| Summary: | Studies on the correlation of long-term PM<sub>2.5</sub> exposure with childhood-onset asthma are limited to western countries. We aimed to study the association between long-term PM<sub>2.5</sub> exposure and childhood-onset asthma in South Korea, which has higher ambient PM<sub>2.5</sub> levels than western countries. We constructed a retrospective cohort of children aged 6–14 years living in seven metropolitan cities using the National Health Insurance service in South Korea from 2011 to 2016. Children who made a hospital visit with asthma from 2008 to 2010 were excluded. A child was diagnosed with asthma incidence if he or she visited the hospital three times or more with a primary diagnostic code of asthma. A time-varying Cox regression model was constructed to investigate the association of long-term district-level PM<sub>2.5</sub> exposure with asthma incidence. Of the 1,425,638 children evaluated, 52,133 showed asthma incidence, with an incidence rate of 6.9 cases/1000 person-years. A 10 µg/m<sup>3</sup> increase in the 48-month moving average PM<sub>2.5</sub> exposure was associated with an elevated risk of asthma incidence, with a hazard ratio of 1.075 (95% confidence interval: 1.024–1.126), and this association was robust for different PM<sub>2.5</sub> exposure levels (12-, 36-, and 60-month moving average). In this study, long-term exposure to PM<sub>2.5</sub> was associated with asthma incidence in school-aged children in South Korea. Policies to reduce environmental PM<sub>2.5</sub> levels and protect children from PM<sub>2.5</sub> are necessary to prevent childhood-onset asthma.
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