Long-term exposure to ambient air pollution from multiple sources and associations with asthma incidence in children born in Denmark 1998-2016

BACKGROUND AND AIM Asthma has been associated with ambient air pollution exposure, mainly in the urban environment. We examined the nationwide associations between early-life exposure and childhood asthma incidence. METHODS Asthma incidence was defined as the first record of a ICD-code for asthma collected from hospital admission, emergency room or outpatient contact among all liveborn singletons born in Denmark between 1998 and 2016. We linked registry data with monthly mean concentrations of particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2), nitrogen oxides (NOX), elemental carbon (EC) and organic carbon (OC), sulphur dioxide (SO2), ozone (O3), sulphate (SO42-), nitrate (NO3-), ammonium (NH4+), secondary organic aerosols (SOA) and sea salt modelled at the home addresses. Associations were estimated with Cox proportional hazard models adjusted for potential confounders using fixed prenatal and time-varying postnatal exposure means. RESULTS Out of the 1,060,154 children included, 6.1% had asthma during the mean follow-up of 8.8 years. The risk of asthma increased with increasing prenatal exposure to all pollutants in adjusted models except for O3 and sea salt. Increased risks were also evident for postnatal exposure to most pollutants and for asthma after age 4. The hazard ratio associated with an interquartile range increase of 2.4 and 8.7 μg/m3 in prenatal exposure was 1.06 (95% CI 1.04-1.08) for PM2.5 and 1.04 (95% CI 1.02-1.05) for NO2. This association with PM2.5 was stable after adjustment for NO2, while it attenuated for NO2 to 1.01 (95% CI 0.99-1.03) after adjustment for PM2.5. OC, for which biomass burning is an important source, appeared to increase risk beyond its contribution to total PM2.5. For a 0.5-μg/m3 increase in the HR was 1.08 (95% CI 1.06-1.10) irrespective of adjustment for PM2.5. CONCLUSIONS The findings strengthen the evidence that early-life exposure to ambient air pollution from multiple sources contributes to asthma development.