Emergency (****); * (*): *-* This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com 107 Emergency (2014); 2 (3): 107-114 ORIGINAL RESEARCH The Effects of Air Pollution on Cardiovascular and Respiratory Causes of Emergency Admission Ali Mohammad Shahi1, Ali Omraninava1, Mitra Goli2, Hamid Reza Soheilarezoomand3, Nader Mirzaei1* 1. Department of Emergency Medicine, AJA University of Medical Sciences, Tehran, Iran 2. Department of Geriatric Nursing, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran 3. Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran Abstract Introduction: Today, air pollution is one of the critical problems in metropolitans and necessary preparations are needed for confronting this crisis. The present study was based on the goal of determining the relationship of air pollutant levels with the rate of emergency admissions for respiratory and cardiovascular patients. Methods: In the present retrospective cross-sectional study, all respiratory and cardiovascular patients, referred to emer- gency department during 2012, were assessed. The meteorological and air pollution data were collected. Infor- mation regarding the numbers and dates (month, day) of admission for respiratory and cardiovascular diseases was achieved from the hospital's electronic registration system. The relation of air pollution and respiratory and cardiovascular admissions were analyzed by generalize additive model (GAM). Results: 5922 patients were as- sessed which included 4048 (68.36%) cardiovascular and 1874 (31.64%) respiratory. Carbon monoxide (CO) level was an independent risk factor of cardiovascular disease on the same day (RR=1.49; 95% CI: 1.25- 1.77; P<0.001), the day before (RR=1.22; 95% CI: 1.02- 1.45; P=0.03), and the last two days (RR=1.3; 95% CI: 1.09- 1.54; P<0.001). The same process was repeated for ozone (O3). In addition, the O3 level on the same day (RR=1.49; 95% CI: 1.25- 1.77; P<0.001), the day before (RR=1.22; 95% CI: 1.02- 1.45; P=0.03), the last two days (RR=1.3; 95% CI: 1.09- 1.54; P<0.001), and the last week (RR=1.004; 95% CI: 1.0007-1.008; P=0.02) were inde- pendent risk factors of respiratory admissions. The increased level of particulate matter less than 2.5 microme- ters in diameter (PM2.5) like O3 led to growth in the admissions to emergency department. Conclusion: The find- ings of the present study suggested that rising levels of CO and O3 during two days leads to a significant increase in cardiovascular admission on the third day. Furthermore, increase in O3, PM2.5, nitrogen dioxide (NO2), and CO levels causes a rise in respiratory admissions to emergency department. Key words: Air pollutant; patient admission; cardiovascular disease; respiratory disease Cite this article as: Mohammad Shahi A, Omraninava A, Goli M, Soheilarezoomand HR, Mirzaei N. The effects of air pollution on cardiovascular and respiratory causes of emergency admission. Emergency. 2014;2(3):107-14. Introduction:1 ir pollution is a complicated and heterogeneous mixture of gases, liquids, and particulate matter (PM) which is known as one of the risk factors of cancers, respiratory and cardiovascular diseases (1-4). During the past 20 years, numerous epidemiologic re- ports have shown a growing concern related to possible dangerous effects of air pollution on cardiovascular dis- eases (5, 6). Among these pollutants, more attention has been paid to carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxide (SO2), ozone (O3), Lead, and thoracic particles such as particulate matter less than 10 mi- *Corresponding Author: Nader Mirzaei; Department of Emergency Medicine, AJA University of Medical Sciences, Bessat Hospital, Tehran, Iran. Tel/Fax +982188028354; Email: nad5406@yahoo.com Received: May 2014; Accepted: June 2014 crometers in diameter (PM10) and fine particles like particulate matter less than 2.5 micrometers in diame- ter (PM2.5) (7-10). The findings of these studies are rep- resentative of the direct relation between the levels of these molecules in the air and the rate of admission as well as mortality of cardiovascular diseases (11). There- fore, the cardiac causes of mortality were exacerbated even with short-term increase in these components (12-14). However, little information has been achieved about the air pollution morbidity most of which relate to decades of 1990 and 2000. Since the levels of SO2, PM10, and PM2.5 have noticeably increased in recent years (15), further assessment of this relationship is highly important. A mailto:nad5406@yahoo.com This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com Mohammad Shahi et al 108 Today, air pollution is recognized as a crisis in metro- politans and necessary preparations are required for confronting this problem. Preparation of health care systems to control and treat diseases risen from air pol- lution is very significant in management of these pa- tients. Awareness of patient admissions has a remarka- ble role in providing proportional equipment and facili- ties fitted with the number of admitted patients. On the other hand, because of differences in pollutant levels of various geographical locations, the pattern of hospital admissions may have a noticeable difference on pollut- ed days (16). Therefore, it is critical for health care sys- tems to be informed of the pattern of hospital admis- sions in polluted days to program more accurate man agent plan. The present study was done based on the goal of determining the relationship of air pollutant levels with the rate of emergency admissions for res- piratory and cardiovascular patients. Methods: Study design and setting: In this retrospective cross- sectional study, the associa- tion between air pollutant levels with respiratory and cardiovascular diseases has been evaluated in patients referred to the emergency department of Bessat hospi- tal, Tehran, Iran, during 2012. Tehran, the capital of Iran, is the most air-polluted city in this country and most of its pollution results from traffic and transport. About 8.5 million people live in this city and make up more than 10% of Iran's population. Patients The studied population were patients referred to the emergency department with diagnosis of non-traumatic cardiovascular and respiratory disease from April 2012 to April 2013. Exclusion criterion was uncertainty of diagnosis. Hospital Admissions Information of daily admissions of respiratory and car- diovascular diseases was achieved from hospital's elec- tronic registration system. At first, the numbers and dates (month, day) of admissions were extracted. Myo- cardial infarction, ischemic attacks, angina pectoris, coronary cardiovascular disease, cardiomyopathy, heart failure, cardiac dysrhythmia, endocarditis, and myocar- ditis were counted as cardiovascular disease. In addi- tion, asthma, emphysema, allergies, bronchitis, obstruc- tive pulmonary disease, respiratory infections, and Table 1: The status of emergency admission and air pollutants during study period  Variable Mean±SD Min Centile 25 Median Centile 75 Max Emergency Admission Cardiac 11.06±2.05 5 10 11 12 16 Respiratory 5.12±1.75 2 3 5 6 10 Total 16.18±1.85 12 15 16 18 21 Air pollutants concentrations CO (µg/m3) 3.7±0.88 1.9 3 3.6 4.3 6.5 NO2 (µg/m3) 47.58±11.68 18 38 47 56 78 O3 (µg/m3) 37.08±11.6 12 27 38 45 70 SO2 (µg/m3) 32.22±4.67 20 29 32 35 50 PM2.5 (µg/m3) 98.77±23.9 42 83 95 110 192 PM10 (µg/m3) 64.72±19.49 24 42 62 71 201 Meteorological measures Temperature (°C) 19.6±9.43 -2 10.3 21.2 28.9 41.3 Humidity (%) 48.97±15.28 20 40 50 60 79 Wind speed (km/h) 17.7±5.55 10 13 17 22 34 SD: Standard deviation; CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate matter less than 10 micrometers in diameter. Table 2: correlation matrix between air pollutants and meteorological variables  CO O3 SO2 NO2 PM10 PM2.5 Temperature Wind speed Humidity CO 1.00 - - - - - - - - O3 0.05 1.00 - - - - - - - SO2 0.42a -0.05 1.00 - - - - - - NO2 0.38 a 0.47 a 0.39 a 1.00 - - - - - PM10 0.43 a 0.03 0.54 a 0.40 a 1.00 - - - - PM2.5 0.23 a 0.39 a 0.36 a 0.54 a 0.54 a 1.00 - - - Temperature -0.23 a -0.38 a 0.01 -0.32 a -0.04 -0.23 a 1.00 - - Wind speed -0.14b -0.16b -0.06 -0.07 0.06 -0.04 0.42 a 1.00 - Humidity 0.12C 0.25 a -0.12C 0.17b -0.03 0.18 a -0.62 a -0.34 a 1.00 a: p<0.001; b: p<0.01; c: p<0.05; CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate matter less than 10 micrometers in diameter. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com 109 Emergency (2014); 2 (3): 107-114 pneumonia were considered as respiratory problems. Air pollution and meteorological characteristics There are 31 air quality-monitoring stations in Tehran city, recording data 24 hours per day. These stations check the levels of CO, PM10, PM2.5, NO2, O3, and SO2 each hour. In the present study, these data were used to cal- culate the average of pollutant concentrations during 24 hours. It is worth noting that meteorology organization of Tehran measures the average of humidity, temperature, and wind speed, hourly. There are 13 meteorology sta- tions in the province five of which are specifically rep- resentative of meteorological situations of Tehran city. The data of these five stations were extracted and used. Statistical analysis Descriptive statistics and correlation among emergency admissions, air pollutants, and meteorology factors (temperature, humidity, and wind speed) were ana- lyzed using SPSS statistical software version 21.0. Rela- tive risks of respiratory and cardiovascular admissions were calculated by generalize additive model (GAM) based on Poisson distribution; because the previous studies showed that the distribution of pollutants’ data didn't follow the normal pattern (5, 6). Each GAM was fitted based on the logarithm of the emergency admis- sion numbers as well as the overall adjusted and linear effects of predictive factors (air pollutants). Therefore, smoothing spline functions were applied to tempera- ture, wind speed, and humidity as confounding factors. Degree of freedom was defined in terms of Akaike’s cri- terion (17). The daily levels of pollutants for the same day (Lag0), the average of the day before and same day (Lag1), average of the last two days (Lag2), and average of the last week (Lag7) were analyzed and the associa- tion of pollutant levels with number of daily emergency admissions were evaluated in each period. All the find- ings were presented as relative risk (RR) and 95% con- fidence intervals (95% CI). In all analyses p<0.05 was defined as significance level. Results: In the present study, the data of daily air pollutant lev- els, humidity, temperature, and wind speed during 2012 were collected. In total 5922 patients were as- sessed including 4048 (68.36%) cardiovascular and 1874 (31.64%) respiratory. The admission numbers of cardiovascular patients in winter was significantly low- Table 3: The correlation between enviromental pollutants and cardiovascular admissions in the studied hospital  Variable Adjusted Relative risk 95% confidence Interval Pa Lag 0 CO 1.49 1.25-1.77 <0.001 O3 1.02 1.01-1.03 <0.001 NO2 1.001 0.98-1.03 0.01 SO2 0.998 0.97-1.03 0.94 PM10 0.99 0.98-1.01 0.41 PM2.5 1.004 0.99-1.02 0.53 Lag 1 CO 1.22 1.02-1.45 0.03 O3 1.01 1.001-1.02 0.03 NO2 1.00 0.98-1.02 0.94 SO2 0.99 0.96-1.02 0.64 PM10 0.98 0.98-1.01 0.06 PM2.5 1.01 0.99-1.02 0.35 Lag 2 CO 1.30 1.09-1.54 <0.001 O3 1.01 1.001-1.02 0.02 NO2 0.99 0.96-1.01 0.25 SO2 0.99 0.96-1.02 0.37 PM10 1.00 0.98-1.02 0.94 PM2.5 1.00 0.99-1.01 0.69 Lag 7 CO 0.97 0.88-1.08 0.60 O3 0.99 0.99-1.002 0.19 NO2 1.01 0.99-1.02 0.13 SO2 1.001 0.98-1.02 0.96 PM10 1.003 0.99-1.01 0.41 PM2.5 1.00 0.99-1.002 0.36 a: Adjusted for temperature, wind speed, humidity and other air pollutants. CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate matter less than 10 micrometers in diame- ter. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com Mohammad Shahi et al 110 er than other seasons (10.3 admissions per day) (df: 3, 362; F=23.24; p=0.0007). The average humidity in the study period was 48.97±15.26 percent and the average of maximum wind speed was 17.7±5.55 kilometers per hour. The findings of this study revealed that with hu- midity increasing (r=0.35; p<0.001) the rate of respira- tory diseases grew, while the wind speed had an in- verse relation with admission numbers of respiratory patients (r=-0.16; p=0.002). The wind speed (r=0.097; p=0.06) and humidity (r=-0.06; p=0.23) did not have a significant correlation with cardiovascular admissions. Table 1 shows the respiratory and cardiovascular ad- mission status along with air pollutant levels during the study period. Table 2 shows the correlation among air pollution indi- cators. Based on this table, PM2.5 and NO2 have a corre- lation with other pollutants. The strongest relationships were seen between PM10 and PM2.5 (r=0.54; p<0.001), NO2 and PM2.5 (r=0.54; p<0.001), and NO2 and O3 (r=0.47; p<0.001). All pollutant levels except PM10 have a significant relation with relative humidity. The air pollutant levels and cardiovascular emergency admissions The findings of the present study showed that cardio- vascular admissions to emergency department have a significant association with increasing pollutant levels (Figure 1 and Table 3). After adjusting the analysis for season, temperature, wind speed, humidity, and other air pollutants, the CO level on the same day (RR=1.49; 95% CI: 1.25- 1.77; P<0.001), the day before (RR=1.22; 95% CI: 1.02- 1.45; P=0.03), and the last two days (RR=1.3; 95% CI: 1.09- 1.54; P<0.001) were independ- ent risk factors of cardiovascular admissions. The same pattern was seen for O3. Other air pollutants did not have any effects on cardiovascular admissions. The air pollutant levels and respiratory emergency admissions Respiratory admissions to emergency department also had a significant relationship with increasing pollutant levels (Figure 2 and Table 4). After adjusting the analy- sis for season, temperature, wind speed, humidity, and other air pollutants, the O3 level on the same day Table 4: The correlation between enviromental pollutants and respiratory admissions in the studied hospital  Variable Adjusted Relative risk 95% confidence Interval Pa Lag 0 CO 1.04 1.002-1.09 0.04 O3 1.01 1.008-1.015 <0.001 NO2 1.01 1.002-1.01 0.01 SO2 0.99 0.985-1.00 0.05 PM10 0.998 0.996-1.001 0.33 PM2.5 1.002 1.001-1.004 0.01 Lag 1 CO 1.04 0.999-1.08 0.06 O3 1.005 1.002-1.009 <0.001 O3 1.002 0.998-1.006 0.24 NO2 0.998 0.99-1.005 0.62 SO2 0.999 0.996-1.002 0.62 PM10 1.002 1.0003-1.004 0.02 Lag 2 CO 1.04 1.001-1.085 0.04 O3 1.007 1.003-1.01 <0.001 NO2 1.0004 0.996-1.005 0.84 SO2 0.995 0.988-1.003 0.21 PM10 0.999 0.997-1.002 0.71 PM2.5 1.002 1.001-1.004 0.01 Lag 7 CO 1.03 0.99-1.07 0.14 O3 1.004 1.0007-1.008 0.02 NO2 1.005 1.0006-1.009 0.03 SO2 0.998 0.993-1.007 0.00 PM10 0.999 0.996-1.002 0.77 PM2.5 1.002 1.001-1.004 0.01 a: Adjusted for temperature, wind speed, humidity and other air pollutants. CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate matter less than 10 micrometers in diameter This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com 111 Emergency (2014); 2 (3): 107-114 (RR=1.49; 95% CI: 1.25- 1.77; P<0.001), the day before (RR=1.22; 95% CI: 1.02- 1.45; P=0.03), the last two days (RR=1.3; 95% CI: 1.09- 1.54; P<0.001), and the last week (RR=1.004; 95% CI: 1.0007-1.008; P=0.02) were independent risk factors of admission. Increased level of PM2.5 led to a growth in the admis- sions to emergency department with respiratory causes just like O3. Altered levels of NO2 and SO2 did not have any effects on respiratory admissions. Discussion: The findings of the present study revealed that air pol- lutant levels have a direct relationship with the number of respiratory and cardiovascular admissions. The in- creased levels of NO and O3 in the two days before ad- mission, caused a significant increase in cardiovascular admissions. Similarly, increased O3, PM2.5, CO, and NO2 levels were associated with more respiratory admis- sions to emergency. In several studies, CO was mentioned as the most effec- tive pollutant on hospital admissions. For example, in a study Qorbani et al. showed that there is a significant relationship between acute coronary syndrome and the CO level in Tehran citizens. Also, it was suggested that there was no correlation between the increased levels of PM10 and PM2.5 and coronary heart disease (7). Hos- seinpoor et al. displayed that the relative risk of angina Figure 1: The relation between cardiovascular admissions and air pollutant levels. ** Statistically Significant at level of p<0.001; * Statistically Significant at level of p<0.05 CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate mat- ter less than 10 micrometers in diameter.  This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com Mohammad Shahi et al 112 pectoris increased with higher CO levels in the two days before admission. This group didn't find any associa- tions between the rate of angina pectoris and other pol- lutants (11). Furthermore, in this study CO level had the closest relation with cardiovascular diseases with O3 in the second rank. Although in some studies (7, 11), there was no association seen between O3 and emergency- Admissions with cardiovascular disease. Destructive effects of these pollutants on cardiovascular system has been recognized since many years ago. For example, a review study showed that O3 was one of the most toxic components of mixed photochemical of air (14). Thus, its increase in air resulting in more the hospital admis- sions is not too far-fetched. On the other hand, because of further urbanization of Tehran, ozone level has re- markably increased in recent years. Qorbani et al. (7) reported the average of ozone levels 9.7 µg/m3 in 2001, while in the present study, the annual average of this component was 37.08 µg/m3. Although some studies revealed that short-term exposure to particulate mat- ters, increases the risk for cardiovascular admission but the present study did not find any relationship between them. Dominici et al. suggested that the levels of these pollutants are associated with increased cardiovascular admissions (18). In contrast, Hosseinpoor et al (11) and Chen et al (19) found no significant effects. This may be due to misclassification of the average population expo- sure to particulate matters (20). In addition, some stud- Figure 2: The relation between respiratory admissions and air pollutant levels. ** Statistically Significant at level of p<0.001; * Statistically Significant at level of p<0.05 CO: carbon monoxide; NO2: nitrogen oxide; O3: ozone; SO2: sulfur oxide; PM2.5: like particulate matter less than 2.5 micrometers in diameter; PM10: like particulate matter less than 10 micrometers in diameter.  This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com 113 Emergency (2014); 2 (3): 107-114 ies have stated that biological variables such as ventila- tion, time spent outdoors, and household characteris- tics, affect the strength of these associations. So, for cer- tain individuals and cohorts the health effects attribut- ed to particulate matter might vary in different geo- graphic areas (21). Air pollution is one of the significant environmental risk factors of acute and chronic respiratory disease. The present project revealed that increased levels of O3, PM2.5, and CO aligned with more emergency admissions due to respiratory diseases. The role of ozone in higher respiratory admissions arose from the effect of this pol- lutant on increasing inflammation in the respiratory tract (12) which is critical in respiratory disease symp- toms. In addition, ozone reduces the respiratory func- tion, increases the activity of respiratory tracts, and asthma. This component leads to increased emergency admissions with respiratory disease in adults and chil- dren (22). Burnett and his colleagues showed that one hour increase in the ozone level during the last five days caused 35% increase in emergency admissions for children less than two years of age (12). Ji et al. in their meta-analysis displayed that 10 parts-per-billion in- crease in ozone leads to 3% increase in respiratory causes of emergency referrals (15). Lin et al. also re- vealed that there is a strong association between in- creased level of ozone and clinical admissions for res- piratory diseases (6). PM2.5, like ozone, has a remarkable role in respiratory symptoms. Dominici et al. showed that PM2.5 has a close association with increased emergency admissions be- cause of obstructive lung diseases and lung infections (18). Similarly, Lall et al. suggested that there is an in- terconnection between PM2.5 level and emergency ad- missions for respiratory diseases (23). The present study showed similar results, too. There are several disagreements regarding the correla- tion between levels of other pollutants and respiratory emergency admissions. Chen et al. presented that NO2, PM10, and SO2 levels don't have a correlation with res- piratory diseases in Shanghai, China (19). Whereas Tao et al. showed that a strong association exists between these three pollutants and respiratory diseases in Lan- zhou, China. This issue was raised due to the difference among pollutant levels in various urban areas. NO2 had a significant relation with respiratory admissions in this study. Conclusion: The findings of the present study suggested that raised levels of CO and O3 during two days leads to a signifi- cant increase in cardiovascular admissions. Further- more, increased O3, PM2.5, NO2, and CO levels cause a rise in respiratory admissions to emergency depart- ment. Acknowledgments: The authors appreciate the insightful cooperation of the staff in the emergency department of Bessat hospital in Tehran, Iran. Conflict of interest: None Funding support: None Authors’ contributions: All authors passed four criteria for authorship contribu- tion based on recommendations of the International Committee of Medical Journal Editors. References: 1. Zhou M, Liu Y, Wang L, Kuang X, Xu X, Kan H. Particulate air pollution and mortality in a cohort of Chinese men. Environ Pollut. 2014;186:1-6. 2. Beverland I, Carder M, Cohen G, Heal M, Agius R. Associations between short/medium-term variations in black smoke air pollution and mortality in the Glasgow conurbation, UK. Environ Int. 2014;62:126-32. 3. Dimakopoulou K, Samoli E, Beelen R, et al. Air pollution and nonmalignant respiratory mortality in 16 cohorts within the ESCAPE project. Am J Respir Crit Care Med. 2014;189(6):684- 96. 4. Masjedi MR, Naghan PA, Taslimi S, et al. Opium could be considered an independent risk factor for lung cancer: a case- control study. Respiration. 2013;85(2):112-8. 5. Tao Y, Mi S, Zhou S, Wang S, Xie X. Air pollution and hospital admissions for respiratory diseases in Lanzhou, China. Environ Pollut. 2014;185:196-201. 6. Lin Y-K, Chang S-C, Lin C, Chen Y-C, Wang Y-C. Comparing ozone metrics on associations with outpatient visits for respiratory diseases in Taipei Metropolitan area. Environ Pollut. 2013;177:177-84. 7. Qorbani M, Yunesian M, Fotouhi A, Zeraati H, Sadeghian S, Rashidi Y. Relation between Air Pollution Exposure and Onset of Acute Coronary Syndrome in Tehran Heart Center Using a Case-Crossover Design. Iran J Epidemiol. 2007;3(1):53-9. 8. Heo J, Schauer JJ, Yi O, Paek D, Kim H, Yid S-M. Fine Particle Air Pollution and Mortality. Epidemiology. 2014;25(3):379- 88. 9. Moolgavkar SH, McClellan RO, Dewanji A, Turim J, Luebeck EG, Edwards M. Time-series analyses of air pollution and mortality in the United States: a subsampling approach. Environ Health Perspect. 2013;121(1):73-8. 10. Cesaroni G, Badaloni C, Gariazzo C, et al. Long-term exposure to urban air pollution and mortality in a cohort of more than a million adults in Rome. Environ Health Perspect. 2013;121(3):324-31. 11. Hosseinpoor AR, Forouzanfar MH, Yunesian M, Asghari F, Naieni KH, Farhood D. Air pollution and hospitalization due to angina pectoris in Tehran, Iran: A time-series study. Environ Res. 2005;99(1):126-31. 12. Burnett RT, Smith-Doiron M, Stieb D, et al. Association between ozone and hospitalization for acute respiratory diseases in children less than 2 years of age. Am J Epidemiol. 2001;153(5):444-52. 13. Dockery DW, Pope CA, Xu X, et al. An association between air pollution and mortality in six US cities. N Engl J Med. 1993;329(24):1753-9. 14. Srebot V, Gianicolo E, Rainaldi G, Trivella MG, Sicari R. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Copyright © 2014 Shahid Beheshti University of Medical Sciences. All rights reserved. Downloaded from: www.jemerg.com Mohammad Shahi et al 114 Ozone and cardiovascular injury. Cardiovasc Ultrasound. 2009;7:30-9. 15. Ji M, Cohan DS, Bell ML. Meta-analysis of the association between short-term exposure to ambient ozone and respiratory hospital admissions. Environ Res Lett. 2011;6(2):024006. 16. Chay KY, Greenstone M. The impact of air pollution on infant mortality: evidence from geographic variation in pollution shocks induced by a recession. Q J ECON. 2003;118(3):1121-67. 17. Akaike H. Factor analysis and AIC. Psychometrika. 1987;52(3):317-32. 18. Dominici F, Peng RD, Bell ML, et al. FIne particulate air pollution and hospital admission for cardiovascular and respiratory diseases. JAMA. 2006;295(10):1127-34. 19. Chen R, Chu C, Tan J, et al. Ambient air pollution and hospital admission in Shanghai, China. J Hazard Mater. 2010;181(1):234-40. 20. Chang C-C, Tsai S-S, Ho S-C, Yang C-Y. Air pollution and hospital admissions for cardiovascular disease in Taipei, Taiwan. Environ Res. 2005;98(1):114-9. 21. Sarnat JA, Schwartz J, Catalano PJ, Suh HH. Gaseous pollutants in particulate matter epidemiology: confounders or surrogates? Environ Health Perspect. 2001;109(10):1053. 22. Shrey K, Suchit A, Deepika D, Shruti K, Vibha R. Air pollutants: the key stages in the pathway towards the development of cardiovascular disorders. Environ Toxicol Pharmacol. 2011;31(1):1-9. 23. Lall R, Kendall M, Ito K, Thurston GD. Estimation of historical annual PM 2.5 exposures for health effects assessment. Atmos Environ. 2004;38(31):5217-26.