Cigarette smoking and sperm density: a meta-analysis**Supported through Cooperative Agreement RR13485–01 between Health Effects Research Laboratory, United States Environmental Protection Agency and the University of North Carolina, Department of Epidemiology. Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and, therefore, does not necessarily reflect the view of the Agency and no official endorsement should be inferred.