Exposure to air pollution in early childhood and the association with Attention-Deficit Hyperactivity Disorder

Background: Exposure to air pollution in early life has been linked to cognitive deficits and adverse neurodevelopmental effects. However, studies examining associations between air pollutants and Attention-Deficit/Hyperactivity Disorder (ADHD) have had conflicting findings. Methods: Individuals born in Denmark 1992–2007 (n = 809,654) were followed for the development of ADHD from 1997 to 2013. Data on daily concentrations of nitrogen dioxide (NO ₂) and fine particulate matter (PM _2.5) from air-modeling data at a 1 km × 1 km resolution at residences within the first five years of life, was linked with population-based data from the Danish national registers, including data on clinical diagnoses of ADHD. We estimated incidence rate ratios (IRRs) with 95% confidence intervals (CI) for ADHD, according to increases in exposures, adjusting for age, year, sex, and parental education and income. Results: Exposure to NO ₂ and PM _2.5 during early life was associated with a significantly increased risk of ADHD: IRR of 1.38 (Cl: 1.35 to 1.42) per 10 μg/m ³ increase in NO ₂ and an IRR of 1.51 (Cl: 1.41 to 1.62) per 5 μg/m ³ increase in PM _2.5. In two-pollutant models, the association between NO ₂ and ADHD did not change (IRR 1.35; 95% CI: 1.31 to 1.39), while the association with PM _2.5 was substantially attenuated (IRR 1.07; 95% CI: 0.98 to 1.16), although in stratified models an elevated association with PM _2.5 was found in the lowest quintile of NO ₂ exposure. Conclusions: In this large nationwide prospective cohort study, residential air pollution exposure, specifically NO ₂, during early childhood was associated with the development of ADHD, even when adjusted for parental level of income and education.