Objectives: To investigate the association between mobile phone exposure and semen parameters.

Design: A nationwide cross-sectional study.

Setting: Andrology laboratories in close proximity to 6 army recruitment centers.

Patients: In total, 2886 men from the general Swiss population, 18-22 years old, were recruited between 2005 and 2018 during military conscription.

Intervention: Participants delivered a semen sample and completed a questionnaire on health and lifestyle, including the number of hours they spent using their mobile phones and where they placed them when not in use.

Main outcome measures: Using logistic and multiple linear regression models, adjusted odds ratios and β coefficients were determined, respectively. The association between mobile phone exposure and semen parameters such as volume, sperm concentration, total sperm count (TSC), motility, and morphology was then evaluated.

Results: A total of 2759 men answered the question concerning their mobile phone use, and 2764 gave details on the position of their mobile phone when not in use. In the adjusted linear model, a higher frequency of mobile phone use (>20 times per day) was associated with a lower sperm concentration (adjusted β: -0.152; 95% confidence interval: -0.316; 0.011) and a lower TSC (adjusted β: -0.271; 95% confidence interval: -0.515; -0.027). In the adjusted logistic regression model, this translates to a 30% and 21% increased risk for sperm concentration and TSC to be below the World Health Organization reference values for fertile men, respectively. This inverse association was found to be more pronounced in the first study period (2005-2007) and gradually decreased with time (2008-2011 and 2012-2018). No consistent associations were observed between mobile phone use and sperm motility or sperm morphology. Keeping a mobile phone in the pants pocket was not found to be associated with lower semen parameters.

Conclusion: This large population-based study suggests that higher mobile phone use is associated with lower sperm concentration and TSC. The observed time trend of decreasing association is in line with the transition to new technologies and the corresponding decrease in mobile phone output power. Prospective studies with improved exposure assessment are needed to confirm whether the observed associations are causal.