A coalition of consumer groups is recommending the U.S. Department of Agriculture get tuna out of school lunchrooms after tests of canned tuna sold to schools found highly variable levels of mercury, in some cases higher than federal guidelines.

Tuna industry groups countered that canned tuna is safe and wholesome. The real public health issue is that "we don't eat enough" seafood, says Gavin Gibbons of the National Fisheries Institute, a seafood industry group in McLean, Va.

The Mercury Policy Project of Montpelier, Vt., is a non-profit working to reduce mercury in the environment. It tested 59 samples of tuna in institution-size cans and foil pouches from 11 states. The levels of methylmercury were in general close to previous tests done by the Food and Drug Administration. However, levels of mercury varied widely, even from the same can or pouch. The average methylmercury content ranged from 0.02 to 0.64 parts per million in light tuna and between 0.19 and 1.27 parts per million in albacore tuna.

"On any given day in a given school, children eating the same meal could get mercury doses that vary by tenfold," just because of the variability of the chunk of meat in the packet," says Edward Groth, author of the report, released Wednesday. It was sponsored by several groups, including the Center for Science in the Public Interest and Physicians for Social Responsibility.

Current federal dietary guidelines urge Americans to eat seafood twice a week because seafood is a healthy protein and contains omega-3 fatty acids, important for metabolism, but most people eat it once a week or less, says Gibbons.

"To suggest we're eating too much is almost comical," he says. Scaring children away from tuna "at a point in their life when they're developing their nutrition habits and their palates" is damaging.

Diane Pratt-Heavner of the School Nutrition Association in National Harbor, Md., says she doesn't believe tuna is a big issue because it's not popular on school lunch menus. She only sees it as an item in deli-style counters, mostly in high schools, where it's one choice among many.

Groth agrees that tuna isn't a huge part of school lunches, but wants to make sure kids aren't getting too much. And parents need to be aware of how much tuna their children eat, he says. Kids who eat a tuna sandwich a month aren't at risk but some children, "we don't know how many there are," love tuna and eat a lot of it, he says. Even four times a month could have "subtle adverse effects" on some children. "We're trying to put those kids on the map," he says.

The Environmental Protection Agency's maximum acceptable dose for methylmercury, a potent neurotoxin, is one-tenth of a microgram per kilogram of a person's body weight. Even tiny levels of methylmercury have been linked to learning disabilities and developmental delays in children, according to EPA scientists.

To ensure that the brains of fetuses and children aren't exposed to levels high enough to damage them, the EPA and FDA said in 2004 that women who are pregnant or might be pregnant can eat up to two meals, or 12 ounces, of fish and shellfish a week. Children should eat "smaller portions," the guidelines said.

Since the EPA adopted that standard, some studies indicate it may be too high. "Our research suggests that this limit should be decreased by 50%," says Philippe Grandjean, a professor of environmental studies at Harvard University who studies mercury in seafood. "If anything, [the Mercury Project] report underestimates the risks associated with regular tuna intake."

By the Mercury Project's measure, a 44-pound child who ate just two ounces of albacore tuna at levels the project found in some tuna would be getting almost half, 47%, of the standard. Based on the emerging evidence, the report recommends that children not eat albacore tuna, which can have more mercury, and that young children eat canned light tuna only once a month and older children only twice a month. They also suggest school lunch programs limit canned tuna servings to twice a month and phase it out, moving toward lower-mercury seafoods such as salmon and shrimp.

Fish become contaminated when mercury in industrial pollution enters waterways. Bacteria transform the mercury into methylmercury, a more biologically active and dangerous form of the element, according to the Food and Drug Administration. Fish eat the bacteria and the mercury accumulates in the largest and oldest fish, which is why long-lived and large species such as tuna have higher levels. Canned light tuna comes from skipjack species of tuna, which are smaller and often younger. They haven't been around long enough to accumulate as much methylmercury in their systems. Albacore is harvested older and therefore contains more.