Early-life exposure to ambient air pollution and asthma in the children of the Danish National Birth Cohort – does asthma definition and adjustment for cohort-specific covariates matter?

BACKGROUND AND AIM Ambient air pollution exposure has been associated with asthma, but evidence from children is mixed. We examined associations between early-life exposure to air pollution and childhood asthma. METHODS We included singletons of the Danish National Birth Cohort born in 1997-2003 to women with complete address, registry-based, self-reported data on health and asthma risk factors (e.g. smoking, breastfeeding, pets, mold) collected from pregnancy and follow-up questionnaires until age 11. We defined asthma incidence from hospital admission, emergency room or outpatient contact using ICD-10 codes and prevalence of doctor-diagnosed asthma from parental recalls at age 7. RESULTS Out of the 22,084 children included, 1,475 children had asthma according to the ICD-10 code, of which 51% had asthma according to parental recall of a doctor-diagnosis, whilst 35% of children with a parental recall of doctor-diagnosis also had an ICD-10 code for asthma. The prenatal mean (SD) exposure to PM2.5 and NO2 at the home addresses was 11.5 (1.4) and 18.6 (7.3) µg/m3, respectively. A 5 µg/m3 increase in prenatal exposure to PM2.5 was associated with an HR of 1.05 (95%CI: 0.81, 1.36) for ICD-10-defined asthma and an OR of 0.85 (0.70, 1.04) for doctor-diagnosed asthma in the adjusted models. The corresponding effect estimates associated with a 10 µg/m3 increase in prenatal exposure to NO2 were 1.03 (0.92, 1.15) and 0.95 (0.87, 1.03), respectively. All point estimates were higher among children with the same address at birth as at age 11 when data on home characteristics was collected than movers. Results from models adjusted with register-based covariates were similar to models above that included cohort-specific covariates. CONCLUSIONS These findings suggest that the associations between ambient air pollution and asthma in children depend more on the asthma definition than further adjustment for cohort-specific variables in settings like Denmark with rich registry data.