Comparison of childhood asthma incidence in 3 neighbouring cities in southwestern Ontario: a 25-year longitudinal cohort study.

Background: Air pollution is a known trigger for exacerbations among individuals with asthma, but its role in the development of new-onset asthma is unclear. We compared the rate of new asthma cases in Sarnia, a city with high pollution levels, with the rates in 2 neighbouring regions in southwestern Ontario, Canada.

Methods: Using a population-based birth cohort design and linked health administrative data, we compared the hazard of incident asthma among children 0 to 10 years of age between those born in Lambton (Sarnia) and those born in Windsor and London-Middlesex, for the period Apr. 1, 1993, to Mar. 31, 2009. We used Cox proportional hazards models to adjust for year of birth and exposure to air pollutants (nitrogen dioxide, sulphur dioxide [SO], ozone and small particulate matter [PM]), as well as maternal, geographic and socioeconomic factors.

Results: Among 114 427 children, the highest incidence of asthma was in Lambton, followed by Windsor and London-Middlesex (30.3, 24.4 and 19.8 per 1000 person-years, respectively; < 0.001). Relative to Lambton, the hazard of asthma, adjusted for socioeconomic and perinatal factors, was lower in Windsor (hazard ratio [HR] 0.72, 95% confidence interval [CI] 0.67-0.77) and London-Middlesex (HR 0.65, 95% CI 0.61-0.69). Inclusion of air pollutants attenuated this relative difference in both Windsor (HR 0.79, 95% CI 0.62-1.01) and London-Middlesex (HR 0.89, 95% CI 0.64-1.24).

Interpretation: We identified a higher incidence of asthma among children born in Lambton (Sarnia) relative to 2 other regions in southwestern Ontario. Higher levels of air pollution (particularly SO and PM) in this region, as experienced by children in their first year of life, may be contributory.