In Wednesday's issue of the journal Human Reproduction, researchers said they found a decline in the concentration, size and shape of sperm in a sample of 26, 609 men between 1989 and 2005.
"To our knowledge, it 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," the researchers wrote.
"This constitutes a serious public health warning. The link with the environment particularly needs to be determined."
The study focused on the male partners of women who were either missing both fallopian tubes or had blocked tubes. They were going to fertility clinics for the first cycle of assisted human reproductive technology. Fresh semen was analyzed twice for each man.
Joelle Le Moal of the Institut de Veille Sanitaire in Saint Maurice, France, and her co-authors analyzed data on three measures of sperm quality.
They said they found a "significant and general decrease in sperm concentration" of 32 per cent during the study period. The number of sperm in one millilitre of the average man's semen decreased nearly two per cent.
The percentage of sperm that were of normal size and shape also decreased.
Global sperm decline under debate
The researchers didn't find an overall trend for motility — the ability of sperm to move properly towards an egg.
Whether sperm concentrations are decreasing globally is debated, the researchers said.
Previous reviews of the medical literature pointed to a global decrease. A 1998 study of Canadian sperm quality also found a significant downward trend in sperm concentration at university fertility centres.
The researchers said the trends they observed were the same when men over 50 were excluded.
They weren't able to take socioeconomic status into account. The study's authors argued the findings should apply to the general population, where smoking and excess weight may impair sperm quality more than among the study population.
Men participating in the study were asked to abstain for three to five days, but the researchers weren’t able to adjust for abstinence time. Concentration increases with abstinence time, according to previous research.
The authors didn't seek any funding.