Men with high levels of three common pesticides in their urine are more than 10 times as likely to have low-quality sperm than men who haven\'t been exposed to the chemicals, according to a groundbreaking new study published today.

Researchers cautioned that the study by Shanna Swan of the University of Missouri at Columbia is far from definitive, and relied on a small sample size - just 86 men in Missouri and Minnesota.

But they said her results were so powerful that many scientists will quickly seek to test her study\'s suggestion that the widely-used weed killers alachlor and atrazine and the insecticide diazinon may be affecting the fertility of men who drink water tainted with the compounds.

\'You just don\'t see [effects] this large in environmental epidemiology very often. This is interesting and intriguing,\' said Dr. Russ Hauser of the Harvard School of Public Health.

The study adds new fuel to a decade-long controversy over whether toxic chemicals are responsible for the fact that male infertility is increasing in some parts of the world, including portions of Northern Europe, which has become a hotbed of research into the trend.

It also comes at a time when atrazine, the nation\'s most heavily used pesticide, is under intense scrutiny by the U.S. Environmental Protection Agency. A study last year suggested that the weed-killer is making male frogs in the Midwest grow female gonads, prompting an ongoing EPA review.

Atrazine, alachlor and diazinon have been found in drinking-water supplies throughout North America, including the Northeast, but the greatest contamination has tended to be in farm belt states like Missouri.

Swan\'s study, published today in the online edition of the journal Environmental Health Perspectives, is the first to look specifically at the impact of those heavily used pesticides on sperm quantity and quality in men who drink water tainted with those chemicals.

She found that the strongest effect was from alachlor, a herbicide widely used on corn and soybeans. Men with unusually high levels of the weed killer in their urine were 30 times more likely to have lower sperm counts and less vigorous sperm than men who had much lower exposures.

Lesser but still very high risks were found for men with high exposures to atrazine and to the bug-killer diazinon. Until the EPA ordered it phased out in 1990, diazinon was the nation\'s top lawn pesticide.

Other chemicals tested showed correlation with sperm problems, including the DEET and malathion.

\'Our study finds a very, very large risk, and I\'m very anxious to try to replicate it,\' Swan said. \'Even if the next study finds an effect one-third as big, it will still be of the same magnitude as smoking and lung cancer.\'

While the sperm problems identified in the lab tests were serious, they did not prevent the men from having children. In fact, all of the chosen men had conceived children already. They were part of a larger multigenerational study.

Several researchers cautioned yesterday that there could be other explanations for Swan\'s results, including genetic differences.

Dr. Harry Fisch, an associate clinical professor of urology at Columbia University, said the study can\'t prove a cause-and-effect relationship because the men may have had poor sperm quality before they were ever exposed to the pesticides.

In fact, he said, medical science still knows very little about why sperm quality and quantity can vary so much from man to man. \'Until we realize what naturally causes changes in semen quality, it\'s hard to point to something in the environment and say that\'s the cause,\' he said.

Even so, Fisch added, \'I think this is a provocative finding, and there is cause for more research. For sure we need to look to see if pesticides are causing a drop in fertility.\'