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 amount 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 hroughout the 17-year period, suggesting that it could be continuing.
The latest research was conducted in France but British experts say it has global implications. The scientists said the results constituted a "serious public health warning" and that 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 have doubled in the last 30 years – and in other male sexual disorders such as undescended testes, which are indicative of a "worrying pattern", scientists say.