Consumer Reports finds "unpredictable" mercury spikes in canned tuna

A Consumer Reports investigation out Thursday morning found "unpredictable" spikes of mercury levels in five popular canned tuna brands — and suggests that pregnant people "avoid canned tuna altogether."

"While canned tuna, especially light varieties, has relatively low average levels of mercury, individual cans can sometimes have much higher levels," Consumer Reports said. 

"From can to can, mercury levels can spike in unpredictable ways that might jeopardize the health of a fetus," said James Rogers, director of Food Safety Research and Testing at Consumer Reports.

The mercury levels Consumer Reports found were within FDA standards, which say pregnant people can eat canned tuna in limited quantities.

CBS News reached out to all five companies in the report. Chicken of the Sea, Safe Catch, StarKist and Wild Planet said their products are safe and within FDA standards. Bumble Bee did not get back to us as of early Thursday morning but noted to Consumer Reports the "health benefits of consuming seafood far outweigh any potential risk, including concerns about mercury."

Mercury is a neurotoxin — a compound that affects neurodevelopment, said CBS News medical contributor Dr. David Agus. Possible health risks from mercury include impaired brain function, loss of smell and developmental delays in children.

If a developing fetus is exposed to a high mercury level, cognitive issues could be seen later in life, said Agus.

"Young children and pregnant women especially need to keep mercury away from those neurons that are developing," he said. 

Consumer Reports tested 10 products, including albacore and light tuna from each of the five tuna brands. In total, 30 samples — all tuna products that were packed in water — were tested. 

Light tuna in general has less mercury than albacore, which comes from larger fish. "But you can't tell by just looking how much mercury a specific can has," said Michael Hansen, a senior scientist at Consumer Reports.

Of the 30 samples tested, Consumer Reports said it found six individual spikes in mercury content — amounting to one in five cans — "that would change the FDA's recommendation about how often someone should eat that particular tuna." 

"What this report showed is that while light has much less mercury than other tuna, it's variable, and you're gonna get a batch here and there with a higher level," said Agus. 

Consumer Reports said the tests give insight into what consumers "may experience at a moment in time when eating these brands of tuna, and underscore the importance of making safer choices in their daily routines."

"One big takeaway is that albacore has much more mercury than light or skipjack tuna, regardless of the brand," Consumer Reports said. "That's not surprising, since albacore is larger and lives longer than the tunas that make up the light tuna or skipjack tunas. But the disparity was quite wide: The albacore products had three times more mercury, on average, than the others."

Mercury can't be removed through cooking. It is an impurity in coal; when coal is burned, elemental mercury goes to the clouds and comes down when it rains, ending up in the ocean as methylmercury, explained Agus. Marine animals consume the contaminant. 

"Fish eat it and then large fish eat the smaller fish, and that's how they accumulate mercury," Agus said. 

Fish that contain higher levels of mercury include shark and swordfish. Very little mercury is found in smaller fish at the surface, like trout and salmon, Agus noted.

Consumer Reports suggests adults who aren't pregnant aim for 8 to 12 ounces per week of fish that is relatively low in mercury.

"That could include up to three servings of light or skipjack tuna. ... You can eat albacore, but only one 4-ounce serving per week," Consumer Reports says.

It suggests that kids stick to light or skipjack tuna and eat low-mercury fish in limited quantities.

Editor's note: This story has been updated to correct a line comparing light tuna to albacore. Light tuna in general has less mercury than albacore.