Semen quality and male reproductive health: the controversy about human sperm concentration decline

Concern about the effect of environmental changes on male reproductive heal th has grown in recent years to become a major preoccupation in some develo ped countries. A possible decline in human sperm concentration was suggeste d in the early seventies following studies in the US. In 1992 a meta-analys is of 61 articles published by Carlsen et al. concluded that the mean sperm count of healthy men had declined by 1% per year over the previous 50 year s. From 1995 and onwards, some retrospective, longitudinal analyses of the sperm count of fertile or infertile men contradicted this while others did not. The demonstration of a geographical variation in sperm concentration, between and within countries or regions, appears to be less controversial. The amplitude of the difference observed cannot only be explained by method ological or confounding factors, and must to some extent be attributed to e thnic. genetic or environmental factors. As many of the published studies s uffer from imprecision regarding the description of population characterist ics and confounding factors, and were not designed with controlled and stan dardised methodology, the debate remains open. Prospective studies in well- defined cohorts of men in various populations are required to evaluate the potential effect of external factors on male reproductive health. These stu dies should not be limited to the analysis of sperm concentration, as this may not be the best biomarker of testis function and human fertility.