Should you worry about being exposed to environmental chemicals that can act like human hormones--from natural plant estrogens found in many foods to pesticides and industrial contaminants such as DDT and PCBs?

So far, there is no solid evidence from human studies that low-level exposure to hormone-mimicking chemicals in the environment causes disorders such as cancer or infertility, according to a new report issued last week.

But that doesn't mean there's no problem. Chemicals such as PCBs, if ingested by a woman before or during pregnancy, can be stored in her tissues and can affect her infant's brain development, leading to a lower IQ and a poorer short-term memory, according to the report by an expert committee of the National Research Council.

"I would say those are effects that . . . should be taken seriously," said Ana M. Soto, an associate professor of cellular biology at Tufts University School of Medicine and a member of the committee.

The report also found that hormonally active chemicals have been clearly linked with a wide range of adverse health effects in wildlife and in laboratory animals, including reproductive problems, abnormal development of the nervous system, weakening of the immune system and tumors of certain glands.

Because it is possible that these chemicals--which are ubiquitous in the environment--may cause similar effects in people, the committee called for extensive research, including studies that would monitor some human populations from conception through adulthood to find out whether exposure--particularly during fetal development or infancy--could cause health problems years later.

"As we really got into this thing, this subject became bigger and bigger," said Ernst Knobil, a professor at the University of Texas-Houston Medical School, who chaired the expert committee. "It really addresses just about all of biology when you come right down to it."

Many chemicals have hormone-like activity, and everyone is exposed to some degree. Natural hormone-like substances include phytoestrogens found in many food plants, including soybeans, nuts, plant oils, grains, berries, vegetables and tea. Synthetic chemicals with hormonal activity are, of course, components of certain drugs (such as birth control pills), but are also present in some herbal supplements as well as in many consumer products, including cosmetics, plastics, dental sealants and household cleaners.

In addition, various pesticides and industrial chemicals that persist for years in soil, water or the atmosphere can have hormone-like actions. Those include the banned pesticides DDT, chlordecone (kepone) and dieldrin and the currently marketed pesticide methoxychlor, as well as PCBs and dioxin. Manufacture of PCBs ceased in the United States in 1977 but the chemicals still contaminate many rivers and lakes. Dioxin is a byproduct of wood-burning and of many industries, including pulp and paper mills.

The amounts of such chemicals to which people are exposed in daily life, as well as their hormonal potency and how long they remain in the body, vary from one substance to another. For instance, the typical diet contains a gram of plant estrogens a day but only two one-thousandths of a microgram of PCBs. On the other hand, genistein, the major phytoestrogen in soy, is cleared by the body within a couple of days, while studies in monkeys show PCBs can remain in body tissues for more than seven years.

Environmentalists, as well as some wildlife biologists and other scientists, have expressed alarm in recent years about the potential health effects of hormone-like environmental chemicals. A 1996 book by zoologist Theo Colborn of the World Wildlife Fund and two coauthors popularized the term "endocrine disruptors" and suggested that exposure to such substances might cause breast cancer in women and low sperm counts in men.

As early as the 1960s, biologists wondered whether hormone-like actions were the cause of some of the reproductive disorders--such as thinning of eggshells and abnormal sexual behavior--seen in birds exposed to DDT and other pesticides, Knobil said.

Then in 1971, eight cases of a rare cancer of the vagina were reported in young women whose mothers had taken the synthetic estrogen DES during pregnancy. Subsequent studies in animals confirmed that estrogen exposure before birth could lead to cancer and reduced fertility in later life, fueling the theory that environmental chemicals acting like estrogen or other hormones could have long-term adverse health effects.

But the committee found that in many cases where a chemical has been clearly linked to an effect--such as DDT and eggshell-thinning--it's unclear whether a hormone-like action of the chemical is the cause, Knobil said. For that reason, he said, the panel rejected the term "endocrine disruptors" and recommended further studies to determine precisely how pesticides, PCBs and other chemicals act in the body.

Among the report's major conclusions:

* CANCER. Although hormone-mimicking chemicals have been associated with tumors of the thyroid, pituitary and adrenal glands in animals, the evidence to date does not support a link between adult exposure and an increased risk of cancer (including tumors of hormone-sensitive organs such as the breast, prostate, testicles and uterus). One study did find an association between dieldrin exposure and breast cancer. Further research is needed.

* REPRODUCTION AND DEVELOPMENT. Consumption of PCB-contaminated freshwater fish by pregnant women has been associated with lower birth weights and premature births as well as IQ and memory deficits and delayed neuromuscular development in infants. Many animal studies have found that hormonally active chemicals can produce abnormalities of reproductive organs, and field studies document abnormal sexual development and behavior in exposed fish, alligators and other wildlife.

However, the committee found no sign of an overall downward trend in human sperm counts or of a link between sperm count and exposure to hormone-like chemicals. Knobil said studies on sperm counts have shown large regional differences. "These things are yo-yoing up and down," he said. "There's no evidence that these hormonally active agents are involved."

* NERVOUS SYSTEM. In American, European and Asian studies, children exposed to PCBs before birth have shown persistent problems with memory and intellectual function. Monkeys, rats and mice exposed to these chemicals before birth also have impaired learning and motor ability. How the chemicals produce these effects is unknown.

* IMMUNE SYSTEM. Studies of birds, seals and other animals show that DDT and other pesticides suppress the immune system, but it is not known whether this occurs through a hormone-like mechanism. Few studies have been done on such chemicals' effect on the human immune system.

The NRC report was commissioned and funded by the Environmental Protection Agency, the Department of the Interior and the Centers for Disease Control and Prevention. The EPA is screening an array of chemicals for hormonal effects, and research in its laboratories has produced new information on how some substances affect the body, said Frederick S. vom Saal, a biology professor at the University of Missouri and a panel member.

For example, he said, DDT was long believed to act as an estrogen-like hormone. But a 1995 study by EPA scientists showed that the chemical into which DDT is converted by the body acts as an anti-androgen, inhibiting the action of the male sex hormone testosterone. That probably explains why male birds, alligators and other animals exposed to DDT fail to develop normal male sex organs and often behave like females.

"It is almost as potent [an inhibitor of testosterone] as flutamide, the drug used to treat prostate cancer," vom Saal said.

Although the new report offers no recommendations for consumers, vom Saal said he thought the evidence on PCBs was strong enough to justify warning women of childbearing age, including teenagers, against eating freshwater fish, because so many bodies of water in the United States are contaminated.

"You can have these chemicals in your body as a result of eating fish years before your pregnancy, because you don't clear them," he said.

The report, "Hormonally Active Agents in the Environment," is available online at http://www.nap.edu. Copies (at $64.95 each) can also be ordered from the National Academy Press at 202-334-3313 or 1-800-624-6242.