Associations of Early-Life Exposure to Submicron Particulate Matter With Childhood Asthma and Wheeze in China

IMPORTANCE: Exposure to particulate matter (PM) has been associated with childhood asthma and wheeze. However, the specific associations between asthma and PM with an aerodynamic equivalent diameter of 1 μm or less (ie, PM(1)), which is a contributor to PM(2.5) and potentially more toxic than PM(2.5), remain unclear. OBJECTIVE: To investigate the association of early-life (prenatal and first year) exposure to size-segregated PM, including PM(1), PM(1-2.5), PM(2.5), PM(2.5-10), and PM(10), with childhood asthma and wheeze. DESIGN, SETTING, AND PARTICIPANTS: This cross-sectional study was based on a questionnaire administered between June 2019 and June 2020 to caregivers of children aged 3 to 6 years in 7 Chinese cities (Wuhan, Changsha, Taiyuan, Nanjing, Shanghai, Chongqing, and Urumqi) as the second phase of the China, Children, Homes, Health study. EXPOSURES: Exposure to PM(1), PM(1-2.5), PM(2.5), PM(2.5-10), and PM(10) during the prenatal period and first year of life. MAIN OUTCOMES AND MEASURES: The main outcomes were caregiver-reported childhood asthma and wheeze. A machine learning–based space-time model was applied to estimate early-life PM(1), PM(2.5), and PM(10) exposure at 1 × 1-km resolution. Concentrations of PM(1-2.5) and PM(2.5-10) were calculated by subtracting PM(1) from PM(2.5) and PM(2.5) from PM(10), respectively. Multilevel (city and child) logistic regression models were applied to assess associations. RESULTS: Of 29 418 children whose caregivers completed the survey (15 320 boys [52.1%]; mean [SD] age, 4.9 [0.9] years), 2524 (8.6%) ever had wheeze and 1161 (3.9%) were diagnosed with asthma. Among all children, 18 514 (62.9%) were breastfed for more than 6 months and 787 (2.7%) had parental history of atopy. A total of 22 250 children (75.6%) had a mother with an educational level of university or above. Of the 25 422 children for whom information about cigarette smoking exposure was collected, 576 (2.3%) had a mother who was a current or former smoker during pregnancy and 7525 (29.7%) had passive household cigarette smoke exposure in early life. Early-life PM(1), PM(2.5), and PM(10) exposure were significantly associated with increased risk of childhood asthma, with higher estimates per 10-μg/m(3) increase in PM(1) (OR, 1.55; 95% CI, 1.27-1.89) than in PM(2.5) (OR, 1.14; 95% CI, 1.03-1.26) and PM(10) (OR, 1.11; 95% CI, 1.02-1.20). No association was observed between asthma and PM(1-2.5) exposure, suggesting that PM(1) rather than PM(1-2.5) contributed to the association between PM(2.5) and childhood asthma. There were significant associations between childhood wheeze and early-life PM(1) exposure (OR, 1.23; 95% CI, 1.07-1.41) and PM(2.5) exposure (OR, 1.08; 95% CI, 1.01-1.16) per 10-μg/m(3) increase in PM(1) and PM(2.5), respectively. CONCLUSIONS AND RELEVANCE: In this cross-sectional study, higher estimates were observed for the association between PM with smaller particles, such as PM(1), vs PM with larger particles and childhood asthma. The results suggest that the association between PM(2.5) and childhood asthma was mainly attributable to PM(1).