Identifying PM2.5 flexion points for environmental diseases in susceptible and vulnerable groups: A nationwide cohort in South Korea

To evaluate the health impacts of PM 2.5 exposure on sensitive and vulnerable populations in South Korea and to identify flexion points—concentration levels at which the risk of environmental diseases increases significantly. A retrospective cohort study using nationwide health insurance claims data linked with environmental monitoring data. Statistical analysis included Cox proportional hazards regression and piecewise linear regression to estimate disease risks and identify flexion points. South Korea, using data from the National Health Insurance Service–National Sample Cohort (NHIS-NSC) and national air quality monitoring stations. A total of 1,031,517 individuals aged 0–79 years who were covered by the national health insurance system between 2017 and 2019 and had no prior diagnosis of the target diseases during the 2016 washout period. Primary outcomes were the incidence and exacerbation of four environmental diseases: chronic obstructive pulmonary disease (COPD), asthma, stroke, and heart failure. Exacerbation was defined as hospitalization or emergency department visits due to these diseases. Higher PM 2.5 exposure was significantly associated with increased incidence and exacerbation of all four diseases. Flexion points were identified at 30 µg/m³ for incidence and between 24 and 26 µg/m³ for exacerbation. Subgroup analyses showed lower flexion points among elderly participants. The findings suggest that current PM 2.5 standards in South Korea may not sufficiently protect vulnerable populations. Operational flexion points identified in this study may serve as evidence-based thresholds to guide targeted public health interventions and air quality regulations.