Associations of Compound Exposure to Heat Waves and Ozone During Pregnancy with Preterm Birth: A Birth Cohort Study

Background: Previous studies have associated heat waves and ozone (O3) with preterm birth (PTB), a leading factor of neonatal morbidity and mortality. However, the impact of compound exposure to the two factors has rarely been examined. Methods: We included 77,529 mother–infant pairs from a birth cohort in Foshan, China. Heat wave and O3 exposures were estimated at each participant’s residence using spatiotemporal models. Logistic regression was applied to examine the associations of individual and compound exposure to heat waves and O3 with the risks of PTB. Relative excess odds due to interaction (REOI) were used to evaluate the potential synergistic effects of heat waves and O3. Results: Among the cohort, 5214 PTBs (6.7%) were observed. Heat wave exposure was associated with an increased risk of PTB, whereas the associations for O3 were less evident. By comparison, the compound exposure demonstrated stronger associations, and significant synergistic interactions between heat waves and O3 were observed (REOIs > 0), particularly for exposures in 0–1 days before delivery. Conclusion: We found that heat waves and O3 could interact synergistically to increase the risks of PTB, suggesting that mitigation of both heat wave and O3 exposures may provide greater benefits in PTB prevention.