Traffic pollution contains a number of chemicals which could cause irritation and potential damage to the lungs. In particular, the acidic gas nitrogen dioxide has been shown to decrease ciliary function and increase the response of asthmatics to allergens; and inhalation of diesel exhaust particles has been shown to cause immune changes in mice.
Studying the effects of exposure to traffic pollution on the risk of developing asthma is challenging, since it can be difficult to quantify the lifetime exposure of an individual to traffic fumes. A number of modelling approaches can be used including taking into account high pollution versus low pollution areas, the distance to roads of a subject’s home, traffic counts in the local area and vehicle miles travelled. However, modelling exposure based on geographical location is not ideal since it can be difficult to separate the effects of exposure to traffic from the effects of poverty.
Conflicting Evidence Collected
The potential link between asthma and road traffic pollution has been extensively studied, but research has yielded conflicting reports. For example, a British study based on routine hospital admissions found no association between hospital admissions for asthma and exposure to traffic pollution. The study studied asthma-related admissions in comparison to a control group consisting of other admissions. Scientists estimated the exposure of each patient to traffic pollution by calculating the distance of the nearest busy main road from their house, and by estimating traffic volume within 150 meters of the patient’s home using Geographical Information System techniques.
When investigating the potential for traffic pollution to cause asthma, it is important to consider a number of factors including the severity of the condition. It is quite possible that traffic pollution exposure could cause an increase in the number of children suffering from mild asthma without significantly affecting asthma hospital admissions: the focus for this particular study.
Nitrogen Dioxide Affects Respiratory Health
A Californian study published in 2005 examined possible associations between exposure to traffic pollution and childhood asthma. Traffic pollution exposure was measured by taking outdoor nitrogen dioxide readings outside each subject’s home in the summer and winter, together with determining the distance to the nearest freeway and the traffic volume within 150 meters. The study found children exposed to higher levels of traffic pollution were significantly more likely to suffer from asthma.
Early Exposure May be Significant
Scientists in France conducted a case-control study in five urban areas between 1998 and 2000. The researchers calculated lifelong exposure to traffic pollution of 217 pairs of matched asthma cases and controls aged between 4 and 14 years. Having adjusted the results to allow for confounding factors such as exposure to tobacco smoke, the researchers found there was no association between lifelong exposure to traffic and asthma.
However, the study found there was a significant association between exposure to traffic in the first three years of life and the development of asthma. This suggests the very early years of life may be critical in the development of sensitivity to respiratory conditions, and that limiting exposure to air pollution during these years may help prevent the onset of asthma.
Does Traffic Pollution Cause Asthma?
It is well established that traffic pollution can exacerbate the symptoms of children already suffering from asthma. However, evidence that air pollution can actually cause the condition is less compelling. Further studies and meta-analyses are required to confirm any possible link between exposure to traffic pollution and the onset of asthma and other respiratory conditions.
 EC Ferguson, R Maheswaran, M Daly “Road-Traffic Pollution and Asthma - Using Modelled Exposure Assessment for Routine Public Health Surveillance” International Journal of Health Geographics 3:24 (2004)
 P Wilkinson, P Elliott, C Grundy “Case-Control Study of Hospital Admission With Asthma in Children Aged 5-14 Years: Relation With Road Traffic in North West London” Thorax 54:1070-1074 (1999)
 WJ Gauderman, E Avol, F Lurmann “Childhood Asthma and Exposure to Traffic and Nitrogen Dioxide” Epidemiology 16:737-743 (2005)
 D Zmirou, S Gauvin, I Pin “Traffic Related Air Pollution and Incidence of Childhood Asthma: Results of the Vesta Case-Control Study” Journal of Epidemiology and Community Health 58:18-23 (2004)
 MI Gilmour, MS Jaakkola, SJ London “How Exposure to Environmental Tobacco Smoke, Outdoor Air Pollutants, and Increased Pollen Burdens Influences the Incidence of Asthma” Environmental Health Perspectives 114:627-633 (2006)