August 18, 2014 — The Environmental Protection Agency recommends consuming a daily maximum of 0.1 micrograms of mercury for each kilogram of your body weight. That would limit a 176-pound adult (the national average) to 8 micrograms of mercury each day.
Toxicologists have a saying: “The dose makes the poison.” In other words, there is no such thing as “toxic” or “non-toxic”—it always depends on how much of a substance you consume.
So what’s a toxic level of mercury in your diet? This has long been a concern, because many fish contain measurable levels of mercury, which can cause profound neurological disease and death if consumed in sufficient amounts. The issue gained new urgency last week when a study in the journal Nature showed that mercury concentration at the ocean surface has tripled since the beginning of the industrial era.
How does mercury get into fish, anyway?
In the 19th and 20th centuries, factories dumped massive amounts of methylmercury—the most dangerous form of mercury, bonded to carbon and hydrogen—directly into waterways. The most infamous example occurred in Japan in the 1950s and ’60s, when industrial mercury poisoned more than 2,000 people who ate fish from Minamata Bay. (The neurological syndrome caused by severe mercury poisoning is called “Minamata disease” after the tragedy.)
Mercury dumping has been a problem in the United States, too. I grew up a few miles from New York’s Onondaga Lake, where the Allied Chemical Company disposed of as much as 20 pounds of mercury per day in the mid-20th century. “America’s most polluted lake” is still recovering.
Mercury can also take a less direct route to the sea. Burning fossil fuels releases mercury into the air—around 160 tons per year in the United States. The mercury settles to the ground, where rains eventually wash it to the ocean. There, elemental mercury turns into methylmercury. Scientists aren’t quite sure how this conversion occurs, but it probably involves the metabolism of a small but abundant living creature. When that creature is eaten by bigger creatures, the methylmercury travels up the food chain, collecting in animal tissue in larger and larger amounts. That’s why predators like tuna have troubling levels of mercury—they eat a lot.
Read the full story from OnEarth at Salon.com