Defective Sperm, Pesticide Tie Found
Men exposed to pesticides widely used on crops are many times more likely to have defective sperm and low sperm counts than males with little or no exposure, according to a scientific study published today .
The study provides new evidence supporting a theory that pesticides and other chemicals which mimic estrogen or block testosterone are harming human reproductive systems. It is the first time that scientists have shown a link between environmental contaminants in men’s bodies and large decreases in the number and quality of their sperm.
A team led by University of Missouri-Columbia reproductive epidemiologist Shanna Swan compared men from central Missouri who had higher concentrations of two herbicides and one insecticide in their bodies with men from Missouri and the Minneapolis area who had low levels.
“Within Missouri, the pesticide score [of the men] was strongly associated with semen quality,” the authors reported in the journal Environmental Health Perspectives, which is published by the National Institute of Environmental Health Sciences.
None of the men in the new study worked at or lived next to farms, where the pesticides are most commonly used. They were most likely exposed through drinking water supplied by aquifers, Swan said.
The number of men tested for pesticides -- 50 from Missouri and 36 from Minnesota -- is considered small, but scientists said the findings warrant close attention because some of those tested were found to be 30 times more likely to have defective sperm. That degree of risk is in the same range as the odds of contracting lung cancer from a lifetime of smoking cigarettes.
All the tested men, in their 20s and 30s, were fertile and recently fathered children.
“What this means is that it’s harder for these men to conceive. It takes them longer,” Swan said. “We also wonder what else it is doing to these men, and what it is doing to the rest of the family, the women and children?”
Sally Perrault, a reproductive toxicologist at the U.S. Environmental Protection Agency’s national research lab, said the study “really raises our antennae.”
Still, she said, Swan’s team “hasn’t proven that [the pesticides] came from the water” and that more men should be tested before the safety of the pesticides is questioned. “We should look into this more, rather than drawing real definite conclusions now,” she said.
A scientific debate about sperm counts has been waged since 1992, when Danish researchers reported that men on average have half as much sperm as they did a half-century earlier, based on 61 sperm count studies, mostly in Europe and North America. Some scientists challenged the findings because there was no universal pattern.
Declining sperm counts had been reported in European, but not American, men. Since then, reproductive experts have tried to determine if chemical exposures or geographic patterns could explain the differing sperm counts.
Last fall, Swan broke new ground by reporting major differences in sperm between rural and urban areas. Men in Columbia, Mo., have significantly lower numbers of sperm -- as much as 44% less -- than men in Minneapolis, New York and Los Angeles, according to her research.
“We had trouble finding men in Missouri with good sperm quality,” Swan said in an interview. “The counts in Missouri are really low.”
Rex Hess, a reproductive toxicologist at University of Illinois-Urbana who was not involved in the study, said the research was so well-documented and found such a high risk that it leaves “no doubt in [his] mind that there is a link” between semen and the three pesticides, alachlor, atrazine and diazinon.
“What this means is if your sperm counts are low, [these pesticides] ought to be a top candidate,” he said. “Water is so important that everybody ought to be conscious of this.”
Environmentalists hope the study will increase pressure on the Bush administration to adopt stricter regulations governing the use of herbicides and insecticides. But the authors acknowledged that confirmation of the results using larger numbers of men and those from other areas is warranted.
“Given the widespread use of these pesticides, if further study confirms these findings, the implications for public health and agricultural practice could be considerable,” the researchers wrote in the journal.
Scientists say it has already been well-documented over the last 20 years that farm workers and herbicide sprayers have poor semen quality.
But the new study is the first to note the condition in men who weren’t working directly with pesticides. “We’re not looking at exposure through home or occupational use. This is an environmental exposure of which people had no knowledge,” Swan said.
The pesticide industry said Tuesday that the researchers “do not provide clear evidence” that pesticides caused the sperm differences. Instead, they could be within the normal, random variation found in men, said Ray McAllister, vice president for science at CropLife America, a group representing pesticide manufacturers.
Dr. Christina Wang, a co-author of the study from Harbor-UCLA Medical Center, said it is “unlikely” that natural variation or genetics would explain the large differences in sperm, but acknowledged that she would need to see similar results in more men to confirm her suspicion that pesticides are to blame. The research team is now expanding its testing to another agricultural area, Iowa City.
“The main problem was that the study was done comparing only Missouri and Minneapolis, mainly because those places showed the largest difference in sperm counts,” Wang said.
The scientists wrote in the study that they adjusted the data for factors known to affect sperm, including age, race, smoking, abstinence and diseases, which they said left pesticide exposure as the only known culprit. Seasonal temperature differences can also affect sperm, but that effect would not be so large, Perrault and Wang said.
In numerous studies during the last few years, researchers have found evidence that estrogen-mimicking chemicals in the environment are feminizing or emasculating wild animals. Laboratory tests also show that when a male animal is exposed to high levels of pesticides as a fetus, its sperm is affected.
But people generally are exposed to much lower doses, and some scientists are skeptical that the levels commonly found in the environment can harm people.
The human evidence has been growing, however. Other recent studies have linked sperm quality of men with industrial compounds called PCBs and chemicals found in plastics called phthalates.
The highest risk was associated with alachlor, a popular weedkiller in the Midwest used on primarily on corn, soybeans and peanuts, the study said. About 20 million pounds of the weedkiller are used annually in the United States.
The rate of sperm defects was 30 times greater for Missouri men with the highest levels of alachlor in their urine, compared to Missouri men with low levels.
Men exposed to higher levels of the insecticide diazinon were 17 times more likely to have poor semen quality. Diazinon, used widely on lawns, was banned for residential use last year, but it is still legal on many crops.
Men exposed to atrazine, used primarily on corn, were 11 times more likely to have defective sperm, although fewer men had detectable levels of atrazine than the other two chemicals.
“Almost everybody who had any atrazine in his body had very poor semen quality,” Swan said.
In January, after a review that lasted several years, the EPA announced that atrazine, the country’s most heavily used herbicide, could still be used safely on crops, but that the manufacturer must monitor some water systems in the Midwest and Southeast. Environmental groups argue that the chemical should be banned because it exceeds federal standards in some drinking water. They also want all commercial use of diazinon eliminated.
Perrault of the EPA said the results of the new study are surprising and important because the exposures to the three pesticides were fairly low among the Missouri men and would be routine in many parts of the country.