For Related Articles and More Information, Please Visit OCA’s Food Safety Research Center Page.

One of the most dangerous yet confusing toxic pollutants is mercury in seafood. Mercury is very bad for developing fetuses and children, and seafood is very good for them. But mercury is in all seafood. Like I said: confusing.

Last summer a friend caught a quite large bigeye tuna, over 200 pounds. He gave me a big chunk, about 20 pounds. Knowing that such a big, old fish would be pretty high in mercury, I started whittling away at it just a little at a time. My friend was eating a lot of this fish for the next several months, so much so that I advised him to get his blood tested. There’s no sharp line between “safe” and “unsafe” levels of mercury in the body, but the average adult has a blood level of about 1 microgram per liter, and anything above 5 micrograms per liter is considered too high. When he called me saying he had over 40 micrograms per liter, I went to my doctor. My level was 24. I’m now off fish for several months. OK, so I was headed toward vegetarian anyway.

I recently spoke with Ned Groth, an environmental health scientist formerly with Consumers Union, and Michael Bender, co-founder of the Mercury Policy Project (MPP). Bender’s group is suing the federal government in an attempt to update mercury guidelines that Michael says are out of date and not reaching the folks who need them most — pregnant women and heavy fish eaters. It was time for me to understand more about mercury in seafood. What I learned might help clear up some confusion over risks, and how to eat seafood safely.

Mercury in ocean fish comes from natural and human sources. About two-thirds of each year’s new mercury comes from human sources, especially coal combustion. Mercury is an impurity in coal.

From the air it falls into water where bacteria convert it to a form called methylmercury that gets into living things and builds up in them. Plankton with mercury get eaten by little fish, and little fish get eaten by bigger fish. A big fish has gotten a dose of mercury from every little fish it’s ever eaten. If you eat it, it will pass that combined dose on to you. So hint #1: bigger fish and older fish have more mercury, because it accumulates up the food chain.

Ned says that among commonly consumed fish and shellfish, some kinds have 100 times more mercury than other kinds. The highest levels are found in larger, longer-lived predatory fish such as shark, swordfish, tuna, marlin, and grouper.

The developing brain is especially sensitive to mercury’s effects, so pregnant women and young children are the main at-risk group. But the danger falls mostly on people who eat larger than average amounts of fish, and those who eat higher-mercury varieties like those just listed.