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 percent, totalling 32.2 percent.
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 percent 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 percent.
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."
Worries about falling male fertility were first triggered by a Danish probe in September 1992, which found semen quality in Danish men halved between 1938 and 1991.
But the evidence that has emerged since then has been mixed.
A study in Finland likewise pointed to a decline but another -- Danish -- study among 5,000 18-year-old men examined for military service found no change. In the United States, relatively small-scale investigations have seen either stability or a fall.
Fertility specialists say one reason for the conflicting picture is that the semen samples invariably come from fertility clinics or men recruited as sperm donors, and thus are not representative of men in the general population.
Also, the way in which sperm is assessed in the lab has changed over the decades, which means that scientists today find it hard to compare like with like. The UN's World Health Organisation (WHO) published new guidelines in 1989 and 2005 in a bid to standardise procedures.
"A very unclear picture has been painted about whether semen quality has actually changed and, importantly, whether or not we should be worried about any environmental threats to male fertility," said Allan Pacey of Britain's University of Sheffield in a commentary.
"Unfortunately, almost all of the studies published to date have inevitable flaws."
The new investigation was unable to take socio-economic factors into account.
Smoking and weight, both of which are associated with social background, can affect semen quality and concentration.
But one of the authors, Joelle Le Moal at the Institute for Health Surveillance (InVS), said IVF treatment, although universally available in France, is mainly used by the better educated, who tend to smoke less and be less overweight.
As a result, "the decreases" in sperm quality "could possibly be stronger" when extrapolated for the general population.