French scientists are warning that men's sperm counts are plummeting at record rates.
A study of nearly 27,000 French men found the average sperm concentration fell more than 32 percent over the 17-year study period between 1989 and 2005. That's almost 2 percent each year.
While the study only looked at French men, the research supports other studies that showed sperm counts were falling worldwide, potentially raising risk for fertility problems in couples.
"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," wrote the researchers, led by Dr Joëlle Le Moal, an environmental health epidemiologist at the Institut de Veille Sanitaire in Saint Maurice, France. "This constitutes a serious public health warning. The link with the environment particularly needs to be determined."
The study was published Dec. 4 in Human Reproduction.
The researchers examined sperm concentration and quality in sperm samples that were given by men who were the partners of women undergoing fertility treatments for Fallopian tube problems. The men had no known problems with their sperm.
The researchers found for men who were an average age of 35, semen concentrations declined from an average of 73.6 million per milliliter in 1989 to 49.9 million per milliliter in 2005. Concentration is a measure of sperm per millimeter, while total "sperm count" refers to the actual amount of sperm in ejaculate.
"The 2005 values are lower than the 55 million per millimeter threshold (from the World Health Organization), below which sperm concentration is expected to influence the time it takes to conceive," Le Moal said in a statement.
The researchers also looked at sperm quality and found a 33 percent decrease in the percentage of normally-formed sperm over the same period. However the study also found the proportion of sperm that moves -- called motility -- increased slightly between 1989 to 2005.
Could chemicals in the environment be to blame for drops in sperm concentration and normal sperm?
The researchers say other studies have pointed to the role so-called endocrine disrupters, which are chemicals that disrupt the body's natural hormone balance, play in affecting sperm. Problems with male sperm or female eggs tend to suggest environmental stresses, according to Le Moal, because these are the first cells from which human beings are built. Early exposures may impact adult health.
He also said the effect could be cumulative, so "the observed trends could be the result of several generations' changes."
"Clearly if this type of decrease continues, we're going to find that we're going to have young men that have low sperm counts," Grace Centola, president of the Society for Male Reproduction and Urology in Birmingham, Alabama, told Reuters. She was not involved in the study.
The study was unable to control for other factors that may have influenced sperm concentration and quality, such as whether men were smokers or obese.
One expert questioned the study's methodology in comparing sperm concentrations across a period of time when collection methods have changed over the years.
"In the paper, the authors claim that the methods for measurement of sperm concentration and motility 'have not changed noticeably during the study period', yet to me this is an odd thing to say as in my experience they have changed remarkably everywhere else in the world," Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield in the U.K., told the BBC. "I would argue that the 'jury is still out' on this issue."