Exposure to ultrafine particles and the incidence of asthma in children: A population-based open cohort study in Montreal, Canada

Abstract

BACKGROUND AND AIM: Asthma is the most prevalent chronic respiratory disease in children and represents an important burden on health care systems. The association between air pollution and asthma is well established, however the role of ultrafine particles (UFPs) remains unclear. We evaluated the association between long-term exposure to UFPs and childhood-onset asthma in a population-based birth cohort in Montreal, Canada. METHODS: The cohort was constructed from the Quebec Integrated Chronic Disease Surveillance System and included all children born in Quebec between 2000 and 2015 who were resident of Montreal. Case definitions consisted of at least two physician claims with a diagnosis of asthma within a two-year period or one hospital discharge with a primary or secondary diagnosis of asthma. Annual concentrations of UFPs and of nitrogen dioxide (NO2) were estimated from land use regression models developed for Montreal and assigned at participants’ residence over the follow-up using their residential postal codes. The association between UFPs and childhood asthma onset was estimated with a Cox proportional hazards model stratified for sex and adjusted for age, neighbourhood material and social deprivation, calendar year and co-exposure to NO2. RESULTS:The cohort included 352,966 children, with 30,825 incident asthma cases. The annual average concentrations were 24,427 particles/cm3 for UFPs and 18.55 ppb for NO2. In the single pollutant model, the adjusted hazard ratio (HR) per interquartile range (IQR) increase in UFPs was 0.990 (95% CI: 0.983, 0.997). The two-pollutant model showed no association for UFPs (HR per IQR = 0.986; 95% CI: 0.979, 0.997) but a positive association for NO2 (HR per IQR = 1.038; 95% CI: 1.013, 1.051). CONCLUSIONS:In this population-based birth cohort, childhood asthma onset was not associated with UFPs but positively associated with NO2 estimated at participants’ residential location. KEYWORDS: Air pollution, Particulate matter, Asthma, Respiratory outcomes, Children's environmental health