Health - How much mercury is in fish we eat ?

Russell McLendon

Global warming isn't fossil fuels' only dirty trick. While it is the most sweeping and civilization-threatening side effect of our carbon economy, there are lots of toxins lurking in every lump of coal and drop of oil. And one especially scary fossil-based toxin is also now embedded in your mahi-mahi: mercury.

While atmospheric CO2 starts a chain reaction that disrupts delicate natural processes across the planet, mercury is a more hands-on pollutant. It injects itself into the food chain, accumulates as it moves up and then attacks our bodies directly when we eat contaminated animals, namely fish.

Mercury poisoning can cause severe brain, kidney and lung damage to children and adults, but it’s even more dangerous to fetuses. It prevents nerve cells in the brain from forming correctly, which can damage attention span, fine motor function, language skills, visual-spatial abilities and verbal memory. That’s why the FDA suggests women who are pregnant or plan to become pregnant avoid fish with the highest levels of mercury: shark, swordfish, king mackerel and tilefish.

But why do some fish have more mercury than others? They are what they ate.

Mercury is released into the air just like CO2 when fossil fuels are burned, and coal-fired power plants are the main source. Rather than drifting up into the atmosphere, though, mercury accumulates in clouds and falls to earth with rain. Most of it drains into streams, rivers, lakes and oceans, where plankton absorb it and convert it to the dangerous methylmercury. Small fish like minnows spend their lives eating this plankton, and the merthlymercury builds up in them. Bigger fish eat the minnows, and each fish up the food chain accumulates more and more of it; that’s why smaller fish like anchovies and tilapia have the lowest mercury levels and predator fish like shark and swordfish have the highest. That's also why women who aren't pregnant but plan to eventually have kids should avoid predator fish, because mercury builds up in humans, too.

The United States leapt forward in combating mercury pollution with the 2005 Clean Air Mercury Rule and now contributes only about 3 percent of annual human emissions worldwide. But that doesn’t mean U.S. fish are mercury-free. That 3 percent is still significant — more than5 million pounds in 2006 — and much of our fish comes from waters polluted by other countries' mercury emissions. Furthermore, all that mercury doesn't just go away each year; it adds up over time as it climbs the aquatic food chain from plankton to people. In November, theNew York Times reported that bald eagles in the Catskills were showing increasing levels of mercury — not enough yet to further threaten that species, but an indicator nonetheless that the toxin is still contaminating wild fish and moving up the food chain.