More than half of canned tuna samples from three national brands exceeded the U.S. Environmental Protection Agency’s safety level for human consumption, according to a new study published in Environmental Toxicology and Chemistry. Only 5 percent of the tuna studied contained more mercury than what the U.S. Food and Drug Administration considers safe.

The researchers, who analyzed more than 300 cans of tuna purchased from a grocery store in Las Vegas from November 2005 to February 2006, also found that solid white tuna, or albacore, had higher mercury concentrations than light tuna—a result that’s consistent with previous studies. And packaging medium—oil versus water—didn’t make a difference when it comes to mercury concentrations, researchers found.

The FDA has less stringent safety standards than the EPA regarding mercury in tuna; thus, the reason why one study sample could be deemed safe be the FDA and not by the EPA. The EPA’s action level for mercury in tuna is 0.5 ppm, whereas the FDA’s is 1.0 ppm. As a result of their findings, study authors asked for stricter regulation of mercury in the canned tuna industry and called on federal agencies to make designations of safe concentrations of mercury consistent.

“For at-risk populations such as women, infants and children, these guidelines should be modified and explicitly stated,” the study authors reported, adding that their findings suggest “the need for a long-term monitoring program to ensure the safety of tuna that we consume.”

Americans consume about 1 billion pounds of canned tuna a year, accounting for 25 percent to 35 percent of all U.S. seafood consumption, according to the study authors. Fish consumption is considered the primary way humans get exposed to methylmercury, which can cause numerous health effects like central nervous system damage, hearing loss, vision problems and even death.