Role of traffic-related air pollution in asthma: Is this just a matter of big cities?

Background

Traffic-related air pollution has been linked to asthma in large cities. It is however unclear whether such associations exist elsewhere. Herein, we examined relationships between traffic-related air pollution and asthma in urban, semi-urban and rural areas.

Methods

Data were from a representative population-based sample of 2000 adults living in Eastern Townships (Quebec, Canada, 2011). This region is divided into 66 local communities and contains a mix of urban, semi-urban, and rural areas. It includes 315,000 individuals from which half live in one central city named Sherbrooke (Quebec’s 6th largest city). Participants were asked to complete a telephone survey (response rate=63.5%). Among investigated issues, there were: 1) diagnosed asthma and 2) asthma symptoms. Using postal codes, community-level road density (km of roads per km² of land area), a proxy for traffic-related air pollution, was computed for each participant. Chi-square tests, logistic regressions, and multilevel analyses were run.

 Results

 A positive gradient was observed between road density and diagnosed asthma (p=0.02), but not with asthma symptoms (p=0.28). Multivariate models were similar for both variables. Independent of smoking and socioeconomic status, road density was associated with diagnosed asthma (Q5/Q1 odds ratio[OR]=1.81; 95% confidence intervals[CI]=1.09-3.02) and asthma symptoms (OR=2.10;95%CI=1.11-3.95). However, when controlling for being a tenant, the associations were attenuated and became non-significant (diagnosed asthma: OR=1.66;95%CI=0.97-2.89; asthma symptoms: OR=1.93;95%CI=0.99-3.79).

 Conclusion

Previous literature linking traffic-related air pollution to asthma may apply to smaller cities. However, potential confounders (e.g. being a tenant, housing quality) have to be considered given their relation with both outcomes.