Cigarette Smoking and Semen Quality: A New Meta-analysis Examining the Effect of the 2010 World Health Organization Laboratory Methods for the Examination of Human Semen

Objective: Approximately 37% of men of reproductive age smoke cigarettes, with Europe having the highest tobacco use among all the World Health Organization (WHO) regions. Toxins from tobacco smoking can potentially affect sperm development and function, with a negative effect on semen parameters. Given the high prevalence of smoking and recent changes in the WHO laboratory methods for the examination of human semen, the role of this exposure in face of new WHO methods needs to be clarified.

Evidence acquisition: We conducted a systematic review, followed by a meta-analysis, to determine whether cigarette smoking affects human semen parameters. PubMed, Saint Joseph's University Discover, and Google Scholar were used to identify relevant studies published after release of the latest WHO methods for laboratory evaluation of human semen. Participants were from fertility/urologic clinics and andrology laboratories. The outcome measures were semen volume, sperm concentration, motility, and morphology, the parameters usually used in clinical settings to assess fertility.

Evidence synthesis: Twenty studies with 5865 participants were included in the meta-analysis. Exposure to cigarette smoking was associated with reduced sperm count (mean difference [MD]: -9.72×10⁶/ml; 95% confidence interval [CI], -13.32 to -6.12), motility (MD: -3.48%; 95% CI, -5.53 to -1.44), and morphology (MD: -1.37%; 95% CI, -2.63 to -0.11). Subgroup analyses indicated that effect size was higher in infertile men than in the general population and in moderate/heavy smokers than in mild smokers. The overall effect size on semen volume, sperm count, and motility remained similar when 2010 and earlier WHO manuals were used for semen analysis but was lower with regard to sperm morphology.

Conclusions: Our results suggest that cigarette smoking has an overall negative effect on semen parameters. The latest WHO laboratory methods for the examination of human semen had a minimal impact on the magnitude of effect size, thus confirming the observed negative effect of smoking on conventional semen parameters.

Patient summary: A new systematic review and meta-analysis comprising 5865 men shows that cigarette smoking is associated with reduced sperm count and motility. Deterioration of semen quality is more pronounced in moderate and heavy smokers.