ny time. They have been in the news because two groups that periodically convene scientists to decide whether something is a carcinogen issued new reports. Last month, the International Agency for Research on Cancer, part of the World Health Organization, said there is a possibility cellphones raise the risk of brain tumors. "The operative word is `possibility,'" said Lichtenfeld, who among others has pointed out the thin evidence for this and the fact that cancer rates have not risen since cellphones came out. Last week, the National Toxicology Program, part of the National Institute of Environmental Health Sciences — both of which Birnbaum heads — issued its report. It adds to the list of known carcinogens formaldehyde, which is in building materials and some hair-straightening products, though Birnbaum said on-the-job exposure is the main concern. The list also adds a plant substance in some "natural" arthritis remedies, aristolochic acid. Six other things were dubbed "reasonably anticipated" to be carcinogens, including styrene and another herbal medicine ingredient, riddelliine (rih-DELL-een). Since 1971, the international cancer agency has evaluated more than 900 substances. Just over 100 have been deemed carcinogens, 59 are called probable carcinogens, and 266 others are possible ones. In this last category of possibles — besides the electromagnetic energy from cellphones — are coffee, engine exhaust and talc-based body powder. Talc in its natural form may contain asbestos, though products sold for home use since the 1970s have been asbestos-free. Again, most risk is thought to involve occupational or unusual exposure to natural talc. The evidence on coffee has gone back and forth for years, with no clear sign of danger and some suggestions of benefit. However, known carcinogens include alcoholic beverages, estrogen treatments for menopause symptoms, birth control pills, certain viruses and parasites, and even some drugs used to treat cancer, such as cyclophosphamide and tamoxifen. "Most people would probably be shocked to see the number of things they interact with every day" on these lists, Lichtenfeld said. Here's the problem: The agencies that pass judgment on a carcinogen don't regulate it or determine what levels or routes of exposure are a concern and for whom. "People immediately assume it's going to cause cancer at any exposure level, and that's simply not true," said A. Wallace Hayes, editor of the scientific journal Food and Chemical Toxicology, and an industry consultant. The rule is "RITE" — Risk Is equal to Toxicity times Exposure — and "they've left out half of the equation" by not saying how much exposure is a concern, Hayes said. "The organizations that list these substances as possibly carcinogenic have to be conservative. That means if there's any reasonable evidence, way before it's a sure thing, they have to say, `Let's be cautious.' That's their job — to raise the flag," said David Ropeik, a consultant and author of "How Risky Is It Really? Why Our Fears Don't Always Match the Facts." It's human nature to fear risks we didn't choose, such as hazardous