PARIS - The sperm count of French men plunged by a third between 1989 and 2005, a finding which fuels concern that environmental pollutants or lifestyle are crimping fertility, scientists said on Wednesday.
Exceptional in scope, the study is believed to be the first country-wide, long-term probe into sperm quality, the team said.
"This constitutes a serious public health warning. The link with the environment particularly needs to be determined," they warned in the European journal Human Reproduction.
Researchers examined data for semen samples provided by 26,609 men at 126 in-vitro fertilisation (IVF) centres in France over 17 years.
The men were partners of women who had blocked or missing Fallopian tubes, meaning that the couples' infertility was not caused by any problem with the man's sperm.
Over this period, the sperm count - the millions of spermatazoa per millilitre of semen - fell continuously, by an average annual rate of about 1.9 per cent, totalling 32.2 per cent.
In men aged 35 on average, concentrations declined from an average of 73.6 million per millilitre in 1989 to 49.9 million per millilitre in 2005.
By comparison, 55 million sperm per millilitre is the threshold below which sperm concentration is likely to influence the time it takes to conceive, and 15 million per millilitre is a common threshold of infertility.
During the 17 years of the study period, there was a 33.4 per cent fall in the percentage of normally formed sperm. In contrast, the motility of the sperm - its ability to move satisfactorily, a key factor in conception - increased slightly, from 49.5 to 53.6 per cent.
Speculating on the source of the declines, the scientists point to suspects which have already been fingered in lab research.
They could be chemical pollutants called endocrine disruptors that change hormone levels.
"They might also be linked to other known semen-altering factors that would have changed over the study period, like an increase in body-mass index, stress, nutrition or infections."