Ambient air pollutant exposure and semen quality in healthy male undergraduates: A prospective refined multi-window panel study.

Global declines in male semen quality highlight the need to define environmental reproductive risks, yet panel studies in homogeneous 18-22-year-old undergraduates remain limited. Reproductive associations between ambient air pollutants and the 15-69-day meiotic spermatogenic stage remain poorly characterized, and no prior studies have stratified this window into biologically distinct sub-stages. We aimed to quantify associations between air pollutant exposure and semen quality, identify sensitive windows, and pinpoint key pollutants. We conducted a 1-year prospective pilot panel study of 32 undergraduates, with four quarterly follow-ups covering the 90-day spermatogenic cycle. Using daily average concentration and cumulative exposure dose metrics, we stratified the 15-69-day meiotic window into three sub-stages, and applied linear mixed-effects models, two-pollutant interaction models, and Bayesian Weighted Quantile Sum (BWQS) regression. Each interquartile range increment in full-cycle O3 exposure was associated with a 15.45% reduction in sperm concentration (95% confidence interval [CI]: -20.95%, -9.58%). O3 exerted consistent adverse associations across all meiotic sub-stages (peak at 31-45 days: -13.30%, 95% CI: -18.09%, -8.24%); NO2 showed the strongest positive association at 15-30 days, whereas CO effects were concentrated in the 31-45-day and 46-69-day windows. Exploratory signals of synergistic additive interactions were detected for gaseous pollutant pairs, and BWQS identified O3 and SO2 as core contributors to adverse mixed-exposure effects. Our findings suggest ambient air pollutants are associated with variations in semen quality in a window- and pollutant-specific manner. Refined 15-69-day stratification enables precise susceptibility mapping, highlighting O3 mitigation to safeguard male reproductive health.