Assessing the Risks of Mercury Exposure

Mercury Depositions
Michigan's utility companies are responsible for 2 percent of the four tons of mercury deposited annually in the state.

During the Clinton Administration, the EPA conducted research on the risks of mercury exposure and concluded that there are no health risks from mercury in the air in the vicinity of coal-fired power plants, or from ingesting crops grown in the vicinity of power plants.

The primary health concern with mercury releases is human consumption of methylmercury. Consuming large quantities of seafood with extremely high levels of methylmercury over an extended period of time may lead to neurological damage. Children and fetuses are believed to be the most sensitive to the potentially toxic effects of methylmercury. However, the extent of harm from current levels of mercury in fish and other seafood is a matter of considerable debate.

Various national and international groups have developed estimates for what constitutes a “safe” level of mercury consumption. Most safety thresholds are based on studies of populations that were accidentally exposed to very high levels of methylmercury, or studies of island populations for whom seafood is a dietary mainstay. The levels declared to be safe all contain a considerable margin of safety — that is, the safety standard is set far below the levels at which health effects result.

The EPA’s standard for chronic exposure is 0.1 microgram of methylmercury per kilogram of body weight per day. The U.S. Agency for Toxic Substances and Disease Registry has a minimum risk level of 0.3 microgram of methylmercury per kilogram of body weight per day. The World Health Organization has a “permissable tolerable weekly intake” recommendation of 3.3 micrograms per kilogram of body weight (which translates into a daily rate of 0.47 microgram per kilogram of body weight per day). Thus, there is a considerable range in the government’s safety levels, with the EPA’s being the most stringent.

State and federal regulators consider safety levels — as well as surveys of actual mercury levels in fish — when deciding whether to issue public advisories that recommend limits on fish consumption. Michigan has had a statewide fish consumption advisory for inland lakes since 1988. The advisory warns against eating more than one meal a week of rock bass, perch or crappie over nine inches in length, or any size largemouth bass, smallmouth bass, walleye, northern pike or muskie from inland lakes. Women of childbearing age and children under age 15 are advised not to eat more than one meal of these fish per month.

The EPA, in setting federal mercury standards, analyzed various studies that tested for effects on children who had been chronically exposed to mercury while in vitro by mothers who consumed large quantities of seafood. In each of the three studies analyzed by the EPA, a battery of neurological tests was administered. Two of the three studies found subtle effects on some neurological functions. The results of these studies were used by the EPA to establish the federal safety threshold of mercury exposure — a threshold level at least 10 times more stringent than the level at which the subtle health effects were observed. This standard is expressed as the amount of daily exposure to mercury that is unlikely to produce deleterious effects in the course of a lifetime.

For the federal mercury rule, the EPA considered IQ to be a surrogate for the neurological effects of mercury exposure since deficits in IQ can be monetized. None of the studies found statistically significant effects on IQ from methylmercury exposure. Subsequently, the EPA commissioned a further analysis that combined the results of all three studies as one. The combined analysis showed a small effect on IQ.

The EPA does not claim that methylmercury degrades IQ. Instead, agency policy holds that children who are exposed to methylmercury in vitro may be at increased risk for poor performance on tests that measure attention, verbal memory and visual-spatial abilities.