Linear and non-linear relationships between sulfur dioxide and semen quality: A longitudinal study in Anhui, China

Existing evidence indicates that ambient air pollutants pose a threat to human semen quality; however, these findings are sparse and controversial. Besides, their non-linear dose-response relationship has not yet been well investigated. This study aimed to explore the linear and non-linear associations of gaseous air pollutants exposure with semen quality based on a large longitudinal cohort. A total of 15,112 males (with 28,267 semen tests) from the Anhui prospective assisted reproduction cohort were analyzed. Individual air pollutants exposure before semen tests in four exposure windows (i.e., 0-9, 10-14, 70-90, and 0-90 days) were estimated by inverse distance weighting interpolation. Linear mixed-effects models, cubic spline analysis and piecewise regression were used to test the potential linear and non-linear dose-response relationships. Ambient SO₂ exposure was negatively associated with all semen quality parameters (all p values < 0.05), except for the progressive motility in the 0-90 and 70-90 days exposure windows. There were 'J' or 'U' shaped dose-response relationships of ambient SO₂ exposure with total sperm count, progressive motility, total motility, progressively motile sperm count, and total motile sperm count (p values for non-linearity < 0.05), but not sperm concentration. Piecewise regression analysis also indicated a negative association of SO₂ exposure with semen quality only when SO₂ exposure was below the cut-off points identified by cubic spline analyses, which were all smaller than 40 μg/m³, the 2021 updated WHO air quality guideline level for SO₂ exposure. Overall, we found that SO₂ exposure was negatively associated with semen quality. Ambient SO₂ exposure could reach the maximum hazardous dose even below the WHO air quality guideline level for SO₂ exposure, suggesting a refinement to the current guideline.