Background: Mounting evidence has linked asthma with environmental and meteorological factors, yet any effects due to combined exposure during pregnancy on childhood asthma are unknown.

Objective: To examine the association between asthma and prenatal exposure to both ambient air pollution and temperature, in order to identify any critical time windows for exposure, and their combined effects in asthma development.

Methods: A cohort study of 2,598 preschool children was performed in Changsha, China during the period 2011-2012. A questionnaire was developed to survey each child’s doctor-diagnosed asthma and exposure to home environmental factors. Each mother’s exposure to ambient air pollutants (PM₁₀, SO₂, and NO₂) and temperature (T) was estimated for the three trimesters and for the whole pregnancy. Associations between asthma and exposure to air pollutants and environmental temperature were assessed using logistic regression models in terms of odds ratio (OR) with 95% confidence interval (CI).

Results: Childhood asthma (6.7%) was significantly associated with exposure to industrial- and traffic-related air pollutants (SO₂ and NO₂) in utero particularly during the 1^st and 2^nd trimesters, with ORs (95% CI) = 1.46 (1.13-1.90) and 1.67 (1.24-2.26) respectively for per IQR increase in SO₂ and NO₂ exposure. Childhood asthma was also positively associated with higher temperature exposure during the 1^st trimester, OR (95% CI) = 2.29 (1.09, 4.80) for per IQR increase in T exposure. We also found that exposure to air pollution with low T posed a significantly higher risk compared to exposure at high T during the 1^st trimester. Furthermore, asthma risk of exposure to air pollution was significantly higher among children experiencing high T in the 3^rd trimester. Sensitivity analysis suggested that boys were more susceptible to the interaction effects between environmental temperature and air pollution on asthma risk.

Childhood asthma; Pregnancy; Trimesters; Ambient air pollution; Environmental temperature; Interaction effect