Long-term exposure to air pollution and risk of adult-onset asthma

Air pollution is a risk factor for adult-onset asthma, yet evidence on black carbon (BC) remains limited and inconsistent. To assess the association between long-term exposure to air pollution, focusing on BC, and asthma incidence. We followed 28,731 female nurses from the Danish Nurse Cohort (1993/1999–2018) for first-ever hospital contacts for asthma. Annual mean concentrations of BC, fine particulate matter (PM_2.5), and nitrogen dioxide (NO₂) were modeled via the Danish Air Geographic Information System and linked to time-varying residential addresses. Time-varying Cox models were used to assess the association between 3-year running mean air pollution and incident asthma, adjusted for individual- and area-level covariates. Stratified analyses were conducted to examine effect heterogeneity by age, calendar time, smoking, obesity, and comorbidity (cardiovascular disease [CVD], chronic obstructive pulmonary disease [COPD], and diabetes). Over a mean follow-up of 23 years, 633 nurses developed asthma. Exposure to all three air pollutants was associated with increased asthma risk, with hazard ratios of 1.06 (95% confidence interval, 1.00–1.13) per 0.3 μg/m³ BC, 1.12 (0.95–1.32) per 2.5 μg/m³ PM_2.5, and 1.15 (1.04–1.28) per 8.0 μg/m³ NO₂. The associations with BC were consistent across all subgroups, while associations with NO₂ and PM_2.5 were stronger in recent years and among individuals with COPD or CVD. Long-term exposure to air pollution, particularly BC and NO₂, is linked to an increased risk of adult-onset asthma. Our findings suggest that PM_2.5 toxicity may have escalated in recent years and that individuals with COPD or CVD are especially vulnerable, underscoring the need for targeted public health interventions.