Air pollution drives concentrated clinical severity of respiratory illness in early childhood

Exposure to fine particulate matter (PM2.5) increases paediatric respiratory morbidity, but its impact on the intensity and economic burden of care in early childhood remains unclear. Here we link nationwide health records for 3,210,381 South Korean children (99,261,945 child-month observations, 2015-2019) and exploit quasi-random variation in PM2.5 induced by thermal inversions to estimate its causal effects on respiratory care and expenditure. A 1-standard-deviation increase in monthly PM2.5 (7.23 micrograms per cubic metre) raises respiratory visits broadly, peaking at age 1 (0.29 additional visits per child-month) and remaining significant through age 6. In contrast, expenditures rise disproportionately among children under 3, increasing by 46.7%, 43.0%, and 35.4% at ages 0, 1, and 2 and becoming indistinguishable from zero beyond age 4. This shift towards higher-intensity emergency care translates into a 32.2% increase in annual respiratory spending among children under 5. Air pollution thus imposes a dual burden, amplifying both the frequency and severity of illness among the youngest infants.