Background: For decades, environmental pollutants have been known to play a major role in occurrence of atopic dermatitis (AD). However, no population-based cohort study had assessed the relationship between outdoor air pollutants and aggravation of AD. Objectives: The aim of this study is to investigate the relationships between outdoor air pollutants and incident AD cases using National Health Insurance Service-National Sample Cohort (NHIS-NSC) database. Methods: We included 209,168 subjects of general population not diagnosed with AD from NHIS-NSC from 2008 to 2013. Hourly air pollutant data (particulate and gaseous) and climate data were collected. Results: During 1,030,324 person·years, incident AD was observed in 3,203 subjects (310.9/100,000 person·year). There was significantly association between incident AD and average fine particulate matter (PM2.5) concentration for 1- [HR= 1.113 (95% CI 1.040-1.191) for 10 μg/m3 increments], 2- [HR= 1.118 (95% CI 1.032-1.212) for 10 μ g/m3 increments] and 4-weeks [HR= 1.192 (95% CI 1.084-1.310) for 10 μg/m3 increments] after adjusting for other pollutants and climate variables. Conclusion: This study demonstrated that exposure to PM2.5 is an independent risk factor for developing AD.