Mercury: how much is safe?

June 23, 1998

By Peter Montague
A fight is shaping up over the amount of toxic mercury that the US government will call "safe" in the human diet. The outcome will determine how strictly the government will control mercury emissions from incinerators,

By Peter Montague

A fight is shaping up over the amount of toxic mercury that the US government will call "safe" in the human diet. The outcome will determine how strictly the government will control mercury emissions from incinerators, coal-burning power plants and other industrial sources. The outcome will also directly affect the commercial and recreational fishing industries. And of course it will affect the health of people — especially children — who eat mercury-contaminated food.

Humans ingest mercury mainly by eating fish. More than 95% of the mercury in fish takes the chemical form called methyl mercury, which is the most toxic form of the element.

In 1996, the US Environmental Protection Agency set a new guideline for methyl mercury in the diet: 0.1 micrograms of mercury per kilogram of body weight per day (0.1 µg/kg/day). This is 4.7 times as strict as the World Health Organisation's standard of 0.47 µg/kg/day.

(The EPA's guideline is officially called a "reference dose"; it is the amount of methyl mercury that people are thought to be able to eat without any harmful effects.)

EPA recently published results of two surveys of mercury in freshwater fish, one completed in 1985 and one completed in 1994. The 1985 survey found an average of 0.11 parts per million mercury in freshwater fish, and the 1994 survey found an average of 0.26 ppm mercury.

Ocean fish have average mercury levels of 0.21 ppm. Taken together, freshwater and marine fish have an average mercury concentration of about 0.2 ppm.

'Safe' levels

An average woman weighing 60 kilograms can ingest 60 x 0.1 = 6 micrograms of mercury per day without exceeding the EPA reference dose. If each gram of fish contains 0.2 micrograms of mercury, our average woman could only eat 6/0.2 = 30 grams of fish per day without exceeding the EPA reference dose. Thus one good-sized serving of fish per week is about all that is safe, if the fish are contaminated at average levels.

EPA says that the fish species that people prefer to eat are contaminated at a level of only 0.12 to 0.14 ppm. At this level of contamination, our average woman could safely eat 46 grams of fish per day, or 322 grams per week.

Of course, some people prefer to eat species of fish that accumulate large amounts of mercury: shark, swordfish, sea bass, walleye and largemouth bass can contain 0.5 to 1.0 ppm or more. At these levels of contamination, consumption must be strictly limited to remain within EPA's guidelines.

The Food and Drug Administration has set an "action level" of 1.0 ppm mercury for fish in interstate commerce. If an "action level" is exceeded, FDA can issue a warning, but warnings are not posted where consumers might see them (for example, at fish markets); they appear only in FDA publications. Furthermore, FDA's 1.0 ppm mercury limit was established to protect adults, not children.

How much do people eat? EPA recently addressed this question. In 1990, there were 248.7 million people in the United States. Some 177 million were adults. Of these, 58% (102 million) eat fish once a week. Between 13% and 23% eat fish or shellfish two or three times a week. An estimated 1% (1.8 million people) eat fish or shellfish daily.

Several populations eat more fish than average: people who fish commercially or for recreation tend to eat far more than average. Native people, people from the Caribbean, African-Americans, Asian-Americans and Pacific islanders tend to eat far more fish than average.

EPA believes that the critical population is women of child-bearing age (15 to 44), of which there are 58.6 million in the US. Among women in this age group who report eating any fish at all, EPA estimates that half exceed the reference dose, 25% are ingesting twice the reference dose, 10% are ingesting four times the reference dose, and 5% are ingesting five times the reference dose.

Another critical population is young children. Mercury damages the nervous system, and the nervous system continues to develop through at least age six. There are 15 million children in the US between the ages of three and six; EPA estimates that 20% of them exceed the reference dose for methyl mercury.

Fish is an important source of animal proteins and other nutrients. Fish and shellfish are low in saturated fats, and they provide antioxidants such as selenium and vitamin E. They also offer beneficial omega-3 polyunsaturated fatty acids, which are important for optimal development of motor skills, the brain and vision.

Based on the benefits that humans derive from eating fish, some authorities are urging that EPA relax its reference dose, to allow more methyl mercury into our diets. For example, the Agency for Toxic Substances and Disease Registry has proposed that the US adopt the WHO's mercury standard of 0.47 g/kg/day.


An estimated 1600 to 4000 tonnes of mercury enter the atmosphere from natural sources each year. Rain and snow then bring it to earth, where it ends up in streams and lakes, and ultimately the oceans.

Humans roughly double this amount, contributing somewhere between 2000 and 6000 tonnes of mercury to the atmosphere each year. The main human sources are waste incinerators, coal combustion, cement kilns and the manufacture of chlorine.

The human contribution of mercury to the atmosphere is increasing at least 2% each year. So long as these increases continue, mercury will continue to accumulate in the bodies of fish.

As it is, 47 states in the US issued a total of 11,531 fish advisories for some or all of their waters during 1996, warning residents to limit their consumption of fish. Of these warnings, 64% were for mercury. The mercury problem is serious now and is getting steadily worse.

Dubious assumptions

Making the mercury health standard more permissive is a dubious public health proposition.

The EPA's reference dose, as strict as it seems to some people, was developed based on certain questionable assumptions.

First, EPA based its estimates on the assumption that mercury has a half-life in the human body of 70 days. This is true, but the biological half-life of mercury in the brain is 230 days — and the brain is the main organ that mercury attacks. (The biological half-life is the time it takes for the human body, or one of its organs, to rid itself of half of its burden of mercury.)

Second, EPA set its reference dose based on data derived from an acute poisoning incident in Iraq, not on the kind of chronic poisoning that produced so many mentally defective children near Minamata Bay in Japan. The mercury poisoning of some 10,000 people who lived around Minamata Bay during 1956 to 1974 showed that children can be poisoned by daily ingestion of fish polluted at only 0.11 ppm.

Lastly, EPA's reference dose was developed on the assumption that people who ingest mercury in fish are not also ingesting other toxicants. This is obviously a false assumption.

A recent study by the Natural Resources Defense Council shows unmistakably that fish in the US are contaminated not only with mercury but also with numerous organochlorine compounds such as PCBs, DDT, chlordane and dioxins. And of course lead is a constant threat to children.

Based on the dismaying history of poisoning children with lead and dioxins, we should be very cautious about declaring that we know what is a "safe" dose of mercury for anyone.

[From Rachel's Environment & Health Weekly. Like Green Left Weekly, Rachel's is a non-profit publication which distributes information without charge on the internet and depends on the generosity of readers to survive. If you are able to help keep this valuable resource in existence, send your contribution to Environmental Research Foundation, PO Box 5036, Annapolis, Maryland 21403-7036, USA. In the United States, donations to ERF are tax deductible.]

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