I haven’t taken a biology class since high school. With all the news about the danger of mercury emissions, I decided it was time to understand how mercury works in our bodies. Now I see why our young are so vulnerable to mercury, a truly terrible poison.

The trouble begins with coal plants, the primary source of mercury emissions in the United States. They spew many poisons into the air, including other toxic metals such as arsenic.

Mercury drifts through the air and settles into water–reservoirs, ponds, streams, rivers, marshes–and onto the land, where it is washed by rain into water. In the water, mercury is converted by certain microorganisms to a highly toxic form known as methylmercury. That bacteria make the mercury “bio available”–able to be taken up by the creatures that consume it.

Methylmercury is absorbed by shellfish and fish, through their gills; it is dispersed by their blood through their bodies, and accumulates in their fatty tissue. The contaminated fish is eaten by other fish, and birds and mammals–including humans. The biggest source of mercury exposure in humans comes from eating fish and shellfish.

Typically, the longer a fish lives, and the larger it is, the more mercury accumulates in its flesh; grouper, swordfish, orange roughy, halibut and tuna have been identified as having high levels of mercury in them. Across the country, states have instituted warnings about mercury in certain types of fish. Pregnant or nursing women are told not to eat tuna more than once a week.

Once eaten, mercury goes directly into the highest lipid-containing organs in our bodies–including breasts and brains. Breast milk, which is packed with nutrients and high in lipid content, can contain mercury.

In addition, mercury can cross the blood-brain barrier–a nearly impermeable membrane made of high-density cells that helps to safeguard the brain. The blood-brain barrier is extremely effective at protecting the brain from many common bacterial infections, for instance. But it cannot keep methylmercury out.

Unborn babies and young children are especially vulnerable to methylmercury in their bloodstreams. Methylmercury, like many toxicants, does cross the placental barrier, and has been detected in measurable amounts in umbilical cords.

Fetuses and young children are constantly developing; they are much more vulnerable to the harms of neurotoxicants than adults. Developmental neurotoxicants can impair the growth of the brain in ways that interfere with learning and thinking.

Once in our bodies, mercury stays there for many months, and most people have some amount of mercury in their tissues at all times.

Mercury exposure at high levels can harm the brain, heart, kidneys, lungs and immune system of people of all ages.

We are never exposed to any one toxic pollutant alone. We’re exposed to all of them at the same time. Some of them have synergistic effects–they cause more trouble in combination.

That’s why we have to stop coal plants from spewing mercury. Write to the EPA and tell them you support the new Mercury and Air Toxics Standards. And write to the owners of the polluting coal plants to demand they clean up their emissions. For the sake of our children.