"Several surveys from recent years indicate that 10-15% of non-contracepting couples do not obtain a clinical pregnancy within one year. Almost 50% of the prevalence of infertility is more or less attributable to male factors. Most often impaired semen quantity and quality is the only sign of male reproductive disorder. There has been an ongoing controversy for more than 25 years about whether sperm count in the general male population has been declining. In Denmark the discussion was initiated already in 1983 by a paper demonstrating a substantial downward shift in sperm concentration in infertility clients across a 20 years period. In the US the discussion was temporarily closed by McLeod who in 1979 arrived at the conclusion that no change could be demonstrated in US. But interest in male reproductive health was renewed in 1993 when Skakkebæk and his group claimed a major decline in sperm count worldwide based on analysis of semen studies published during the past 50 years. Although several additional studies and intensive discussion have not provided a definitive answer to the question about temporal and regional trends in semen quality, few would disagree about the need for concern and additional research. If sperm count has changed dramatically in one or few decades it is obvious to look for environmental causes. The evidence on environmental impact on male reproductive function stem first of all from the occupational setting where exposure levels can be high."(From the introduction)

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