Over the years, many companies have used mercury to manufacture a range of products including thermometers, thermostats and automotive light switches. Although the metallic mercury in these products rarely poses a direct health hazard, industrial mercury pollution becomes a serious threat when it is released into the air by power plants, certain chemical manufacturers and other industrial facilities, and then settles into oceans and waterways, where it builds up in fish that we eat. Children and women of childbearing age are most at risk.

Mercury in Fish

Once mercury enters a waterway, naturally occurring bacteria absorb it and convert it to a form called methyl mercury. This transition is particularly significant for humans, who absorb methyl mercury easily and are especially vulnerable to its effects.

Mercury then works its way up the food chain as large fish consume contaminated smaller fish. Instead of dissolving or breaking down, mercury accumulates at ever-increasing levels. Predatory fish such as large tuna, swordfish, shark and mackerel can have mercury concentrations in their bodies that are 10,000 times higher than those of their surrounding habitat.

Mercury and Human Health

Humans risk ingesting dangerous levels of mercury when they eat contaminated fish. Since the poison is odorless, invisible and accumulates in the meat of the fish, it is not easy to detect and can't be avoided by trimming off the skin or other parts.

Once in the human body, mercury acts as a neurotoxin, interfering with the brain and nervous system.

Exposure to mercury can be particularly hazardous for pregnant women and small children. During the first several years of life, a child's brain is still developing and rapidly absorbing nutrients. Prenatal and infant mercury exposure can cause mental retardation, cerebral palsy, deafness and blindness. Even in low doses, mercury may affect a child's development, delaying walking and talking, shortening attention span and causing learning disabilities.

In adults, mercury poisoning can adversely affect fertility and blood pressure regulation and can cause memory loss, tremors, vision loss and numbness of the fingers and toes. A growing body of evidence suggests that exposure to mercury may also lead to heart disease.

Mercury and a High-Fish Diet

A July 2005 report from the federal Centers for Disease Control and Prevention (CDC) concluded that one in 17 women of childbearing age have mercury in their blood above 5.8 micrograms per liter of blood -- a level that could pose a risk to a developing fetus. This is an improvement from the prior report in 2003 which showed that one out of 12 women had mercury in their blood at this level. Newer science indicates, however, that mercury actually concentrates in the umbilical cord blood that goes to the fetus, so mercury levels as low as 3.4 micrograms per liter of a mother’s blood are now a concern. Nearly one in 10 women of reproductive age in the United States has mercury in her blood at or above this level, according to the new CDC study.

Dr. Jane Hightower, a doctor of internal medicine at the California Pacific Medical Center in San Francisco also linked fish consumption to elevated mercury levels when she tested her own patients. Her 2003 study found that 89 percent of the participating patients -- chosen because of their fish-heavy diets -- had elevated mercury levels. Many had levels as much as four times that which the Environmental Protection Agency considers safe.

The good news is that Dr. Hightower and other health professionals conclude that high mercury levels are reversible: cutting consumption of mercury-contaminated fish causes blood mercury to drop, though it can take six months or more.

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