution rate should be slow Chen and Papadopoulos (2020).

In our test, more than 10% of the nicotine in ETS was taken away by the condensate. This is an interesting result about the testing of air-condition condensate from the smoking room. Few researchers have noticed the problem of indoor air pollutants being discharged outdoors through the condensate before. The air-conditioner could produce about 0.6 L condensate in an hour in both case 3 and case 4. For the smoking case, about 0.6 mg nicotine was discharged from the room through the condensate in 15 minutes. It is about a third of the nicotine yield in the smoke of a cigarette Connolly et al. (2007). Therefore, the air-conditioner can help to remove indoor nicotine at a measurable level. It should be noted that the condensation process is affected by the operating condition of the air-conditioner compressor, which was an uncontrolled factor in our tests. In respect of the amount of condensate, the impact of the air-conditioner compressor was found to be minimal, indicated by differences between cases. However, for situations where the indoor latent load is high or the environmental temperature is low, the air-conditioner compressor will work in a very different condition from this study. The nicotine removal effect of the air-conditioner will change accordingly.

The condensate from the air-conditioner has attracted people’s attention on its reuse. It is claimed to have three characteristics: higher water quality, lower temperature and considerable volume Nannan and Li (2021). Currently, the potential ways of recovering and utilizing are used as water resources and cooling water (Akram et al. 2018; da et al. 2019; Nannan and Li 2021). The properties of the condensate such as hardness, alkalinity, turbidity, electrical conductivity, counting of bacteria and fungi, were experimentally ascertained by related researches (Akram et al. 2018; Alipour, Mahvi, and Rezaei 2015). However, there is no research on the detection of nicotine in air-conditioning condensate and the analysis of its impact on condensate recycling. If the condensate from the air-conditioner of a smoking zone is used as irrigation and green water or urban landscape water, it might cause secondary pollution of the environment. And if it is used as cooling water, dissolved nicotine may have chemical reaction with equipment parts, affecting the operation of the system. Therefore, caution should be paid to this kind of condensate recovery and utilization. Furthermore, condensate containing nicotine will eventually enter the municipal sewage system, and it may affect the accuracy of tobacco monitoring in communities by wastewater analysis (Castiglioni et al. 2014; Chen, Venkatesan, and Halden 2019).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Additional information

Funding

This work was supported by the Chinese Chinese Nature Science Foundation [51876185].

Notes on contributors

Ruoyi Xie

Ruoyi Xie is a former Master student.

Yiyang Xu

Yiyang Xu is an engineer in Huadong Engineering Corporation Limited, Power Construction Corporation of China, Hangzhou, China.

Guangming Chen

Guangming Chen is a retired professor.

Shaozhi Zhang

Shaozhi Zhang is an associate professor in Institute of Refrigeration and Cryogenics at Zhejiang University, Hangzhou, China.