The influence of the urban forest on the association between fine particulate air pollution and onset of childhood asthma

Abstract

BACKGROUND AND AIM: Fine particles (PM2.5) have been associated with childhood asthma onset. Trees have been suggested to reduce PM2.5 levels in urban settings. However, the influence of the urban forest on asthma onset is unclear as trees can reduce air pollution but also emit pollen and biogenic organic compounds. We aim to characterize the influence of the residential tree canopy on the association between PM2.5 and asthma onset in children. METHODS: We used an open cohort of all children born in Montreal (Canada) between 2000 and 2015 created from linked administrative databases; children were followed up to 12 years. New asthma cases were defined with a validated algorithm. Yearly PM2.5 levels estimated from satellite images and a chemical transport model were attributed to children during their follow up through linkage with their residential six-digit postal code. Crown areas (i.e. canopy) of all trees in buffers around the postal codes of the children for the leafy and the pollen seasons were estimated based on LiDAR point cloud data. Cox models for PM2.5 and asthma onset, adjusted or not for crown areas and with an interaction term with PM2.5 levels were developed. Age was the time axis of the models which were stratified for sex, year, health region, and adjusted for material deprivation. RESULTS:352,966 children were followed for a total of 1,701,797 person-years; 30,752 children developed asthma. Mean PM2.5 exposure was 9.01 ± 1.119 (IQR:0.733) µg/m3. PM2.5 levels were not correlated with the deciduous tree canopy. The association between PM2.5 and asthma onset was 1.03 (95%CI: 1.02-1.04) per IQR; it was 1.04 per IQR (95%CI: 0.96-1.03) with the tree canopy included; no effect modification was detected. CONCLUSIONS:Results suggest that the residential urban forest does not influence positively nor negatively, the association between regional PM2.5 and childhood asthma onset. KEYWORDS: Green space, Asthma, Particulate matter, Children's environmental health