Sperm counts are dropping.

The average sperm count of men in the United States and Europe has plummeted by more than 50 percent since the late 1930s, according to a recent analysis by University of Missouri epidemiologist Shanna Swan. The finding fuels ongoing concerns that male reproductive health may be deteriorating, and that environmental pollutants may be the cause.

Based on 61 studies published since 1938 - involving a total of nearly 15,000 subjects - Swan found that average sperm counts among healthy American men have dropped from 120 million sperm per milliliter (million/ml) of semen in 1938 to just over 50 million/ml in 1988, a decline of 1.5 percent per year. In Europe, sperm counts have fallen to roughly the same level, though twice as fast, by 3.1 percent each year between 1971 and 1990.

Though only one sperm is required to fertilize an egg, once sperm count drops below a certain level, infertility becomes increasingly common.

Considerable controversy revolves around the proposed causes of declining sperm counts, though the prevailing explanation implicates environmental chemicals called endocrine disrupters that masquerade as hormones. Since hormones, such as estrogen or androgen, orchestrate the development and everyday functions of organisms, exposure to hormone-mimicking chemicals can disrupt these development signals.

Specifically, synthetic chemicals that mimic the female sex hormone estrogen may influence male development in utero or during the formative years of early childhood when hormone sensitivity is high. Lab animals exposed to even traces of estrogen-mimicking chemicals develop reproductive disorders ranging from testicular cancer to infertility. Exposing laboratory rats to dioxins - pollutants generated in paper production and waste incineration - caused testicular abnormalities, feminine sexual behavior, and reduced or no sperm production in male offspring.

Concerns over declining human sperm counts were first voiced in the mid-1970s, and since then several studies have suggested a substantial drop since mid-century. A study of 1,350 Parisian men showed sperm counts declining from 89 million/ml in 1973 to 60 million/ml in 1992, a 2.1 percent drop per year. Studies from Canada, Sweden, Greece, Italy, Belgium, and other European nations have demonstrated similar declines in recent decades.

Although a scientific consensus is emerging that sperm counts have decreased in certain areas, controversy remains over the global extent of the decline. Indeed, there appears to be considerable geographic variation in sperm counts. For example, while sperm count declines have been demonstrated in Danish men, Finnish sperm counts appear unchanged. And a recent study suggests that sperm counts may vary considerably even within the city of London.

Skeptics have also focused on possible "confounding factors," such as subject abstinence time before sampling, that can influence the accuracy of sperm count data. Requirements for study participation - proof of fertility through current or recent paternity - have also changed over time, with the potential to skew the sample population.

Nonetheless, other distressing signs of deteriorating male reproductive health have surfaced. Data from the industrial world show sperm quality, measured in terms of morphology and vigor, declining since mid-century, with healthy sperm comprising a decreasing share of total sperm produced. Since 1960, rates of testicular cancer in men under age 50 have grown two- to fourfold in Great Britain, the Nordic and Baltic countries, Australia, New Zealand, and the United States. The incidence of undescended testicles and other disorders of the male reproductive system have also soared.

And examples of similar reproductive havoc in wildlife populations have also been attributed to estrogen-mimicking chemicals. In one of the best documented examples, alligators in Florida's Lake Apopka, contaminated with organochlorine insecticides with known estrogenic properties, were severely demasculinized - many males developed without proper sexual organs and fertility levels plummeted.

Currently, endocrine-disruptive activity has been demonstrated for roughly 60 chemicals, though only a tiny fraction of the estimated 80,000 manufactured chemicals in use today have been screened for endocrine-disrupting effects. These common chemicals include certain pesticides, like endosulfan, the most widely used insecticide in North America; components in plastics, detergents, cosmetics, and fabrics; and other industrial products and by-products. While many of the chemicals now known to disrupt reproductive and hormone systems have been banned in the industrial world, their use continues grow in developing nations.

Pinning down the extent - and possible causes of - declining sperm counts will require better data on human exposure to endocrine disrupters. Several initiatives are underway to identify geographic variation in sperm counts and possible environmental causes.