1. Mercury morass

2. Danger signs

3. Mercury mystery

4. Safety: in eye of beholder?

Warnings about mercury pollution affect more than 2 million acres in South Florida. Photo: USGS

Mercury in rain or snow is heaviest along the Atlantic Seaboard, from Maine to Virginia. Image: USGS

After all, toxicology lives by the maxim, "The dose makes the poison."

Is eating a lot of fish hazardous to your health? Perhaps, especially if you eat predators, where mercury tends to concentrate. Photo: William B. Folsom, NOAA

You can safely down an aspirin every day to keep your blood moving. But gobble the whole bottle, and you will rot your digestive pipes. At the recommended dose, aspirin is medicine. At the whole-bottle overdose, it's poison.

But if the mercury poisoning in Minamata does not prove the danger of low doses, nor does it prove their safety, either.

The next mercury disaster struck in Iraq in 1961, where villagers ate donated grain seeds that had been treated with mercury fungicide (the seed bags bore a warning label -- in English). Again, the symptoms included nerve problems -- mercury, like lead and many other heavy metals, is toxic to nerve cells. But again, the doses were higher than those you can get from eating fish, and while 35 people died, the low-dose question remained unanswered.

Mercury: Thermometer rising
In the 1980s, using ever-more accurate chemical detectors, scientists found widespread, low-level mercury contamination, even in "pristine" places like the Wisconsin northwoods, miles from sources of industrial pollution. In the open ocean, tuna and other predators were also carrying a burden of mercury.

Tests for mercury in blood and hair showed that mercury, AKA "quicksilver," was also finding its way into another top predator, Homo sapiens. How did mercury get there, and what was it doing?

The answer to the first question came quickly: Almost all human exposure to methylmercury, which is apparently the most dangerous form, came from eating fish and shellfish.

Here's a historical irony for you. During the 1980s and '90s, a long campaign to restrict use and dumping of PCBs was starting to bear fruit. And so as public workers started taking down some of the "Caution: Fish polluted with PCB" signs, others began nailing up placards with a different warning: "Caution: Fish contain mercury. Don't eat if you're pregnant, thinking about getting pregnant, or a young child."

Today, as we've mentioned, 45 states warn about mercury pollution in some lakes or rivers. In 19 states, these "advisories" apply to all freshwaters.

Murky mercury?
Although even people who eat some fish usually have low levels of contamination, Michael Gochfeld, of the Environmental and Occupational Health Sciences Institute in New Jersey, says clinics do see poisoning in the mercury-rich Eastern United States. Gochfeld, who chaired the New Jersey Mercury Pollution Task Force, says, "Periodically we see people who have eaten a lot of fish, and actually have some evidence of mercury impairment," such as difficulty with memory or muscle control. After they slack off on the fishwiches, he adds, "their blood mercury has gone down, their symptoms have gone down."

In the late 1980s, to probe the health effects of the chronic, low-level exposures that could be affecting hundreds of millions around the world, three major studies began looking at diet, mothers' mercury levels, and child health.

Their results are interesting, but confusing.

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