Scientists have been debating the question for more than a decade: Are men less fertile than they used to be? Some scientists insist sperm counts are plummeting -- by up to 50% since 1940, according to a 1992 Danish study. Skeptics say that much of the data used to support the disappearing-sperm theory is flawed; some even claim that sperm counts are increasing.
But one indisputable fact has emerged from this debate: Scientists on both sides of the argument agree that men in some countries have higher sperm counts than men in other nations. In fact, sperm counts can vary dramatically from one city to the next within the same country. Some scientists say these geographic variations offer evidence that a man's environment can influence his fertility.
Consider: A World Health Organization study found a wide range of average sperm counts among men in nine countries; healthy males in Melbourne, Australia, for instance, had twice as much sperm in their semen as did their counterparts in Bangkok, Thailand. Studies involving men from France, Denmark and Finland have produced similar disparities. In the United States, Columbia University researchers found in 1996 that sperm counts among men in New York City were nearly twice as high as those recorded among similar men in Los Angeles, while guys in Minneapolis fell somewhere in the middle.
Shanna Swan, an epidemiologist at the University of Missouri-Columbia, noticed that most of these studies involved men from urban areas. She wondered whether rural living influenced fertility, so she recruited men who were visiting their wives at prenatal clinics in Boone County, Mo., a center of corn, wheat and soybean farming. Swan and her team compared semen samples from those men to samples from men in Los Angeles, New York and Minneapolis.
The results, published last fall in the journal Environmental Health Perspectives, surprised Swan, who surmised that males from the heartland would have the heartiest semen. Instead, they had the lowest sperm counts, by far. New Yorkers had 75% more sperm than the men in Boone County, on average. Males in Minneapolis had 67% more sperm, while study subjects in Los Angeles had 38% more. Also, the sperm of men in cities demonstrated better motility, or motion, a positive indicator of fertility.
Why did the men in rural Missouri have such low sperm counts? "Our leading suspect was agricultural chemicals," says Swan, even though few of the men from Boone County she studied were farmers. Herbicides have been detected in ground and surface water throughout the Midwest. Beyond that, Swan points out the possibility that a non-farmer who lived downwind from a farm could be exposed to potentially hazardous chemicals, such as weed and bug killers, every day. "There's no question these chemicals are in our environment and are in our bodies," says Swan.
The problems may not be limited to the farm. A recent Harvard study found that men whose urine contained high levels of substances known as phthalates, a commonly used manufacturing chemical found in such products as after-shave lotion and garden hoses, had unusually large amounts of damaged DNA in their sperm. Some scientists theorize that estrogen-like compounds in agricultural chemicals and other man-made products interfere with the way sperm is made and behaves in the body. Animal research lends support to that theory. Last July, British researchers reported that mouse sperm is rendered incapable of fertilizing eggs when it's exposed to synthetic chemicals, including herbicides and pesticides.
The results of test-tube studies don't necessarily reflect what goes on in the human body, however, says chemist Angelina Duggan, director of science policy for CropLife America, a Washington, D.C.-based industry group representing agricultural chemical companies.
What's more, urologist Harry Fisch, who led the 1996 Columbia University study, says there are a number of possible explanations for why sperm counts are lower in Boone County than in the other three communities. Farm chemicals may be one culprit, he says. But genetics is another factor, since each city has its own melting pot of ethnic heritages from around the world. "Different cultures have different sperm counts," says Fisch. "Nobody has a clue why." Many other factors -- including smoking, drug use, diet and several medical conditions -- can affect male fertility, too.
Swan acknowledges that more study is needed to establish a convincing link between agricultural chemicals and diminished sperm. She notes that all of her subjects from the clinics were eventually able to impregnate their wives. But some may have struggled because of low sperm counts, while her investigation may have missed men in Boone County who were completely infertile. These days, Swan thinks it's a mistake to worry about whether sperm counts are declining. "I'd like to move from the historical-trend question," she says, "to the question of: Right now, are there chemicals in the environment that are making us less fertile?"
Massachusetts freelance writer Timothy Gower can be reached by e-mail at email@example.com. The Healthy Man runs the second Monday of the month.