Introduction
Asthma is the most common inflammatory disease that poses a significant burden on both clinical and public health [1, 2]. Recent reports have highlighted asthma as the 12th leading cause of death among children under 5 years of age, and the 20th cause of death in children between 5 and 14 years of age [3]. The prevalence of asthma varies widely across countries at different levels of development. Over the past years, there has been an increase in the incidence of asthma particularly among children who are at a higher risk of developing the condition [4]. The rapid growth of urban population, industrialization, and the intensification of road traffic present significant challenges to ambient air quality [5]. Studies have identified those vehicles as the primary source of air pollution in most cities [5, 6]. Children with respiratory conditions, such as allergies, asthma, and chronic obstructive pulmonary disease are particularly vulnerable to adverse respiratory effects from exposure to traffic-related air pollutants [7, 8]. While there is mounting evidence linking living near heavy traffic to higher rates of asthma, some well-designed studies have found only weak or no associations [9, 10]. Given the ongoing debate surrounding the role of air pollution in the development of new-onset asthma, and its contribution to the current pandemic [11, 12], we conducted a cross-sectional study to investigate the impact of exposure to traffic-related air pollutants on respiratory symptoms among asthmatic individuals in Yazd City, Iran.

Methods
This cross-sectional study involved 7214 adolescents aged 13 to 14 years as a part of the Global Asthma Network 2020 survey, in Yazd Province, Iran. A total of 48 schools were selected through cluster sampling from a total of 48 private and public schools catering to both genders. Informed consent was obtained from participants before completing the electronic questionnaire. The global asthma network questionnaire, which is derived from the international study of asthma and allergies in childhood questionnaire, includes questions about allergic symptoms and related risk factors. The details of the study method referred to previously published [13]. The data on wheezing in the past year, nighttime dry cough, asthma severity, physician-confirmed asthma diagnosis, and exposure to heavy vehicle traffic were collected using the translated Global Asthma Network questionnaire. Children with exposure to smoking, pets, or environmental tobacco smoke were excluded from the study to avoid confounding factors.

Statistical analyses
Data analysis was performed using the SPSS software, version 20. The t-test was used to compare mean differences between quantitative variables. Meanwhile, the Pearson chi-square test was used to assess associations between categorical variables. A P<0.05 was considered statistically significant. The results from the logistic regression model were presented as odds ratio (OR) with corresponding 95% confidence intervals (CI).

Result
A total of 7214 children in the 13–14 years of age groups were included with a response rate of 71%. Among the 5141 students, 3069(59.7%) were females and 2072(40.3%) were males. In this study, 9% of participants reported current asthma symptoms, including wheezing in the past year, while 12.4% reported nighttime dry cough and wheezing after exercise was observed in 15.9% of participants (Table 1). 


Approximately 31% of the children were exposed to frequent and constant heavy goods vehicle traffic. A statistically significant relationship was found between truck traffic and asthma symptoms (P=0.001). Notably, in the multivariate analysis, this relationship remained significant for individuals living on streets with frequent or constant heavy good goods vehicle, traffic compared to those residing on streets where these vehicles did not pass. There was no statistically significant relationship between housing types, floor numbers, and asthma (P=0.15 and P=0.11 respectively) (Table 2).


There was no statistically significant relationship observed in children, with more asthma symptoms who were exposed to higher-intensity truck traffic (P=0.18) (Table 3).


The association between traffic air pollution and asthma in the logistic regression model is detailed in Table 4.


In the multivariable model, intense truck traffic significantly increased asthma symptoms (adjusted OR=2.19 [95% CI, 1.33%, 3.62%]). 

Discussion
The present study identified a significant relationship between high traffic densities and the incidence of respiratory symptoms, aligning with findings from previous studies that have also reported an increase in asthma symptoms with greater exposure to traffic [14-24]. A recent meta-analysis further supported this association by demonstrating a link between higher exposure to truck traffic and childhood asthma [25]. In a study by Janssen et al. positive associations were found between truck traffic density and other indicators of traffic-related air pollution and childhood asthma [26]. However, the consistency of the relationship between traffic exposure and asthma symptoms across different studies is not uniform. Some authors have reported no significant effects, as seen in a study by Pujades-Rodriguez et al. which even suggested a protective effect in 7- to 15-year-old children [27]. A study in New York City, United States, revealed that exposure to high traffic density was associated with an increased risk of asthma emergency department visits, with certain subgroups potentially at higher risk due to physiological susceptibility. Despite adjustment for traffic exposures, racial and demographic disparities persisted among this low-income population, indicating that environmental risk factors, such as local traffic, can disproportionately impact vulnerable groups [28]. As evidenced by a study in Spain the odds of developing severe or exercise-induced asthma were three times higher in 6-7 year-old boys living on streets with frequent heavy-duty vehicle traffic, as opposed to those living on streets where these vehicles do not pass. There was no association between truck traffic and asthma symptoms among girls and 13-14 year-old boys. These findings appear to suggest a differential effect of truck traffic on the development of asthma, particularly in young boys [29]. This study indicated a stronger effect relationship between the severity of asthma and the frequent passing of the truck in the male gender; consistent with previous research findings [29, 30, 31]. This gender difference could be attributed to smaller airway sizes in boys at this age, leading to increased airway reactivity [32, 33]. Cultural and social factors may also contribute to lower acceptance of an asthma diagnosis among females in our country. While some evidence has suggested larger estimates for girls in other studies, our findings highlight a greater impact on males [19, 33, 34]. In addition, our study did not any significant association between traffic exposures and the severity of asthma, consistent with findings reported by other authors [35, 36]. In Shandong, China, a study showed that traffic-related facilities in proximity to residential areas appear to be risk factors for the development of asthma, wheezing, and rhinitis among urban children [37]. Another cross-sectional analysis of a multiracial cohort of children from the environmental influences on child health outcomes cohort revealed a significant association between reported neighborhood traffic and asthma/asthma-like symptoms, both in line with the findings of this study [38].

Conclusion
The results of this study suggest that exposure to traffic-related air pollutants is associated with an increase in asthma symptoms, with the impact varying depending on the sex of the individuals.

Study limitations
This study faced some limitations as the data was collected through a questionnaire without objective measurements of exposure or disease. Additionally, information regarding contaminants from sources other than traffic, such as biomass combustion or certain industries, was not obtained. In conclusion, the results of this study suggest that exposure to traffic-related air pollutants is associated with an increase in asthma symptoms, with the impact varying depending on the sex of the individuals.

Ethical Considerations
Compliance with ethical guidelines
This study received approval from the Ethics Committee of Shahid Sadoughi University of Medical Sciences, Yazd, Iran (Code: IR.SSU.MEDICINE.REC.1400.235).

Funding
This study results were taken from a research project that was funded by Shahid Sadoughi University of Medical Sciences, Yazd, Iran.

Authors contributions
Conceptualization, and data curation: Nasrin Behniafard and Hosein Ghareghahi; Formal analysis: Farimah Shamsi; Funding acquisition: Nasrin Behniafard, and Zahra Nafei; Writing the original draft: Azam Golzar; Methodology, visualization, review and editing: Abdolhamid Jafari nodoshan.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors would like to appreciate the Children Growth Disorder Research Center, Shahid Sadoughi University of Medical Sciences, and Yazd Education Organization staff for their sincere cooperation in this project. Finally, we appreciate the students who participated in this study.


References

1. Brandt S, Perez L, Künzli N, Lurmann F, Wilson J, Pastor M, et al. Cost of near-roadway and regional air pollution-attributable childhood asthma in Los Angeles County. J Allergy Clin Immunol. 2014; 134(5):1028-35. [DOI:10.1016/j.jaci.2014.09.029] [PMID]
2. Brandt SJ, Perez L, Künzli N, Lurmann F, McConnell R. Costs of childhood asthma due to traffic-related pollution in two California communities. Eur Respir J. 2012; 40(2):363-70. [DOI:10.1183/09031936.00157811] [PMID]
3. Ybáñez Zepeda E, Yanes Pérez M. [Homicidio y marginación en los municipios urbanos de los estados más violentos de México, 2000-2005 (Spanish)]. Estudios demográficos y urbanos. 2013; 28(2):291-322. [DOI:10.24201/edu.v28i2.1430]
4. James AL, Knuiman MW, Divitini ML, Hui J, Hunter M, Palmer LJ, et al. Changes in the prevalence of asthma in adults since 1966: The Busselton health study. Eur Respir J. 2010; 35(2):273-8. [DOI:10.1183/09031936.00194308] [PMID]
5. Houston D, Wu J, Ong P, Winer A. Structural disparities of urban traffic in Southern California: Implications for vehicle-related air pollution exposure in minority and high-poverty neighborhoods. J Urban Aff. 2004; 26(5):565-92. [DOI:10.1111/j.0735-2166.2004.00215.x]
6. Pandey B, Agrawal M, Singh S. Assessment of air pollution around coal mining area: Emphasizing on spatial distributions, seasonal variations and heavy metals, using cluster and principal component analysis. Atmos Pollut Res. 2014; 5(1):79-86. [DOI:10.5094/APR.2014.010]
7. Katsouyanni K, Touloumi G, Samoli E, Gryparis A, Le Tertre A, Monopolis Y, et al. Confounding and effect modification in the short-term effects of ambient particles on total mortality: Results from 29 European cities within the APHEA2 project. Epidemiology. 2001; 12(5):521-31. [DOI:10.1097/00001648-200109000-00011] [PMID]
8. Arbex MA, de Souza Conceição GM, Cendon SP, Arbex FF, Lopes AC, Moyses EP, et al. Urban air pollution and chronic obstructive pulmonary disease-related emergency department visits. J Epidemiol Community Health. 2009; 63(10):777-83. [DOI:10.1136/jech.2008.078360] [PMID]
9. Heinrich J, Wichmann HE. Traffic related pollutants in Europe and their effect on allergic disease. Curr Opin Allergy Clin Immunol. 2004; 4(5):341-8. [DOI:10.1097/00130832-200410000-00003] [PMID]
10. Oftedal B, Nystad W, Brunekreef B, Nafstad P. Long-term traffic-related exposures and asthma onset in schoolchildren in Oslo, Norway. Environ Health Perspect. 2009; 117(5):839-44. [DOI:10.1289/ehp.11491] [PMID]
11. Eder W, Ege MJ, von Mutius E. The asthma epidemic. N Engl J Med. 2006; 355(21):2226-35. [DOI:10.1056/NEJMra054308] [PMID]
12. Sarnat JA, Holguin F. Asthma and air quality. Curr Opin Pulm Med. 2007; 13(1):63-6. [DOI:10.1097/MCP.0b013e3280117d25] [PMID]
13. Nafei Z, Behniafard N, Mirzaei M, Karimi M, Akbarian E. Prevalence of allergic rhinitis and eczema in adolescents living in Yazd city: Part of global asthma network survey. Iran J Allergy Asthma Immunol. 2021; 20(3):271-8.[DOI:10.18502/ijaai.v20i3.6331] [PMID]
14. Cesaroni G, Badaloni C, Porta D, Forastiere F, Perucci CA. Comparison between various indices of exposure to traffic-related air pollution and their impact on respiratory health in adults. Occup Environ Med. 2008; 65(10):683-90. [DOI:10.1136/oem.2007.037846] [PMID]
15. Janssen NA, van Vliet PH, Aarts F, Harssema H, Brunekreef B. Assessment of exposure to traffic related air pollution of children attending schools near motorways. Atmos Environ. 2001; 35(22):3875-84. [DOI:10.1016/S1352-2310(01)00144-3]
16. van Roosbroeck S, Wichmann J, Janssen NA, Hoek G, van Wijnen JH, Lebret E, et al. Long-term personal exposure to traffic-related air pollution among school children, a validation study. Sci Total Environ. 2006; 368(2-3):565-73. [DOI:10.1016/j.scitotenv.2006.03.034] [PMID]
17. Brauer M, Hoek G, Smit HA, de Jongste JC, Gerritsen J, Postma DS, et al. Air pollution and development of asthma, allergy and infections in a birth cohort. Eur Respir J. 2007; 29(5):879-88.[DOI:10.1183/09031936.00083406] [PMID]
18. McConnell R, Islam T, Shankardass K, Jerrett M, Lurmann F, Gilliland F, et al. Childhood incident asthma and traffic-related air pollution at home and school. Environ Health Perspect. 2010; 118(7):1021-6. [DOI:10.1289/ehp.0901232] [PMID]
19. Migliore E, Berti G, Galassi C, Pearce N, Forastiere F, Calabrese R, et al. Respiratory symptoms in children living near busy roads and their relationship to vehicular traffic: Results of an Italian multicenter study (SIDRIA 2). Environ Health. 2009; 8:27. [DOI:10.1186/1476-069X-8-27] [PMID]
20. Morgenstern V, Zutavern A, Cyrys J, Brockow I, Koletzko S, Krämer U, et al. Atopic diseases, allergic sensitization, and exposure to traffic-related air pollution in children. Am J Respir Crit Care Med. 2008; 177(12):1331-7. [DOI:10.1164/rccm.200701-036OC] [PMID]
21. O'Connor GT, Neas L, Vaughn B, Kattan M, Mitchell H, Crain EF, et al. Acute respiratory health effects of air pollution on children with asthma in US inner cities. J Allergy Clin Immunol. 2008; 121(5):1133-9.e1. [DOI:10.1016/j.jaci.2008.02.020] [PMID]
22. Gent JF, Koutrakis P, Belanger K, Triche E, Holford TR, Bracken MB, et al. Symptoms and medication use in children with asthma and traffic-related sources of fine particle pollution. Environ Health Perspect. 2009; 117(7):1168-74. [DOI:10.1289/ehp.0800335] [PMID]
23. Middleton N, Yiallouros P, Nicolaou N, Kleanthous S, Pipis S, Zeniou M, et al. Residential exposure to motor vehicle emissions and the risk of wheezing among 7-8 year-old schoolchildren: A city-wide cross-sectional study in Nicosia, Cyprus. Environ Health. 2010; 9:28.[DOI:10.1186/1476-069X-9-28] [PMID]
24. Dales R, Wheeler AJ, Mahmud M, Frescura AM, Liu L. The influence of neighborhood roadways on respiratory symptoms among elementary schoolchildren. J Occup Environ Med. 2009; 51(6):654-60. [DOI:10.1097/JOM.0b013e3181a0363c] [PMID]
25. Gasana J, Dillikar D, Mendy A, Forno E, Ramos Vieira E. Motor vehicle air pollution and asthma in children: A meta-analysis. Environ Res. 2012; 117:36-45. [DOI:10.1016/j.envres.2012.05.001] [PMID]
26. Janssen NA, Brunekreef B, van Vliet P, Aarts F, Meliefste K, Harssema H, et al. The relationship between air pollution from heavy traffic and allergic sensitization, bronchial hyperresponsiveness, and respiratory symptoms in Dutch schoolchildren. Environ Health Perspect. 2003; 111(12):1512-8. [DOI:10.1289/ehp.6243] [PMID]
27. Pujades-Rodriguez M, Lewis S, McKeever T, Britton J, Venn A. Effect of living close to a main road on asthma, allergy, lung function, and chronic obstructive pulmonary disease. Occup Environ Med. 2009; 66(10):679-84. [DOI:10.1136/oem.2008.043885] [PMID]
28. Insaf TZ, Adeyeye T, Adler C, Wagner V, Proj A, McCauley S, et al. Road traffic density and recurrent asthma emergency department visits among Medicaid enrollees in New York State 2005-2015. Environ Health. 2022; 21(1):73.[DOI:10.1186/s12940-022-00885-5] [PMID]
29. Gonzalez-Barcala FJ, Pertega S, Garnelo L, Castro TP, Sampedro M, Lastres JS, et al. Truck traffic related air pollution associated with asthma symptoms in young boys: A cross-sectional study. Public Health. 2013; 127(3):275-81. [DOI:10.1016/j.puhe.2012.12.028] [PMID]
30. Gehring U, Cyrys J, Sedlmeir G, Brunekreef B, Bellander T, Fischer P, et al. Traffic-related air pollution and respiratory health during the first 2 yrs of life. Eur Respir J. 2002; 19(4):690-8. [DOI:10.1183/09031936.02.01182001] [PMID]
31. Carbajal-Arroyo L, Barraza-Villarreal A, Durand-Pardo R, Moreno-Macías H, Espinoza-Laín R, Chiarella-Ortigosa P, et al. Impact of traffic flow on the asthma prevalence among school children in Lima, Peru. J Asthma. 2007; 44(3):197-202. [DOI:10.1080/02770900701209756] [PMID]
32. Molgat-Seon Y, Peters CM, Sheel AW. Sex-differences in the human respiratory system and their impact on resting pulmonary function and the integrative response to exercise. Curr Opin Physiol. 2018; 6:21-7. [DOI:10.1016/j.cophys.2018.03.007]
33. Venn AJ, Lewis SA, Cooper M, Hubbard R, Britton J. Living near a main road and the risk of wheezing illness in children. Am J Respir Crit Care Med. 2001; 164(12):2177-80. [DOI:10.1164/ajrccm.164.12.2106126] [PMID]
34. Kim JJ, Smorodinsky S, Lipsett M, Singer BC, Hodgson AT, Ostro B. Traffic-related air pollution near busy roads: The East Bay Children’s Respiratory Health Study. Am J Respir Crit Care Med. 2004; 170(5):520-6. [DOI:10.1164/rccm.200403-281OC] [PMID]
35. Lewis SA, Antoniak M, Venn AJ, Davies L, Goodwin A, Salfield N, et al. Secondhand smoke, dietary fruit intake, road traffic exposures, and the prevalence of asthma: A cross-sectional study in young children. Am J Epidemiol. 2005; 161(5):406-11. [DOI:10.1093/aje/kwi059] [PMID]
36. Wilkinson P, Elliott P, Grundy C, Shaddick G, Thakrar B, Walls P, et al. Case-control study of hospital admission with asthma in children aged 5-14 years: Relation with road traffic in north west London. Thorax. 1999; 54(12):1070-4. [DOI:10.1136/thx.54.12.1070] [PMID]
37. Liu W, Cai J, Huang C, Chang J. Residence proximity to traffic-related facilities is associated with childhood asthma and rhinitis in Shandong, China. Environ Int. 2020; 143:105930.[DOI:10.1016/j.envint.2020.105930] [PMID]
38. Commodore S, Ferguson PL, Neelon B, Newman R, Grobman W, Tita A, et al. Reported neighborhood traffic and the odds of asthma/asthma-like symptoms: A cross-sectional analysis of a multi-racial cohort of children. Int J Environ Res Public Health. 2020; 18(1):243. [DOI:10.3390/ijerph18010243] [PMID]