Exploring geographical differences in semen quality of young men using spatial dependence analysis.

study questionAre semen quality parameters randomly distributed across space, or do they exhibit spatial patterns that may reflect underlying environmental influences? SUMMARY ANSWER: Semen quality parameters are not randomly distributed in Switzerland but exhibit distinct spatial clustering, with identifiable hotspots indicating areas of comparatively higher values and coldspots indicating areas of comparatively lower values. WHAT IS KNOWN ALREADY: Regional differences in semen quality across Europe suggest environmental influences, but most studies are limited to urban areas with small samples and low geographic precision, leaving gaps in understanding precise geographical patterns at a national level. STUDY DESIGN, SIZE, DURATION: This is a cross-sectional study, including 2677 men aged 18-22 from the general Swiss population who were recruited between 2005 and 2018 during military conscription. PARTICIPANTS/MATERIALS, SETTING,

methodsEach participant provided a semen sample and completed a questionnaire on health and lifestyle, including demographic details on their residence. Georeferenced data were used to compute the level of spatial dependence using the local Getis-Ord Gi* statistic with a 25 km spatial lag to identify clusters of high versus low semen parameter values across Switzerland. The parameters assessed included semen volume, sperm concentration, total sperm count, and sperm morphology. Participants belonging to the same Getis-Ord Gi* class for each semen parameter formed a persistent cluster. To assess differences in environmental exposure between high- and low-persistent clusters, land use data within a 500 m radius of each participant's place of residence were extracted from national registries for the periods of 1992-1997 and 2004-2009. MAIN RESULTS AND THE ROLE OF CHANCE: Semen quality parameters are not randomly distributed across Switzerland, with distinct local clusters of high values (hotspots) and low values (coldspots) identified. A persistent low-value cluster was observed in central western Switzerland. Analysis of national land use data revealed significant differences in agricultural land types between clusters: fodder and field crops accounted for 52.0% of the land use in the coldspot, compared to 28.3% in the hotspot and 30.1% in the non-significant cluster. Additionally, the mean proportion of built areas (housing and infrastructure) near participants' residences was significantly lower in the coldspot (32.6%) compared to the hotspots (53.3%). LIMITATIONS, REASONS FOR CAUTION: Although the study population is nationwide and broadly representative, the sample size used for individual-level spatial analyses limits statistical power. WIDER IMPLICATIONS OF THE

findingsTo our knowledge, this is the first study to apply spatial dependence methods to investigate semen quality parameters using individual-level data at a national scale. This approach can be extended to larger datasets to further explore the relationship between environmental factors and reproductive health. The insights gained may inform public health policy by supporting more targeted environmental monitoring and preventive strategies. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by funding from the Swiss Centre for Applied Human Toxicology-SCAHT to R.R. and to S.N. and from the Département de l'Instruction Publique (DIP) of the state of Geneva to S.N. The authors have no competing interests to declare. TRIAL REGISTRATION NUMBER: N/A.