title: Cigarette smoking and sperm density: a meta-analysis.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill 27599-7400.
authors.forename: M F
authors.initials: MF
authors.lastname: Vine
authors.forename: B H
authors.initials: BH
authors.lastname: Margolin
authors.forename: H I
authors.initials: HI
authors.lastname: Morrison
authors.forename: B S
authors.initials: BS
authors.lastname: Hulka
mesh_terms: D000328:Adult; D006801:Humans; D008297:Male; D012044:Regression Analysis; D012907:Smoking / Q000009:adverse effects*; D013076:Sperm Count