Objective: To quantify, through meta-analysis techniques, the association between cigarette smoking and sperm density.
Methods: The logarithm of the ratio of mean sperm density for smokers to that for nonsmokers for the studies included in this meta-analysis was regressed against a constant, an indicator of study population source (infertility clinic patients or normal men), minimum number of cigarettes smoked per day among smokers (< 10, > or = 10), exclusion of azoospermic men (yes/no), number of semen specimens analyzed (one versus two), and blinding of laboratory personnel to the smoking status of the study participants (yes/no). Regression analyses were performed both unweighted and weighted inversely by study size. A qualitative and quantitative assessment of the relationship between the numbers of cigarettes smoked per day and sperm density was performed.
Results: Results of the meta-analysis indicate that smokers' sperm density is on average 13% to 17% (95% confidence interval = 8.0, 21.5) lower than that of nonsmokers. No other factors besides cigarette smoking were found to be independent predictors of sperm density. No clear dose-response relationships between the numbers of cigarettes smoked per day and sperm density emerged. Research conducted by the authors supports the findings of the meta-analysis.
Conclusion: Cigarette smoking is associated with lowered sperm density. The inconsistency in the literature with regard to this conclusion appears to be the result of small sample sizes in most studies.