Everyone knows that a diet rich in fish is good for you. It’s a lean protein with lots of omega-3 fatty acids, and an important part of the Mediterranean diet that helps prevent heart disease.

Related: Something Fishy Going On

But before you go back to the sushi bar for your second helping of tuna roll, keep this in mind: Big fish — tuna, king mackerel, shark, tile fish, and swordfish — are becoming increasingly polluted with mercury.

You can get tested for mercury at the doctor’s office — or use a kit that tests your hair for mercury and 20 other toxins for $119.

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Mercury is heavy metal that is used in for making thermometers, barometers, electrical switches, batteries, and fluorescent light bulbs. The fumes are hazardous, and eating less than a gram of the stuff can kill you.

Most of us don’t work in factories that use mercury, or in mines that use it to help extract gold. But we are exposed to mercury — or more precisely methylmercury — through ocean fish. This mercury comes from air pollution, especially from coal-burning plants. It eventually settles into the oceans where it is consumed by plankton, which is consumed by small fish that are in turn eaten by bigger fish, and so on.

Related: Fish Tales

“Basically, the bigger the fish the higher the content can be, because as they eat smaller fish it works its way up the food chain,” said Stacy Kennedy, senior nutritionist at the Dana-Farber Cancer Institute and Brigham and Women’s Hospital, both in Boston.

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“Some people are at higher risks than others, but mercury toxicity is a real thing,” Kennedy said.

Those at higher risks include children and pregnant women.

The Centers for Disease Control and Prevention says the milder toxic effects from chronic low-grade exposure to mercury are neurological, and include irritability, depression, muscle pain, tremors, memory loss, speech impairment, and loss of coordination. Other symptoms include shedding of skin or discoloration of the hands and feet — or formication, the feeling that insects are crawling on your skin. More direct exposure damages the lungs and kidneys, and can cause vomiting, diarrhea, shock, renal failure, and death.

Eating too much tuna will not cause sudden shock or renal failure, but like other kinds of metal poisoning it can build up slowly and cause a lot of symptoms that are hard to attribute to mercury.

Related: The Best Tests

“The symptoms can be confused with other autoimmune diseases,” Kennedy told LifeZette. “When you are feeling fatigue, joint pain, and muscle aches, it could be a thyroid condition, or lime tick disease, or fibromyalgia. A lot of the (mercury) symptoms are pretty vague.”

One patient who posted his story on the Internet described an 11-year saga to cure chronic fatigue, only to discover the cause in his mouth full of dental fillings, all using a silver-mercury amalgam, all put in at one time.

To combat all of this, the first line of defense is diet. Kennedy recommends limiting your intake of swordfish to once a month, and tuna steak or ahi tuna to twice a month. If you like your tuna from a can, once a week is acceptable, she said. Chunk light tuna has less of it than the more expensive white-meat albacore.

“People who come to me with mercury toxicity are eating a lot of sushi, or tuna on their salad every day,” Kennedy said.

So don’t.

Your body naturally eliminates mercury, and this process can be aided by staying well hydrated, and eating cruciferous vegetables such as broccoli, cauliflower, cabbage, bok choy, and brussels sprouts.

Another diet tip if you are worried about mercury is to watch where your foods come from. Domestically grown food has better controls for toxins in the water. And then make sure you’re not in an environment where there is a chance of more direct exposure, like in a factory that makes electrical switches, or at a coal-burning power plant.

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If you are still worried, you can get tested for toxic levels of mercury by sending off a sprig of your hair. You can go through your doctor, and use a service like as doctorsdata.com, which tests for mercury and 30 other toxic metals for $135 (plus your doctor’s fees.)

If you want to go it alone, you can go to directlab.com, where they test your hair for mercury and 20 other toxins for $119 and post your results online.

Hair tests mostly pick up the methylmercury from fish, however, and don’t catch the “elemental” mercury that comes from dental fillings. For that you’ll need a urine, blood, or fecal test.

“There are hair tests, but they are not that rigorous. If you are concerned about mercury toxicity, I would see your doctor,” Kennedy said.