But Cal State Northridge biology professor Steven B. Oppenheimer, director of the university's Center for Cancer and Developmental Biology, warns that not all cancer dangers are made by man.

"We all know that our environment is full of man-made carcinogens but few of us realize that our diet is also loaded with natural carcinogens that may pose at least as great a risk as the man-made ones," Oppenheimer said.

His examples included black pepper, mushrooms, celery parsnips, celery, figs and even parsley. All contain natural compounds that have been implicated as possible carcinogens. Alcohol is linked with cancers of the mouth, esophagus, pharynx, larynx and liver.

Some of the molds that grow on poorly stored corn, grains, nuts, breads, cheese, fruit and apple juice produce two major carcinogens, aflatoxin and sterigmatocystin, while molds that are cultured in the production of some cheeses are perfectly safe, Oppenheimer said.

Fruits and vegetables are known to contain many anti-cancer compounds such as vitamin A, beta carotene, vitamin E and vitamin C. However, some vegetablesÑamong them beets, lettuce, spinach, radishes and rhubarbÑsometimes contain high levels of nitrates and nitrites that can be converted in the body into carcinogenic nitrosamines.

Even chocolate and tea contain a compound, theobromine, that may enhance carcinogen action, said Oppenheimer, who invites the public to sit in for free on his "Biology of Cancer" class each fall and learn the latest information about the disease.

"Some estimates suggest we eat 10,000 times more natural toxins in foods than man-made toxins such as pesticides," he said.

A major natural carcinogen that has been cited by the Environmental Protection Agency as the most dangerous naturally occurring health hazard of all time is radon--a radioactive gas that comes up from the ground and concentrates in homes.

"It is impossible to know if your home contains dangerous levels of radon unless you test for it," Oppenheimer said. Unlike termite testing, he noted, radon testing is not currently required before a home is sold.

He added that just because some natural carcinogens may be linked to cancer in humans, people should not be excused from their responsibility for contributing to the number of carcinogens in the environment.

"There is little doubt that millions of workers are exposed to man-made probable industrial carcinogens each year," he said. "Most municipal water supplies contain low levels of many man-made probable carcinogens, such as those associated with the necessary process of chlorination. Air pollution also accounts for our exposure to many man-made carcinogens. Even some of the drugs and medicines we use are possible carcinogens."

So what does all this mean? he asked.

"Both man-made and dietary carcinogens greatly contribute to our cancer risk," he said. "The single most important cause of cancer death in the United States appears to be cigarette smoking, but more and more evidence suggests that the foods we eat and the environment we live in are rich with carcinogens. The more we know about them, the better chance we have of controlling our risk of developing cancer."

Oppenheimer pointed out that in 1775, the British physician Percivall Pott started the field of cancer prevention by noting that cancer can be prevented if one reduces exposure to known carcinogens.