ABSTRACT
The motorcycle taxi drivers of Bangkok have been heavily exposed to high concentrations of PM10 (particulate matter with an aerodynamic diameter ≤10 μm), and the impact of this on their lungs has been neither documented nor studied. This study examines the association between exposure to PM10 and lung function decline among motorcycle taxi drivers. A cross-sectional study was conducted in Bangkok between two groups: a subject group of motorcycle taxi drivers and control group of enclosed vehicle taxi drivers. The findings of the Thailand Pollution Control Department were used to estimate the annual ambient PM10 concentration levels in the metropolis. Pulmonary functions of motorcycle taxi drivers and enclosed vehicle taxi drivers were measured and compared using the Mann-Whitney test. Multiple linear regression analysis was applied to estimate the effects of PM10 exposure on the lung function of motorcycle taxi drivers. A total of 1283 motorcycle taxi drivers and 600 taxi drivers were investigated. The mean forced expiratory volume in 1 sec/forced vital capacity (FEV1/FVC) of the motorcycle taxi drivers was significantly lower than that of the taxi drivers (P < 0.001). The mean FEV1/FVC of motorcycle taxi drivers exposed to ≥50 µg/m3 PM10 was statistically lower (−2.82%; 95% confidence interval [CI]: −4.54% to −1.09%) and the mean % vital capacity (%VC) of those exposed to 40–49.9 µg/m3 PM10 was statistically lower than that of motorcycle taxi drivers exposed to <30 µg/m3 PM10 (−3.33%; 95% CI: −5.79% to −0.87%). Motorcycle taxi drivers were directly exposed to air pollution in their working environment. As a result, their lung function might decrease more than that of enclosed vehicle taxi drivers. With the possible exposure to ≥50 µg/m3 PM10, the vehicular emission standards should be vigorously enforced. Further investigation is warranted to clarify the effect of lung dysfunction on the work and lifestyle of motorcycle taxi drivers.
Implications: Motorcycle taxi drivers are directly exposed to air pollution in their work environment; therefore, their lung function might decrease more than that of enclosed vehicle taxi drivers, especially when exposed to ≥50 µg/m3 PM10. World Health Organization (WHO) vehicular emission standards should be recognized and eventually enforced.
Introduction
In developing countries, metropolitan cities face serious traffic congestion. Motorcycle taxi services have been developed as paratransit, to provide short trips in high-density residential areas and narrow dead-end streets (Raphiphan et al., Citation2014). Currently, around 200,000 motorcycle taxi drivers are registered in Thailand (James et al., Citation2016). Since 2005, each driver has been required to work in a designated area (Oshima et al., Citation2007). Motorcycle taxi drivers often experience traffic accidents, heat stress, and lung diseases (Arphorn et al., Citation2014; Ekpenyong et al., Citation2012). In addition, they are employed by the informal sector, which has no interest in providing either sufficient labor protection or social security under hazardous working conditions (Nankongnab et al., Citation2015).
Motorcycle taxi drivers spend a large portion of their working time at roadsides, which increases their exposure to airborne particulate matter. Particulate matter ≤10 μm in diameter (PM10) enters the lungs, causing a decline in lung function (Abbey et al., Citation1998; Ackermann-Liebrich et al., Citation1997; Sekine et al., Citation2004; Tashkin et al., Citation1994). The U.S. Environmental Protection Agency (EPA) has determined a causal relationship between short-term exposures to coarse particulate matter and respiratory and cardiovascular effects, and mortality (EPA, Citation2009). Previous studies have shown that professional drivers, especially motorcycle taxi drivers, are exposed to higher PM10 concentrations than the general population (Chan et al., Citation2002; Jinsart et al., Citation2012; Lan et al., Citation2013). The World Health Organization (WHO) has recommended that the mean 24-hr PM10 should not exceed 50 µg/m3, in order to prevent adverse effects (World Health Organization, Citation2006). The Thailand National Ambient Air Quality Standards has set the PM10 standards to 120 µg/m3 for the 24-hr mean and 50 µg/m3 for the annual mean (Jinsart et al., Citation2002).
Although exposure to high concentrations of PM10 has been identified as a cause of lung function decline among professional drivers, motorcycle taxi drivers have received little attention. Furthermore, non–air-conditioned vehicles have higher risks of PM10 exposure than air-conditioned vehicles, including taxis, buses, and trucks (Jones et al., Citation2006). Therefore, the aim of this study is to examine the association between PM10 exposure in the work environment and lung function of motorcycle taxi drivers. The findings will assist in better prevention of adverse effects, especially lung function decline.
Methods
Participants
A cross-sectional study was conducted on two groups: a subject group of motorcycle taxi drivers and a control group of enclosed vehicle taxi drivers. Taxi drivers were selected as the control group because they usually work inside air-conditioned vehicles and are employed by the informal sector, which is similar to the conditions faced by motorcycle taxi drivers. Both groups were recruited by the health examination campaign “Health status of Bangkok workers in the informal sectors” between January and August 2015 (Ishimaru et al., Citation2016). The National Health Security Office 13 was responsible for the campaign and conducted examinations at selected temples and public park areas. The examinations were free of charge. Loudspeakers, notice boards, and banners were used to publicize the campaigns.
Motorcycle taxi drivers and taxi drivers aged 18 years or over were included in this study. Females were excluded from the analysis because of the small sample available. This study on health examination campaign has been approved by the Ethical Review Committee for Human Research, Mahidol University (COA. No. MUPH 2014–225, Protocol No. 180/2557).
Procedure
The Thailand Pollution Control Department estimates of annual ambient PM10, sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide (NO2), and ozone (O3) concentrations in 2015 were measured in air pollution at Bangkok’s 28 roadsides and obtained from http://aqnis.pcd.go.th/en. Operationally, motorcycle taxi drivers have restrictions on where they can work due to regulations (Oshima et al., Citation2007). Therefore, our recruitment places were deemed to be locally representative. After matching the air monitoring stations and health examination site districts, 13 air monitoring stations were selected for this study. Jinsart et al. (Citation2002) described the details of the monitoring stations. However, the taxi driver monitoring stations were located near parking areas and taxi drivers would drive long distances after picking up customers. Therefore, annual mean PM10 concentrations cannot enhance their PM10 exposure. In this regard, the taxi drivers group was excluded from the analyses to estimate the effects of PM10 exposure.
Anthropometric measurements, including blood pressure and pulmonary function, were examined. Height and weight were measured using procedures described previously (Ishimaru et al., Citation2016). Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2). Systolic and diastolic blood pressures were measured twice by qualified nurses using calibrated digital sphygmomanometers after the participants were at rest in a sitting position. The average pressure values were then calculated to identify differences.
Closed-circuit spirometry was measured using a calibrated portable spirometer (Spirolab III; Medical International Research, Rome, Italy). Participants performed the pulmonary function test in a standing position. The % vital capacity (%VC), % forced expiratory volume in 1 sec (%FEV1), and forced expiratory volume in 1 sec/forced vital capacity (FEV1/FVC) were measured. The predicted values were calculated in accordance with the National Health and Nutrition Survey III equations (Hankinson et al., Citation1999), adjusted for Asian ethnicity (Hankinson et al., Citation2010).
Statistical analysis
Multicollinearity was assessed using the variance inflation factor (VIF) test, using a cutoff value of ≥10 (Neter et al., Citation1989). The estimated annual ambient SO2, CO, NO2, and O3 concentrations were excluded from the analyses because of potential multicollinearity problems and the considerable amount of data missing on the Web site. Subject and control group variables were compared using the Mann-Whitney test because they were not normally distributed. According to the estimated annual ambient PM10 exposure levels, the motorcycle taxi drivers were classified into four groups: <30 µg/m3; 30–39.9 µg/m3; 40–49.9 µg/m3; and ≥50 µg/m3. In univariate analysis, the Kruskal-Wallis test was used to screen the association between PM10 exposure level and lung function of motorcycle taxi drivers. Multiple linear regression analysis was used to estimate the effects of PM10 exposure on lung function of motorcycle taxi drivers. Age, BMI, and blood pressure were included in the multivariable model. P < 0.05 was considered statistically significant. SPSS version 17.0 (SPSS, Chicago, IL) was used for the statistical analysis.
Results
PM10 exposure level
A total of 1737 motorcycle taxi drivers and 668 taxi drivers participated in the study. The participant data were analyzed after applying inclusion and exclusion criteria; 380 did not join the study near national air monitoring stations and 142 of them were female. illustrates the location of air monitoring stations where health examinations were performed. Annual ambient PM10 concentrations ranged from 18.75 to 59.00 µg/m3 at the 13 sites (). One hundred and forty-six motorcycle taxi drivers (11.4%) were exposed to ≥50 µg/m3 PM10.
Table 1. Number of participants and annual mean PM10 concentrations at air monitoring stations.
Figure 1. Map of the air monitoring stations in Bangkok.
Health examination
summarizes the general characteristics of the participants. The mean age of the motorcycle taxi drivers group was significantly younger than that of the taxi drivers (45.1 and 51.1 yr; P < 0.001). Significantly elevated systolic blood pressure was found in the motorcycle taxi drivers group compared with the other group (P < 0.001). The mean FEV1/FVC of the motorcycle taxi drivers group was significantly lower than that of the taxi drivers (P < 0.001), whereas the mean %VC of the group of motorcycle taxi drivers was higher (P < 0.001).
Table 2. General characteristics of the study participants.
PM10 exposure and lung function of motorcycle taxi drivers
presents the results of univariate and multiple linear regression analyses for the association between PM10 exposure level and lung function of the motorcycle taxi drivers. After adjustment, the mean FEV1/FVC of the motorcycle taxi drivers exposed to ≥50 µg/m3 PM10 was lower (−2.82%; 95% confidence interval [CI]: −4.54% to −1.09%) and the mean %VC of those exposed to 40–49.9 µg/m3 PM10 was lower (−3.33%; 95% CI: −5.79% to −0.87%) than those of the motorcycle taxi drivers exposed to <30 µg/m3 PM10. In contrast, the mean of FEV1/FVC ratio of the motorcycle taxi drivers exposed to 30–39.9 µg/m3 PM10 increased slightly, by 2.09% (95% CI: 0.33% to 3.83%) in comparison with those exposed to <30 µg/m3 PM10.
Table 3. Association between PM10 exposure level and lung function of motorcycle taxi drivers (n = 1283).
Discussion
The mean FEV1/FVC of the motorcycle taxi drivers was lower than that of taxi drivers. The decline in mean FEV1/FVC was associated with work environmental exposure to ≥50 µg/m3 PM10. The results suggest that exposure to high ambient PM10 concentration is a likely cause of lung function decline in motorcycle taxi drivers.
Motorcycle taxi drivers were directly exposed to air pollution; therefore, their lung function decreases more than that of taxi drivers. The finding is consistent with a previous study in Nigeria, which showed that motorcycle taxi drivers had significantly higher respiratory dysfunction and respiratory symptoms than taxi drivers and civil servants (Ekpenyong et al., Citation2012). Motorcycle taxi drivers work in close proximity to vehicular exhaust gas from tail pipes, and physical barriers to inhaled particles is often insufficient. The use of personal protective equipment such as masks may be effective in preventing lung function decline in motorcycle taxi drivers (Patel et al., Citation2016).
Lung function impairment in motorcycle taxi drivers may be attributed to long-term exposure to ≥50 µg/m3 PM10. In the current study, the decline of the mean FEV1/FVC was associated with work environmental exposure to ≥50 µg/m3 PM10. The finding is in agreement with the WHO air quality guideline values (World Health Organization, Citation2006). PM10 concentrations exceeding 50 µg/m3 exposure has been linked to a 3–8% increase in relative risk of death from cardiorespiratory diseases (Frampton, Citation2001). Vahlsing and Smith reported that half the world’s countries provide a national daily air quality standard for PM10; however, the majority are substantially higher than the WHO air quality guideline values (Vahlsing and Smith, Citation2012). Vehicular emission standards should be tougher to prevent chronic respiratory symptoms in heavy traffic locations (Bener et al., Citation1997; Karita et al., Citation2004).
Work environmental exposure to 30–39.9 µg/m3 PM10 was associated with a small increase in the mean FEV1/FVC of motorcycle taxi drivers in this study. The finding may be due to settings with relatively low levels of air pollution and an insufficient variation in air pollution to investigate the hypothesis in the current study. Exposure to PM10 from other combustion sources, such as secondhand smoke, should also be considered (Mehta et al., Citation2013). Additionally, other air pollutants, such as CO, NO2, and O3, have been considered to influence lung disorders (Ierodiakonou et al., Citation2016). Air pollution includes many complex chemicals, and the adverse effects are not uniquely associated with single agents; therefore, epidemiological studies often show different results (Lawin et al., Citation2016). A larger sample size with relevant factor information could help obtain a more comprehensive picture.
Air pollution may induce restrictive lung disorders, rather than obstruction, in motorcycle taxi drivers. This study found that work environmental exposure to PM10 was associated with a decrease in mean %VC but not %FEV1 in motorcycle taxi drivers. The finding is consistent with a study that demonstrated that a 10 µg/m3 increase in PM10 concentration led to a 3.4% decline in FVC (Ackermann-Liebrich et al., Citation1997). However, further research focusing on the type of lung disease is needed to clarify the reason for the restrictive lung disorder.
In the present study, the mean %VC in the motorcycle taxi drivers group was higher than in the taxi drivers. However, the sample group of motorcycle taxi drivers were approximately 6 yr younger than the taxi drivers, which may be the reason for a lesser decline in lung function.
Motorcycle taxi drivers may have an occupational health risk for respiratory and cardiovascular events. The current study showed that motorcycle taxi drivers had higher systolic blood pressure than the taxi drivers. Exposure to air pollution has been linked to increased mortality from cardiovascular conditions in addition to respiratory conditions (Burnett et al., Citation2014; Chen et al., Citation2008). Additionally, unhealthy lifestyles of professional drivers, such as high work demand and sedentary work, should be considered to increase the risk of cardiovascular diseases (Ishimaru et al., Citation2016). Future research should examine professional drivers’ vehicle-based differences in evaluating their cardiovascular disease risks.
A limitation of this study is the lack of information on potential confounding factors such as smoking behavior, past work experience, and medical history, which were not adjusted for in this study. Future research should also examine the effect of the duration of PM10 exposure among motorcycle taxi drivers. In addition, we retrieved ambient PM10 concentration data from the Thailand Pollution Control Department. Air monitoring data were allocated as work environment PM10 exposure levels for each participant based on the health examination sites; air monitoring stations were matched with the districts where each participant was regulated to drive for work. Therefore, the health data do not exactly match the exposure to ambient PM10 concentrations at the work locations. Nevertheless, this study describes one of the first investigations of lung function of motorcycle taxi drivers with a considerable sample size.
Conclusion
Motorcycle taxi drivers are directly exposed to air pollution in their work environment; therefore, their lung function might decrease more than that of enclosed vehicle taxi drivers, especially when exposed to ≥50 µg/m3 PM10. WHO vehicular emission standards should be recognized and eventually enforced. This research design evaluates ambient particulate matter effects without adjusting for major confounding factors. Further investigation is warranted to clarify the lung dysfunction effect on work and lifestyle of motorcycle taxi drivers.
Funding
The “Health status of Bangkok workers in the informal sectors” project was funded by the National Health Security Office 13, Thailand. The funder had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Additional information
Funding
Notes on contributors
Sara Arphorn
Sara Arphorn is an associated professor with the Department of Occupational Health and Safety, Faculty of Public Health, Mahidol University, Bangkok, Thailand.
Tomohiro Ishimaru
Tomohiro Ishimaru is an occupational physician with Nishinihon Occupational Health Service Center, Kitakyushu, Japan, and an adjunct assistant professor with the Department of Occupational Health Practice and Management, University of Occupational and Environmental Health, Kitakyushu, Japan.
Kunio Hara
Kunio Hara is a professor with the Teikyo University Graduate School of Public Health, Tokyo, Japan.
Suwisa Mahasandana
Suwisa Mahasandana is an assistant professor with the Department of Sanitary Engineering, Faculty of Public Health, Mahidol University, Bangkok, Thailand.
References
- Abbey, D.E., R.J. Burchette, S.F. Knutsen, W.F. McDonnell, M.D. Lebowitz, and P.L. Enright. 1998. Long-term particulate and other air pollutants and lung function in nonsmokers. Am. J. Respir. Crit. Care Med. 158:289–98. doi: 10.1164/ajrccm.158.1.9710101.
- Ackermann-Liebrich, U., P. Leuenberger, J. Schwartz, C. Schindler, C. Monn, G. Bolognini, J.P. Bongard, O. Brandli, G. Domenighetti, S. Elsasser, L. Grize, W. Karrer, R. Keller, H. Keller-Wossidlo, N. Kunzli, B. W. Martin, T.C. Medici, A.P. Perruchoud, M.H. Schoni, J.M. Tschopp, B. Villiger, B. Wuthrich, J.P. Zellweger, and E. Zemp. 1997. Lung function and long term exposure to air pollutants in Switzerland. Study on Air Pollution and Lung Diseases in Adults (SAPALDIA) team. Am. J. Respir. Crit. Care Med. 155:122–9. doi: 10.1164/ajrccm.155.1.9001300.
- Arphorn, S., T. Ishimaru, Y. Noochana, S. Bauchum, and T. Yoshikawa. 2014. Working conditions and occupational accidents of informal workers in Bangkok, Thailand: A case study of taxi drivers, motorbike taxi, hairdressers and tailors. J. Sci. Labour 90:183–9.
- Bener, A., J. Brebner, M.N. Atta, J. Gomes, F. Ozkaragoz, and M.Y. Cheema. 1997. Respiratory symptoms and lung function in taxi drivers and manual workers. Aerobiologia 13:11–5. doi: 10.1007/BF02694785.
- Burnett, R.T., C.A. Pope 3rd, M. Ezzati, C. Olives, S.S. Lim, S. Mehta, H.H. Shin, G. Singh, B. Hubbell, M. Brauer, H.R. Anderson, K.R. Smith, J.R. Balmes, N.G. Bruce, H. Kan, F. Laden, A. Pruss-Ustun, M.C. Turner, S.M. Gapstur, W.R. Diver, and A. Cohen. 2014. An integrated risk function for estimating the global burden of disease attributable to ambient fine particulate matter exposure. Environ. Health Perspect. 122:397–403. doi: 10.1289/ehp.1307049.
- Chan, L., W. Lau, S. Zou, Z. Cao, and S. Lai. 2002. Exposure level of carbon monoxide and respirable suspended particulate in public transportation modes while commuting in urban area of Guangzhou, China. Atmos. Environ. 36:5831–40. doi: 10.1016/S1352-2310(02)00687-8.
- Chen, H., M.S. Goldberg, and P.J. Villeneuve. 2008. A systematic review of the relation between long-term exposure to ambient air pollution and chronic diseases. Revi. Environ. Health 23:243–97.
- Ekpenyong, C.E., E.O. Ettebong, E.E. Akpan, T.K. Samson, and N.E. Daniel. 2012. Urban city transportation mode and respiratory health effect of air pollution: A cross-sectional study among transit and non-transit workers in Nigeria. BMJ Open 2(5). doi: 10.1136/bmjopen-2012-001253.
- Frampton, M.W. 2001. Systemic and cardiovascular effects of airway injury and inflammation: Ultrafine particle exposure in humans. Environ. Health Perspect. 109(Suppl 4):529–32. doi: 10.1289/ehp.01109s4529.
- Hankinson, J.L., S.M. Kawut, E. Shahar, L.J. Smith, K.H. Stukovsky, and R.G. Barr. 2010. Performance of American thoracic society-recommended spirometry reference values in a multiethnic sample of adults: The Multi-Ethnic Study of Atherosclerosis (MESA) lung study. Chest 137:138–45. doi: 10.1378/chest.09-0919.
- Hankinson, J.L., J.R. Odencrantz, and K.B. Fedan. 1999. Spirometric reference values from a sample of the general U.S. population. Am. J. Respir. Crit. Care Med. 159:179–87. doi: 10.1164/ajrccm.159.1.9712108.
- Ierodiakonou, D., A. Zanobetti, B.A. Coull, S. Melly, D.S. Postma, H.M. Boezen, J.M. Vonk, P.V. Williams, G.G. Shapiro, E.F. McKone, T.S. Hallstrand, J.Q. Koenig, J.S. Schildcrout, T. Lumley, A.N. Fuhlbrigge, P. Koutrakis, J. Schwartz, S.T. Weiss, and D.R. Gold; G. Childhood Asthma Management Program Research. 2016. Ambient air pollution, lung function, and airway responsiveness in asthmatic children. J. Allergy Clin. Immunol. 137:390–9. doi: 10.1016/j.jaci.2015.05.028.
- Ishimaru, T., S. Arphorn, and A. Jirapongsuwan. 2016. Hematocrit levels as cardiovascular risk among taxi drivers in Bangkok, Thailand. Ind. Health 54:433–8. doi: 10.2486/indhealth.2015-0248.
- James, E., K. Andrew, and R. Rob. 2016. The Experimental City. Oxon, UK: Routledge.
- Jinsart, W., C. Kaewmanee, M. Inoue, K. Hara, S. Hasegawa, K. Karita, K. Tamura, and E. Yano. 2012. Driver exposure to particulate matter in Bangkok. J. Air & Waste Manage. Assoc. 62:64–71. doi: 10.1080/10473289.2011.622854.
- Jinsart, W., K. Tamura, S. Loetkamonwit, S. Thepanondh, K. Karita, and E. Yano. 2002. Roadside particulate air pollution in Bangkok. J. Air Waste Manage. Assoc. 52:1102–10. doi: 10.1080/10473289.2002.10470845.
- Jones, A.Y., P.K. Lam, and E. Dean. 2006. Respiratory health of bus drivers in Hong Kong. Int. Arch. Occup. Environ. Health 79:414–8. doi: 10.1007/s00420-005-0061-8.
- Karita, K., E. Yano, K. Tamura, and W. Jinsart. 2004. Effects of working and residential location areas on air pollution related respiratory symptoms in policemen and their wives in Bangkok, Thailand. Eur. J. Public Health 14:24–6. doi: 10.1093/eurpub/14.1.24.
- Lan, T.T., N.Q. Liem, and N.T. Binh. 2013. Personal exposure to benzene of selected population groups and impact of commuting modes in Ho Chi Minh, Vietnam. Environ. Pollut. 175:56–63. doi: 10.1016/j.envpol.2012.12.017.
- Lawin, H., G. Agodokpessi, P. Ayelo, J. Kagima, R. Sonoukon, B.H. Mbatchou Ngahane, O. Awopeju, W.M. Vollmer, B. Nemery, P. Burney, and B. Fayomi. 2016. A cross-sectional study with an improved methodology to assess occupational air pollution exposure and respiratory health in motorcycle taxi driving. Sci. Total Environ. 550:1–5. doi: 10.1016/j.scitotenv.2016.01.068.
- Mehta, S., H. Shin, R. Burnett, T. North, and A.J. Cohen. 2013. Ambient particulate air pollution and acute lower respiratory infections: A systematic review and implications for estimating the global burden of disease. Air Qual. Atmos. Health 6:69–83. doi: 10.1007/s11869-011-0146-3.
- Nankongnab, N., P. Silpasuwan, P. Markkanen, P. Kongtip, and S. Woskie. 2015. Occupational safety, health, and well-being among home-based workers in the informal economy of Thailand. New Solutions 25:212–31. doi: 10.1177/1048291115589148.
- Neter, J., W. Wasserman, and M.H. Kutner. 1989. Applied Linear Regression Models. New York: Irwin.
- Oshima, R., A. Fukuda, T. Fukuda, and T. Satiennam. 2007. Study on regulation of motorcycle taxi service in Bangkok. J East Asia Soc Transp Stud 7:1828–43. doi: 10.11175/easts.7.1828.
- Patel, D., T. Shibata, J. Wilson, and A. Maidin. 2016. Challenges in evaluating PM concentration levels, commuting exposure, and mask efficacy in reducing PM exposure in growing, urban communities in a developing country. Sci. Total Environ. 543 (Pt A):416–24. doi: 10.1016/j.scitotenv.2015.10.163.
- Raphiphan, P., A. Zaslavsky, and M. Indrawan-Santiago. 2014. Building knowledge from social networks on what is important to drivers in constrained road infrastructure. Procedia Comput. Sci. 35:720–9. doi: 10.1016/j.procs.2014.08.154.
- Sekine, K., M. Shima, Y. Nitta, and M. Adachi. 2004. Long term effects of exposure to automobile exhaust on the pulmonary function of female adults in Tokyo, Japan. Occup. Environ. Med. 61:350–7. doi: 10.1136/oem.2002.005934.
- Tashkin, D.P., R. Detels, M. Simmons, H. Liu, A.H. Coulson, J. Sayre, and S. Rokaw. 1994. The UCLA population studies of chronic obstructive respiratory disease: XI. Impact of air pollution and smoking on annual change in forced expiratory volume in one second. Am. J. Respir. Crit. Care Med. 149:1209–17. doi: 10.1164/ajrccm.149.5.8173761.
- U.S. Environmental Protection Agency. 2009. 2009 Final Report: Integrated Science Assessment for Particulate Matter. Washington, DC: U.S. Environmental Protection Agency.
- Vahlsing, C., and K.R. Smith. 2012. Global review of national ambient air quality standards for PM10 and SO2 (24 h). Air Qual. Atmos. Health 5:393–399. doi: 10.1007/s11869-010-0131-2.
- World Health Organization. 2006. WHO Air Quality Guidelines for Particulate Matter, Ozone, Nitrogen Dioxide and Sulfur Dioxide. Global update 2005. Geneva, Switzerland: World Health Organization. http://www.euro.who.int/__data/assets/pdf_file/0005/78638/E90038.pdf ( accessed March 1, 2017).