With a new food-labeling law set to take effect Jan. 1, the packages of many popular foods -- from candy bars to canned tuna -- are starting to highlight the presence of allergy-triggering substances like peanuts, soy and wheat. Some big food makers like Campbell Soup Co. are going so far as to revamp their recipes to take out allergens before they are required to list them.

This is good news for the millions of Americans who suffer from food allergies, which affect 4 percent of Americans, kill 150 people each year, and send 30,000 more to emergency rooms. Peanut allergies are on the rise among children, with their prevalence doubling from 1997 to 2002, according to a study in the Journal of Allergy and Clinical Immunology.

But the new law is causing confusion, too. It requires allergens to be identified even if they are present only in a tiny amount as ingredients -- an issue the federal Food and Drug Administration hasn't clarified. As many food makers rush to comply by listing every possible allergy-causing substance, consumers who pore over labels are noticing some surprising substances on ingredient lists -- like a clear beef broth that contains milk or a canned tuna that contains soy. What they don't know is whether those amounts are significant enough to trigger an allergic reaction.

"The law just says you label everything," says Steve L. Taylor, a University of Nebraska food-science and technology professor. But he argues that unless the FDA offers further guidance on issues such as allergens that exist in small amounts, "consumers will get more information than what they can deal with."

The law -- the Food Allergen Labeling and Consumer Protection Act -- requires food labels to list in "plain language" whether they contain any of the eight major allergens: tree nuts (including almonds, walnuts, and pecans), milk, eggs, fish, crustacean shellfish, peanuts, soybeans and wheat. Labels must either use parentheses to clarify that, for instance, "whey" or "casein" is a milk product, or they must note at the end of the ingredients list that the product "contains" an allergen.

For most products, it's an obvious call. But some foods might contain tiny amounts of an allergen -- say, as an ingredient of the flavoring. Part of the problem is the issue of whether to label "cross contact" -- when foods might contain trace amounts of an allergen because they are made on the same production line as foods containing allergens. The new law doesn't offer clear guidance but instructs the Department of Health and Human Services to report to Congress by February 2006 -- 18 months after President Bush signed the law -- on how to handle the issue. Until then, many companies are including potential cross contacts on labels -- even though the allergens may not be present.

These issues are crucial for people like Carol Dicker of Sudbury, Mass., whose eight-year-old son, Sean, needs only to touch a tree nut to set off a severe reaction, she says. Recently Sean broke out in "hives all over his body" after sitting on a couch where a cousin, wearing almond oil lotion, had sat the night before, Ms. Dicker says. She carefully reads labels of foods like her son's favorite snack cookies and would like to see labels spell out how much of an allergen the product contains.

"It's really hard to know what to do because there are so many different levels of sensitivities," Ms. Dicker says. The problem with the legislation, she says, is that the allergens will end up on labels, but it will be impossible to tell how much.

Jupiter Yeung, principal scientist at the Food Products Association, an industry group, argues that the food industry shouldn't be required to label trace amounts of an allergen used to process a food. For example, he says, the baking industry widely uses soy lecithin to prevent food from sticking, but only a small amount ends up in the final product.

Robert E. Brackett, director of FDA's Center for Food Safety and Applied Nutrition, says the agency will issue guidance before January with the aim of protecting consumers while holding down companies' cost of complying with the law. But don't count on the FDA guidance to end the dispute, he says. "At what point do you say, 'This is the cutoff?'" Dr. Brackett says. "We may make a decision on thresholds, but I can assure you the debate won't be solved." Last week, the agency published questions and answers about the new labeling on its Web site but didn't address trace amounts of allergens picked up through cross contact.

For many allergy-causing substances, science doesn't offer a clear solution. While studies have been done on specific foods, allergic responses vary widely from person to person, and declaring what level of an allergen will trigger a reaction is difficult.

In the meantime, many food companies have started changing their labels, and they'll likely make further revisions if the FDA issues further guidance, says Alison Kretser, senior director of scientific and nutrition policy at the Grocery Manufacturers Association, a trade group. Two years ago, in anticipation of the new law, ConAgra Foods Inc., whose brands include Peter Pan, Healthy Choice and Kid Cuisine, began listing the eight major allergens -- plus sesame, an allergen that wasn't specified in the law -- under the ingredient list on its food packages, says spokeswoman Tania Graves. The company, based in Omaha, Neb., is now awaiting word from FDA on how to disclose trace amounts of the allergens.

Other companies are even reformulating their food to remove offending substances. Campbell Soup Co. has reformulated about 50 products to eliminate allergens and has updated between 200 and 300 labels to comply with the new law, estimates spokesman John Faulkner. "We can't wait until Dec. 20 to hear from the FDA."

The Camden, N.J., company replaced the tiny amount of fish gelatin used in a coloring agent for beverages and soups. Rather than list fish on the label "and have people say, 'I can't have that product,' we've found an alternative," Mr. Faulkner says. In all, the company has marshaled 100 people -from those in the research and development lab to the insurance experts -- to carry out the labeling law. Kellogg Co. says it has changed manufacturing processes to lessen the risk of allergens ending up in products that are supposed to be allergen-free.

It's not just big companies. Johnsonville Sausage, based in Sheboygan Falls, Wis., has also reformulated its recipes to remove allergens, says research and development director Kevin Ladwig. "The approach for this new legislation is simply: Better safe than sorry," he says.