Drop in quality and numbers over 17-year period part of worrying trend, say experts.
The reproductive health of the average male is in sharp decline, the world's largest study of the quality and concentration of sperm has found.
Between 1989 and 2005, average sperm counts fell by a third in the study of 26,000 men, increasing their risk of infertility. The proportion of healthy sperm was also reduced, by a similar proportion.
The findings confirm research over the past 20 years that has shown sperm counts declining in many countries across the world. Reasons ranging from tight underwear to toxins in the environment have been advanced to explain the fall, but still no definitive cause has been found.
The decline occurred progressively throughout the 17-year period, suggesting that it could be continuing.
The latest research was done in France but British experts say it has global implications. They said the results constituted a serious public health warning and the link with the environment particularly needs to be determined.
The worldwide fall in sperm counts has been accompanied by a rise in testicular cancer rates, which have doubled in the past 30 years, and in other male sexual disorders such as undescended testes, which are indicative of a worrying pattern, scientists say.
They said there was an urgent need to establish the causes so steps could be taken to prevent further damage.
Richard Sharpe, professor of reproductive health at the University of Edinburgh and an international expert on toxins in the environment, said the study was hugely impressive and answered sceptics who doubted whether the global decline was real.
"Now, there can be little doubt that it is real, so it is a time for action. Something in our modern lifestyle, diet or environment is causing this and it is getting progressively worse.
"We still do not know which are the most important factors but the most likely are a high-fat diet and environmental chemical exposures."
Researchers from the Institut de Veille Sanitaire, St Maurice, used data from 126 fertility clinics in France which had collected semen samples from the male partners of women with blocked or missing fallopian tubes. The men, whose average age was 35, had no fertility problems.
The results showed the concentration of sperm per millilitre of semen declined progressively by 1.9 per cent a year throughout the 17 years from 73.6 million sperm per millilitre in 1989 to 49.9 million per millilitre in 2005. And the proportion of normally formed sperm also decreased by 33.4 per cent over the same period.
Allan Pacey, senior lecturer in andrology at the University of Sheffield, said the study was impressive but the apparent fall could be due to changes in methods for counting sperm.
- THE INDEPENDENT