Cumulative Vulnerability to Particulate Matter and Incident Asthma: Joint Effects of Urbanicity, Poverty, and Age in a Nationwide Cohort

Background Long‐term exposure to ambient fine and coarse particulate matter (PM 2.5 and PM 10 ) is a major risk factor for asthma development. However, how urbanicity, poverty, and age jointly modify this association and contribute to disparities in asthma incidence remains unclear. Methods We conducted a nationwide retrospective cohort study of 8,252,498 adults in South Korea using the Korean National Health Insurance Service database. Health records were linked to 1km 2 resolution estimates of long‐term PM 2.5 and PM 10 exposure. Asthma incidence was assessed using Cox proportional hazards models adjusted for demographic, clinical, and environmental covariates. Stratified analyses were performed by age, Medical Aid status, and residential area. Results Long‐term exposure to PM 2.5 and PM 10 was significantly associated with an increased risk of new‐onset asthma. Each 10 μg/m 3 increase in PM 2.5 was associated with a hazard ratio (HR) of 1.33 (95% confidence interval [CI], 1.32–1.34), while a comparable increase in PM 10 was associated with an HR of 1.38 (95% CI, 1.37–1.38). Stratified analyses demonstrated progressive risk amplification across age, Medical Aid status, and residential area, with HRs ranging from 1.16 (95% CI, 1.14–1.18) among younger adults without Medical Aid living in rural areas to 1.80 (95% CI, 1.66–1.94) among older adults and receiving Medical Aid residing in urban areas ( p for interaction < 0.001). Conclusion Long‐term PM exposure significantly increased asthma risk, with the greatest disparities observed among socioeconomically disadvantaged older adults residing in urban areas. Urgent, targeted strategies are needed to mitigate the disproportionate asthma burden attributable to these inequities.