Association of ambient ozone with time to pregnancy and the modifying effect of ambient temperature: a population-based cohort study

Research on the impact of ozone (O₃) on fecundability has been inconclusive, lacking evidence examining the lagged pattern of ozone exposure and temperature modification. Current studies have predominantly been conducted in North America and Europe, leaving a gap in research from the Asian population. This population-based prospective cohort study involved 594,110 couples in Yunnan Province, China, enrolled in the National Free Preconception Health Examination Project. We calculated time-varying cycle-specific O₃ concentration across 1–12 menstrual cycles before conception, according to each female's menstrual cycle length and residential addresses. Time to pregnancy was used as an outcome indicator. A discrete-time Cox regression model integrated with a distributed lag model (DLM) was employed for analysis. We observed that O₃ exposure was negatively associated with fecundability, with the effect diminishing over time relative to conception. The first menstrual cycle before conception appeared most sensitive to O₃ exposure, showing a 6.2 % decrease (HR: 0.938; 95 % CI: 0.936 to 0.941) in fecundability per one IQR increase of O₃. Combined exposure to O₃ and ambient temperature may exert a synergistic effect. The modifying effect of temperature was most pronounced in the first cycle before conception, with HRs of 0.891 (95 % CI: 0.886 to 0.895), 0.859 (95 % CI: 0.853 to 0.865), and 0.833 (95 % CI: 0.826 to 0.840) under low-, median-, and high-temperature conditions, respectively. Targeted policy interventions are needed to mitigate the dual impact of air pollution and rising temperatures.