Low Sperm Counts Blamed on Pesticides in U.S. Water

Stefan Lovgren
for National Geographic News
April 27, 2005
Comparing the reproductive health of men in different areas of the United States, epidemiologist Shanna Swan came up with some baffling results: The sperm count in men from rural Missouri was far lower than in men from cities like New York, Los Angeles, and Minneapolis.

It didn't take long for Swan—a professor at the University of Missouri-Columbia at the time of study, 2003—to hypothesize that agricultural chemicals were a culprit.

Men with higher levels of three commonly used farming pesticides—alachlor, atrazine, and diazinon—in their bodies were much more likely to have a low sperm count than men who showed low levels of the pesticides.

"The risk of poor semen quality was elevated 30 times with higher alachlor levels and 11 times with atrazine," said Swan, who is now at the School of Medicine at the University of Rochester in New York. She cautioned, however, that "the number of men we tested is small, so these numbers are not precise."

But almost none of the men with low sperm counts were farmers who had directly used the pesticides. Swan believes they were likely getting the toxins right from their own kitchen faucets.

The Missouri study bolstered a disturbing claim made by a growing cadre of scientists: Our waters—from our seas to our tap water—are becoming increasingly polluted by chemicals carried in agricultural runoff.

Humans are hardly the only creatures affected. As suggested in an episode of National Geographic's four-part TV series, Strange Days on Planet Earth, which airs tonight on PBS, a wide range of animals—from leopard frogs to Beluga whales—are suffering from waters that may be poisoned by farming chemicals.

Treating Water

In a previous study Swan found that women who drank tap water with elevated levels of certain chlorination by-products had higher miscarriage rates.

"There's no question that there are risks associated with drinking tap water," she said. "On the other hand, you have to [disinfect] the water, and we're a lot better at that in the United States than they are in [many] developing countries."

So what kind of water is the safest to drink?

"The quality of bottled water is very inconsistent," Swan said. "Tabletop filters get out some chemicals but not others. From a public health perspective, it's very difficult to recommend what people should do."

The pesticides found in the Missouri men are often found in drinking water sources. Water treatment plants are not designed to remove these chemicals, so Swan says it is likely that drinking water is the main source of exposure.


The popular weed killer atrazine, widely used by farmers in the U.S. Midwest, may also be the cause of declining populations of the northern leopard frog in the U.S. heartland.

Research by Tyrone Hayes, a biologist at the University of California, Berkeley, has shown that atrazine causes the frog's male hormone testosterone to turn into the female hormone estrogen. As a result, male leopard frogs not only grow eggs but develop ovaries, turning them into hermaphrodites—male-female hybrids.

What's happening in the frogs could have implications for humans. Hormonal functions are similar across most species of animals, including humans. As Hayes warns in the documentary, "Frogs tell us something about us."

(Hayes has received grants from the National Geographic Society's Committee for Research and Exploration and was recently named an Emerging Explorer by the Society.)

The biologist's research also suggests that very little atrazine is needed to induce the deformities in frogs. Larger doses actually appeared less harmful. These findings raise doubts about chemical safety laws and regulations that assume that bigger doses of chemicals are always more harmful.

Chemical Cocktails

Elsewhere, chemical runoff is suspected of killing beluga whales in the northern waters of Canada's St. Lawrence River. The beluga whales there have been found to have some of the highest cancer rates of any wild animals studied.

Some dead belugas have been so full of waterborne toxins that their corpses technically qualify as hazardous waste.

Biologists suspect that a mixture of chemicals—not a single toxin—is causing the belugas to develop such high cancer rates.

Sylvain DeGuise, a University of Connecticut pathobiologist, tested whales' immune cells to a variety of chemicals and found that the chemicals may not be toxic individually but could turn poisonous when mixed.

Testing the toxicity of all chemical mixtures is practically impossible. The U.S. government alone registers an average of 2,000 newly synthesized chemicals each year.

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