The sperm count of French men fell by a third between 1989 and 2005, a study suggests.
The semen of more than 26,600 French men was tested in the study, reported in the journal Human Reproduction.
The number of millions of spermatozoa per millilitre fell by 32.3%, a rate of about 1.9% a year. And the percentage of normally shaped sperm fell by 33.4%.
The average sperm count remained within the fertile range, but experts want to see more research into possible causes.
One of the paper's authors, Dr Joelle Le Moal, an environmental health epidemiologist, said: "To our knowledge, this is the first study concluding a severe and general decrease in sperm concentration and morphology at the scale of a whole country over a substantial period.
"This constitutes a serious public health warning."
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