Yesterday I wrote a story for the paper about how conservation group Oceana released a study that revealed much higher levels of mercury in tuna than the FDA had previously estimated. A study conducted by the New York Times revealed similar findings. For fish-eaters seeking guidance, here’s the skinny:

Q: How does mercury get in fish?
A: Mercury pollution comes from numerous sources like coal-fired power plants and incinerators. In addition to the combustion of fossil fuels, which creates mercury vapor, mercury pollution may come from unlikely sources like the dental industry. Dental amalgams are 49 percent mercury by weight, some of which leaks into our water system.

According to the Monterey Bay Aquarium’s Seafood Watch, bacteria in water transform mercury into methylmercury. Smaller bottom feeders absorb the methylmercury, which they then pass up the food chain in higher concentrations. Top-level carnivores like tuna and swordfish have the highest likelihood of containing harmful amounts of manmade chemicals, stored in their muscles and fatty tissue. These larger, longer-lived species have more time to accumulate dangerous levels of methylmercury.

While mercury is the second most toxic substance known to man (the first being uranium), methylmercury is more damaging than inorganic mercury because it is more easily assimilated into the body.

Q:  What does methylmercury do?
A: According to the FDA, exposure to high levels of methylmercury can harm an unborn baby’s developing nervous system (brain and spinal cord). Young children exposed to high doses of mercury while in the womb or after birth are at risk for limited attention span; poor language, visual-spatial, memory and coordination skills; as well as lower IQ. But adults are at risk as well. Mercury poisoning victims have experienced nervous system effects such as loss of coordination, blurred vision or blindness, and hearing and speech impairment.

Q: Who should be concerned about mercury?
A: In 2004, the Environmental Protection Agency and the FDA formally advised women of childbearing age and young children that they could safely eat up to 12 ounces of low-mercury seafood per week. But according to Dr. Rashid Buttar, head of the Charlotte, NC-based Center for Advanced Medicine and Clinical Research and Chairman of the American Board of Clinical Metal Toxicology, there are other groups that are at risk.

“Anyone who is immunocompromised--diabetics, patients who have gone through a transplant, cancer patients, HIV patients, and anyone over the age of 70 and under the age of 7—should avoid high-mercury fish. Mercury is a very significant immunosuppressive agent. We also find that there are certain genetic predispositions that people have that make them unable to excrete mercury or other metals efficiently. We find that children with developmental delays or people with Alzheimer’s can’t get rid of it.”

Q: Which kinds of fish are OK?
A: Dr Buttar suggests avoiding all big game fish (tuna, swordfish, marlin, etc.). He suggests eating deep ocean fish (cod, for instance) and smaller to midsize fish like tilapia. “Fish from the Gulf of Mexico is more worrisome than fish from the open ocean. In the Gulf there’s more drainage from land masses.” For people eager to maximize the benefits of omega-3 fatty acids but minimize their mercury risk, he suggests wild salmon from Alaska (“not domesticated farm-raised which, because of its feed, has a low omega-3 fatty acid profile”).

Q: But what if I only eat a little—is it safe to eat ahi once a week?
A: “How much high-mercury fish is safe? None, as far as I’m concerned,” says Dr. Buttar. “There’s no safe level of mercury. Mercury levels should be aggressively monitored and documented by the FDA.”