Background The most relevant time of PM10 exposure to affect airway hyperresponsiveness (AHR) and new development of asthma in school‐aged children is unclear. The aims of this study were to investigate the most critical time of PM10 exposure to affect AHR and new diagnosis of asthma from AHR in school‐aged children. Methods Elementary schoolchildren (n = 3570) have been enrolled in a nationwide prospective 4‐year follow‐up survey in Korea from 2005 to 2006. Individual annual PM10 exposure was estimated by using an ordinary kriging method from the prenatal period to 7 years of age. AHR at 7 years was defined by a methacholine PC20 ≤8 mg/mL. Results PM10 exposure during pregnancy and at 1 year of age showed significant effects on AHR (aOR: 1.694, 95% CI: 1.298‐2.209; and aOR: 1.750, 95% CI: 1.343‐2.282, respectively). PM10 exposure during pregnancy was associated with the risk of a new diagnosis of asthma (aOR: 2.056, 95% CI: 1.240‐3.409), with the highest risk in children with AHR at age 7 (aOR: 6.080, 95% CI: 2.150‐17.195). PM10 exposure in the second trimester was associated with the highest risk of a new diagnosis of asthma in children with AHR at age 7 (aOR: 4.136, 95% CI: 1.657‐10.326). Conclusions Prenatal PM10 exposure in the second trimester is associated with an increased risk of a new diagnosis of asthma in school‐aged children with AHR at 7 years. This study suggests that PM10 exposure during a specific trimester in utero may affect the onset of childhood asthma via AHR. Higher PM10* exposure in the second trimester and AHR# at 7 years showed combined effect on the new diagnosis of asthma at school age (aOR 4.136). PM10* exposure in the second trimester may affect the development of childhood asthma via AHR#. *PM10: Particulate matter with an aerodynamic diameter of less than 10 μm. #AHR: Airway hyperresponsiveness.