Sperm density has nearly halved and semen quantity decreased by about a quarter over the past 50 years, a study claimed yesterday.
It concluded that the 'remarkable' decline made it likely that a greater proportion of men would have difficulty fathering children.
Professor Niels Skakkebaek, author of the Danish research which is published in the British Medical Journal, offered reassurance, however.
He said: 'Birth rate in our society is mostly determined by economic and social factors, and human fertility would have to be extremely compromised to have an impact on the number of births.'
His study, based on an analysis of nearly 15,000 men worldwide between 1938 and 1991, showed that the average sperm density had dropped from 113 million per millilitre to 66 million per millilitre and seminal volume decreased from 3.4ml to 2.75ml.
Professor Skakkebaek, of Copenhagen University, said the results may reflect a reduction in male fertility, but a corresponding dramatic increase in genito-urinary abnormalities and testicular cancer supported the theory that environmental factors were to blame. 'It must be something in the environment, although we don't know what it is,' he said.
Some medical experts have linked industrial contaminants such as toxic PCBs to lower sperm counts, while others have suggested testicular cancer could be sparked by exposure to oestrogen or similar substances in the womb.
Professor Skakkebaek said PCBs had a mild oestrogen effect.
He added: 'We believe there should be more interest and research in male reproduction.'Reuse content