The Web site, Health-Track, supported by a grant from the Pew Charitable Trusts, uses cancer death rate figures from the National Cancer Institute and toxic chemical and air pollution information from the Environmental Protection Agency to paint a color-coded map of the U.S.

Although the map breaks new ground by bringing together these two sets of data and making them widely available to the public, it still can''t answer the questions the public has about the effects of environmental hazards on cancer, says Jim O'Hara, executive director of Health-Track.

The aim of the project is to bring together the available information but also to emphasize the need for more public health monitoring, he says. "We're not going to get the answers we need until we do a better job of tracking public health," says O'Hara, a former official with the Surgeon General's office and the Food and Drug Administration.

O?Hara says the missing pieces are:

• More comprehensive, state-by-state data on cancer incidence rates (the number of new cases each year per 100,000 people), not just death.
• State-by-state data on carcinogen exposure. Current data show where toxic chemicals are, but not how much of them have made their way into the bodies of people who are breathing them, absorbing them, or ingesting them. A report by the Centers for Disease Control and Prevention, due this spring, is expected to provide a national snapshot of the degree to which 25 toxic chemicals are found in human tissue. But more detailed tracking is needed, O'Hara says.

"Maps of cancer are potentially very informative because they provide one kind of evidence of how much cancer is, in principle, avoidable," says Michael Thun, MD, vice president for epidemiology and surveillance research for the American Cancer Society (ACS).

"These regional differences in cancer mortality rates are very provocative," Thun adds, "as are the even larger worldwide variations."

Those variations were first analyzed in detail in 1981, when an Oxford University report commissioned by the U.S. Congress came out. It tracked cancer worldwide and found 70% to 80% is caused by environmental factors. That doesn?t mean only environmental pollution, however. Epidemiologists use the term, "environmental" to describe all the factors other than the genes one is born with, Thun says.

"The public thinks of environmental causes being toxic substances and pollution," he says. But environmental factors include diet, smoking, physical activity, infectious agents, cosmic radiation, ultraviolet radiation from sunlight, and cultural practices, Thun says.

The Oxford study looked at differences among different countries, and within countries. For example, stomach cancer ? now rare in the U.S. ? is extremely common in certain parts of China. However, researchers believe that differences in food types, methods of food preservation, and chronic infection with Helicobacter pylori account for most of the variation in stomach cancer incidence rates.

Another example closer to home is the high breast cancer rates in the northeastern U.S., evident on the Health-Track map. Researchers have known about the higher rates in the Northeast for about 40 years. "Most cancer epidemiologists believe that this reflects a tendency for many working women in these areas to delay childbirth until later in life and to have fewer children. The consequence of this reproductive pattern is more ovulatory cycles [menstrual periods] in a woman''s life and greater exposure to estrogen, which appears to increase breast cancer risk," Thun says.

"The cancer prevention community finds map studies hugely helpful in identifying high risk geographical areas and formulating hypotheses about which factors contribute to risk," he adds.