||BACKGROUND: The synergistic effect of allergens and air pollutants on the risk of allergic diseases is unclear. OBJECTIVE: To evaluate the joint effect of outdoor pollutants and indoor allergens on the risk of allergic diseases. METHODS: We enrolled 2661 kindergarten children from the CEAS cohort. Data on allergic diseases and environmental exposure were collected. Skin prick tests were performed. Individual exposure to air pollution was estimated using a geographic information system with the mean concentration of air pollutants. Multiple logistic regression analysis was performed to estimate the association between air pollutants, allergen exposure and the risk of allergic diseases with adjustments for potential confounders. RESULTS: Overall, 12.6% of the children had asthma, 30.0% had allergic rhinitis (AR), and 14.4% had atopic dermatitis (AD). Mite sensitization significantly increased the risk of AD, AR, and asthma (OR (95%CI) 2.15 (1.53-3.03), 1.94 (1.46-2.58), and 2.31 (1.63-3.29), respectively). Exposure to PM10, PM2.5, CO, and O3 was associated with asthma (OR (95% CI) 1.39 (1.03-1.87), 1.45 (1.07-1.97), 1.36 (1.01-1.83), and 0.68 (0.51-0.92), respectively). PM2.5 may have increased the risk of AR (OR (95% CI) 1.54 (1.03-2.32). Mite sensitization showed a synergistic effect with PM2.5 on the development of asthma (p<0.001). Moreover, mite allergens may modify the effect of air pollutants on allergic diseases. CONCLUSION: Dust mites and PM2.5 play an important role on the risk of asthma and AR. Exposure to PM2.5 and mite allergens had a synergistic effect on the development of asthma. Avoiding co-exposure to allergens and air pollutants is important.