Our Preferred Poison. A little mercury is all that humans need to do away with t

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Our Preferred Poison

A little mercury is all that humans need to do away with themselves

quietly, slowly, and surely

By Karen Wright

 

Illustration by Don Foley

 

DISCOVER Vol. 26 No. 03 | March 2005 | Biology & Medicine

 

 

Let's start with a straightforward fact:

 

Mercury is unimaginably toxic and dangerous.

 

A single drop on a human hand can be irreversibly fatal.

 

A single drop in a large lake can make all

 

the fish in it unsafe to eat.

 

 

 

Often referred to as quicksilver, mercury is the only common metal

that is liquid at room temperature. Alchemists, including the young

Sir Isaac Newton, believed it was the source of gold. In the modern

era, it became a common ingredient of paints, diuretics, pesticides,

batteries, fluorescent lightbulbs, skin creams, antifungal agents,

vaccines for children, and of course, thermometers. There is probably

some in your mouth right now: So-called silver dental fillings are

half mercury.

 

 

 

Mercury is also a by-product of many industrial processes. In the

United States coal-fired power plants alone pump about 50 tons of it

into the air each year. That mercury rains out of the sky into oceans,

lakes, rivers, and streams, where it becomes concentrated in the flesh

of fish, shellfish, seals, and whales. Last year the Food and Drug

Administration determined there is so much mercury in the sea that

women of childbearing age should severely limit their consumption of

larger ocean fish. The warning comes too late for many mothers. A

nationwide survey by the Centers for Disease Control shows that one in

12 women of childbearing age already have unsafe blood levels of

mercury and that as many as 600,000 babies in the United States could

be at risk. But that begs a critical question: At risk for what?

 

 

 

Photograph by James Wojcik

 

 

 

TUNA TYPES

 

 

One particularly common source of low-level mercury exposure is tuna.

Because they are large, long-lived predators, tuna accumulate more

mercury in their tissue than smaller, short-lived fish. When tested

for mercury in parts per million, flesh from albacore tuna, which take

five years to mature, was shown to contain about four times as much

mercury as chunk light tuna, which is harvested

 

from younger fish.

Infants born to mothers contaminated by mercury in Japan's Minamata

Bay in 1956 had profound neurological disabilities including deafness,

blindness, mental retardation, and cerebral palsy. In adults, mercury

poisoning can cause numbness, stumbling, dementia, and death. " It's no

secret that mercury exposure is highly toxic, " says toxicologist Alan

Stern, a contributor to a 2000 National Research Council report on

mercury toxicity. But high-level exposures like those at Minamata

cannot help scientists determine whether six silver fillings and a

weekly tuna-salad sandwich will poison you or an unborn child. " The

question is, what are the effects at low levels of exposure? " he says.

 

 

 

Data now suggest effects might occur at levels lower than anyone

suspected. Some studies show that children who were exposed to tiny

amounts of mercury in utero have slower reflexes, language deficits,

and shortened attention spans. In adults, recent studies show a

possible link between heart disease and mercury ingested from eating

fish. Other groups claim mercury exposure is responsible for

Parkinson's disease, multiple sclerosis, Alzheimer's, and the

escalating rate of autism.

 

 

 

How—and in what form—mercury inflicts damage is still unclear. Yet

scientists and policymakers agree that more regulation is imperative.

The Environmental Protection Agency plans to finalize its

controversial first rule on reducing mercury emissions from power

plants this month, and delegates from the United Nations Environment

Programme met in late February to discuss an international convention

limiting mercury use and emissions.

 

 

 

A decade ago researchers and lawmakers agreed that lead, another heavy

metal, was harmful to children at levels one-sixth as high as

previously recognized. But it took scientists decades to establish the

scope and subtlety of lead poisoning. Mercury is now a ubiquitous

contaminant. The average American may have several micrograms of it in

each liter of blood, and the atmospheric burden of mercury has perhaps

tripled since the industrial age. Whatever needs to be done to protect

humanity from its love affair with quicksilver, it had better happen soon.

 

 

 

In August 1996 Karen Wetterhahn, a chemistry professor at Dartmouth

College in Hanover, New Hampshire, spilled a few drops of a laboratory

compound called dimethyl mercury onto one of her hands. She was

wearing latex lab gloves, so she didn't think much of it. A colleague

saw her at a conference the following November. " She said she thought

she was coming down with the flu, " says toxicologist Vas Aposhian of

the University of Arizona. By the time Wetterhahn was diagnosed with

mercury poisoning, in January, it was too late. Despite subsequent

treatment that helped clear the metal from her body, she lapsed into a

vegetative state in February and died the following June.

 

 

 

Scientists are at a loss to explain why mercury often takes months to

exert its effects. " If we knew that, we'd know a lot more about how

mercury poisons the brain, " says Tom Clarkson, a toxicologist at the

University of Rochester Medical Center.

 

 

 

The degree of mercury's toxicity depends on the form and route of

exposure. You can swallow the liquid form of elemental mercury without

much fear because it doesn't easily penetrate the lining of the

stomach and intestines. On the other hand, liquid mercury vaporizes at

room temperature, and when you inhale the vapor it moves right from

the lungs to the bloodstream to the brain. A broken thermometer can

release enough mercury vapor to poison the air in a room—one reason

why some cities and several states discourage the sale of mercury

fever thermometers.

 

 

 

Mercury also binds with other elements in salts and organic compounds

of varying toxicity. Dimethyl mercury, the substance that poisoned the

Dartmouth chemist, is a synthetic form of organic mercury rarely found

outside a lab. A simpler organic compound called methylmercury is of

greater concern because methyl- mercury is the form found in the flesh

of fish.

 

 

 

Seafood is one of the two most common sources of mercury exposure in

adults. Although concentrations of mercury in air and water are

increasing, they are still too small for alarm. But bacteria process

the mercury in lakes and oceans into a form that accumulates in living

tissue. Plankton take in the bacteria and are in turn eaten by small

fish. With each meal, the mercury concentration rises. Then larger

fish eat the small fish, increasing tissue concentrations still more.

Fish at the top of the food chain accumulate the most mercury. The

species singled out by the recent FDA advisory—big predators such as

albacore tuna, shark, and swordfish—can have 100 times more mercury in

their tissues than smaller fish do.

 

 

 

The methylmercury in fish passes readily from the human gut to the

bloodstream and on into all organs and tissues. It seems to act most

powerfully on the brain because the compound is strongly attracted to

fatty molecules called lipids, and the brain has the highest lipid

content of any organ. Methylmercury crosses the protective blood-brain

barrier by binding with an essential amino acid that has dedicated

carrier proteins for shunting it into brain cells. Once inside brain

cells, some of it gets converted to an inorganic form that sticks to

and disables many structural proteins and enzymes essential to cell

function. " It can destroy the biological function of any protein it

binds to, " says Boyd Haley, a biochemist at the University of Kentucky.

 

 

 

Researchers learned how much mercury the body can tolerate from

studies of victims of catastrophic poisoning, such as the Japanese

sickened by eating fish from Minamata Bay and the Iraqis who ate grain

treated with a methylmercury-based preservative in the early 1970s.

But those studies do not reveal how little mercury it takes to cause

harm. At the time of her diagnosis, the Dartmouth chemist had 4,000

micrograms of mercury per liter in her blood. A diet consistently high

in fish can create a blood-mercury level of about 25 micrograms per

liter. That's far below a lethal dose, but it still may not be safe.

 

 

 

 

Visit www.cfsan.fda.gov/~dms/admehg3.html for the EPA's

recommendations regarding seafood consumption.

 

The EPA's Toxics Release Inventory offers easy access to the most

current information on a range of toxins released in this country:

www.epa.gov/tri.

 

Global Mercury Assessment. A United Nations Report:

www.chem.unep.ch/mercury.

 

 

< href= " http://www.epa.gov/tri " a= " " tri<= " " www.epa.gov= " " />

Environmental Defense, an advocacy group, has prepared a list to help

consumers decide which and how much fish is safe to eat:

www.oceansalive.org.

 

 

 

Concerns about low-level toxicity haunt discussions of another

ubiquitous source of mercury exposure: silver dental fillings.

Elemental mercury, which makes up half of silver fillings, releases

mercury vapor, just as liquid mercury does. The vapor from dental

amalgams is the primary source of the one to eight micrograms of

mercury per liter of blood, that is, according to some sources, in the

average American adult. That amount uncomfortably overlaps the

Environmental Protection Agency's current safe level of 5.8 micrograms

per liter. But the EPA's safety level is based on methylmercury

exposure, about which more is known. No human studies have assessed

prolonged exposure to low levels of mercury vapor. One study hints at

subtle neural and behavioral anomalies in dentists, who collectively

use 300 metric tons of mercury in amalgams each year and who often

have two to five times the typical concentration of mercury in their

urine.

 

" I think the methylmercury in fish is probably our least toxic

exposure, " says Haley, who broadcasts the hazards of dental fillings.

 

 

 

Silver-mercury fillings have never been tested for safety. " The

amalgam question will never be solved until we do a clinical trial

like those we do with other medical devices, " says Aposhian.

 

 

 

" It's really unclear what's going on with dental amalgams, " says

Stern, who notes that the issue is complicated by the potential for

panic and lawsuits. " It's a snake pit. "

 

 

 

 

One of the lessons of Minamata is that mercury, like lead, is harder

on fetuses than on the women carrying them, or adults in general. In

the Japanese event, women with no overt symptoms of poisoning gave

birth to severely disabled children. " It was evident there was a major

difference in susceptibility between the developing brain and the

mature brain, " says Philippe Grandjean, an epidemiologist at the

Harvard University School of Public Health. " When we saw serious

poisonings in Minamata, that made us wonder whether mercury could be

like lead. "

 

 

 

Studies of lead have shown that IQ decreases approximately two or

three points for every doubling of prenatal and early postnatal

exposure. To see if mercury has comparable effects, Grandjean, along

with Pál Weihe at the University of Southern Denmark, is conducting

the largest study to date of children's cognition and behavior in a

population routinely exposed to low levels of mercury. His work in the

Faeroe Islands of Denmark includes 1,000 mother-child pairs and spans

almost 20 years. In a typical year, Faeroe islanders consume 1,000

pilot whales, or one whale for every 50 islanders. " They belong to one

of the most fish-eating populations in the world, " says Grandjean.

 

 

 

Whale meat is one of the most highly contaminated seafoods because

whales are at the top of the food chain. Even so, the mercury content

of whale meat is considerably lower than that of the hypertoxic

Minamata fish. An earlier study of shark eaters in New Zealand

suggested that relatively high levels of mercury in a mother's hair

during pregnancy correlated with a loss of three IQ points in her

child. High levels, in that study, were identified as six parts per

million and above in the hair shaft.

 

 

 

Grandjean gave a battery of sophisticated cognitive and developmental

tests to the Faeroese children when they were 7 and 14. His results

indicate that IQ drops 1.5 points for every doubling in prenatal

exposure to mercury. The 2000 National Research Council report

concluded that the risk documented by Grandjean " is likely to be

sufficient to result in an increase in the number of children who have

to struggle to keep up in school. "

 

" We learned there is a response at low levels, " says Grandjean. " It's

not a huge loss, but it's certainly not negligible. "

 

 

 

Yet in another large, long-term epidemiological study conducted on the

Seychelles Islands in the Indian Ocean, Clarkson has so far found no

effect on neurological development from prenatal exposure to low

levels of mercury in seafood. " We can't exclude effects from 20 parts

per million or even 12 parts per million, " he notes. But he concludes

there is no graded risk that extends to the lowest exposure levels.

 

 

 

The 2000 research council report evaluated the Faeroe, Seychelles, and

New Zealand studies and recommended that the EPA set safety standards

based on Grandjean's more sobering findings. The agency did. Then, for

good measure, it added a 10-fold uncertainty factor—a safety margin to

protect against scientific unknowns and individual differences in

response to a toxin. The uncertainty factor lowers the threshold to a

figure of 5.8 micrograms per liter of blood and 1.2 parts per million

in hair.

 

 

 

The problem with safety factors is that they create a toxicological

limbo between demonstrably harmful doses and levels that have been

declared safe. Thus, when Centers for Disease Control surveys find

that one in 12 American women of childbearing age—8 percent—have blood

mercury levels above the safety threshold, the implications aren't

clear, either for them or for the children they bear. Epidemiologist

Tom Sinks says, " It doesn't tell us there's a hazard. "

 

 

 

" The whole idea of a safety factor is to protect people, " Clarkson

says. " You can't turn it around to use as an indication of who's at

risk. If you're just above it, you aren't necessarily in trouble. "

 

 

 

That kind of hedging, along with disagreement among population

studies, leaves regulators with plenty of wiggle room. The FDA, for

example, uses a more relaxed safety standard for mercury based on

studies from the 1970s and 1980s. Where the EPA safety level for daily

exposure is 0.1 microgram per kilogram (about 2.2 pounds) of body

weight, the FDA's standard is about 0.4 microgram per kilogram per

day. The difference is four times as much mercury.

 

 

 

Concern about early exposure to mercury doesn't end at birth. Until

recently, many infants received regular injections of mercury on a

state-mandated, medically sanctioned schedule. The mercury came from a

compound called thimerosal that has been used as a preservative in

vaccines and other medicines since the 1930s. In 1999 the FDA

recommended that thimerosal no longer be used in pediatric vaccines,

and manufacturers have removed it from all but the influenza vaccine.

But some scientists and many more aggrieved parents are convinced that

thimerosal in childhood vaccines has already caused, or at least

catalyzed, the U.S. epidemic of autism.

 

 

 

An estimated 400,000 Americans today have autism, a once rare

neurological disorder characterized by social withdrawal, difficulty

communicating, and involuntary, repetitive movements. Although the

exact numbers are in dispute, the rate of diagnosis seems to have

climbed sharply in the last decade. In California the incidence of

autism was six times higher in 2002 than in 1987.

 

 

 

During that period, federal health officials added four new kinds of

vaccines to the childhood immunization schedule, and the amount of

mercury routinely administered to infants in the first six months of

life more than doubled. Throughout the 1990s, a 3-month-old baby might

receive as much as 63 micrograms of mercury in a single visit to a

doctor—roughly 100 times the daily EPA safety level. By the age of 6

months, properly immunized children were exposed to at least 188

micrograms of mercury in a series of at least nine injections.

Although the 1999 FDA action minimized such exposure, some infant flu

vaccines still contain 12.5 micrograms of mercury per dose—more than

10 times the daily EPA safety level for a 20-pound baby.

 

FAVORED FISH

 

 

 

An inspector at a California cannery in 1953 spot-checks canned tuna.

In the United States, canned tuna is the third most commonly purchased

food item, after sugar and coffee, based on dollar sales per amount of

shelf space devoted to the product. An EPA study reported the median

amount of mercury, measured in parts per million, in the following

varieties of canned tuna: chunk light: .08 parts per million; canned

albacore tuna: .34 ppm; fresh or frozen tuna: .30 ppm. A 2004 EPA

advisory mentions five types of fish and shellfish that are low in

mercury: shrimp, canned light tuna, salmon, pollack, and catfish. The

advisory also warns consumers not to eat shark, swordfish, king

mackerel, and tilefish because they all contain high levels of mercury.

 

 

Visit www.cfsan.fda.gov/~dms/admehg3.html for the EPA's

recommendations regarding seafood consumption.

 

The EPA's Toxics Release Inventory offers easy access to the most

current information on a range of toxins released in this country:

www.epa.gov/tri.

 

Global Mercury Assessment. A United Nations Report:

www.chem.unep.ch/mercury.

 

 

< www.epa.gov= " " tri<= " " a= " " href= " http://www.epa.gov/tri " />

Environmental Defense, an advocacy group, has prepared a list to help

consumers decide which and how much fish is safe to eat:

www.oceansalive.org.

header_litres

 

 

Toxicological Effects of Methylmercury. National Research Council, 2000.

 

Evidence of Harm. David Kirby. St. Martin's Press, 2005. An

investigation of the medical controversy over the use of mercury in

vaccines.

 

 

Circumstantial evidence also implicates mercury in autism. Some of the

symptoms of autism and mercury poisoning are similar, and Haley has

garnered evidence from hair samples that autistic children do not

clear mercury from their bodies as efficiently as most kids do. They

may have a genetic susceptibility that allows more mercury to

accumulate in their tissues, he says. That could make them more

vulnerable to mercury-laced vaccines and the continuous low-level

exposure from their mothers' dental fillings. " It is amazing to me

that no one has taken the tissue of autistic children to see if there

is excess mercury there, " Aposhian told a committee at the Institute

of Medicine in Washington, D.C., last year. " That's one thing that

really has to be done. "

 

 

 

There are other sources of uncertainty. The form of mercury in

thimerosal—an organic compound called ethyl mercury—is the least

studied of all mercury's incarnations. When scientists argue about its

toxicity, they typically rely on data from methylmercury, which may

not be an equivalent form of exposure. Experts even disagree about

whether ethyl mercury can cross the blood-brain barrier. (It probably

does.) " There are no good ways to measure ethyl mercury in tissue, "

toxicologist Polly Sager of the National Institute of Allergy and

Infectious Diseases told the Institute of Medicine committee.

 

 

 

The Institute of Medicine concluded last May that no claim could be

made for a causal link between mercury-laced vaccines and autism, but

several independent researchers had complained that their access to

federal vaccine databases, which could provide evidence of a link, had

been repeatedly blocked. A few scientists, including Haley and

neuropharmacologist Richard Deth of Northeastern University in Boston,

continue to study possible mechanisms for the connection. Deth

reported last year, for example, that in human nerve cells thimerosal

blocks a chemical reaction called methylation that is critical to gene

activity and that is also disabled by exposure to lead.

 

 

 

 

The report that first triggered worries about a connection between

vaccines and autism was published in the British medical journal The

Lancet in 1998. It described eight children whose behavioral problems

surfaced within two weeks of receiving the measles-mumps-rubella

vaccine. The Lancet and most of the article's coauthors ultimately

disowned the study because its lead author had not divulged that he

was also being paid to conduct research for parents seeking to sue

vaccine manufacturers. Nonetheless, the number of parents in the

United Kingdom willing to immunize their babies with the vaccine

dropped from 90 percent in 1998 to less than 80 percent in 2004.

 

 

Health officials in the United States addressed suspicions about

immunization by recommending that thimerosal be removed from pediatric

vaccines. Thimerosal might yet prove harmless, they reasoned, but the

threat to public health posed by a drop in immunization rates was not

worth risking. The same balance of risks exists regarding the issue of

mercury in fish. The current Federal Dietary Guidelines Advisory

Committee Report recommends at least two fish meals a week. Fish are

high in omega-3 fatty acids, which have proven benefits in preventing

heart disease, the number one killer in the United States. " We know

mercury is a hazardous substance, " says the CDC's Sinks. " We know that

less is better than more. We know that fish and shellfish are the

principal source of methylmercury. But we also know that fish and

shellfish are pretty nutritious food: high in protein, high in

vitamins. They contain healthy fats. "

 

 

 

But troubling evidence suggests that methylmercury in fish might cause

heart disease. A seven-year study of more than 1,800 men in Finland

showed that those who ate the most fish doubled their risk of heart

attack compared with those whose diets had less fish. The same men

showed the same increase in risk for death from coronary and

cardiovascular disease. And Grandjean's Faeroe Islands study found

that prenatal exposure to mercury caused significant increases in

blood pressure among 7-year-olds.

 

 

 

The most troubling aspect of this controversial heart-disease data is

that deleterious effects occur at mercury-exposure levels equal to or

lower than for any other toxicological outcome, including the subtle

neurological symptoms in the Faeroe Islands study. In Grandjean's most

recent examination of 14-year-olds, he has found a doubling of certain

neurotoxic effects at five parts per million in hair samples. In the

Finnish study, the men with the doubled risk of heart attack had hair

samples with only two parts per million of mercury. They were eating

little more than an ounce of fish a day. Stern speculates that 10

percent of American men may already eat enough fish to raise their

risk of heart attack.

 

 

 

" There's this interaction between mercury and fish oils that makes it

very complicated because they both come from the same place, " he says.

 

 

The National Research Council report noted that low levels of mercury

contamination might also harm the immune and reproductive systems. And

mercury is being investigated in relation to Alzheimer's, Parkinson's,

attention deficit disorder, and multiple sclerosis. But many low-level

developmental effects will be difficult to identify, Stern says,

because the compromised organ or function still falls within the range

of normal. The intelligence scores of the Faeroese children, for

example, were not pathologically low; it took rigorous statistical

analyses to prove they were simply lower than they would have been

otherwise. Likewise heart disease, as the nation's leading killer, has

plenty of confounding variables. " You're looking to pull a signal out

of a lot of noise, " Stern says.

 

 

 

That signal might soon get a lot stronger. While mercury contamination

is no longer a threat in most childhood vaccines, it is likely to get

worse in fish. " Because of the beneficial effects of fish consumption,

the long-term goal needs to be a reduction in the concentrations of

[methylmercury] in fish rather than a replacement of fish in the diet

by other foods, " said the council's report.

 

 

 

That goal is nothing less than unrealistic.

 

 

 

Mercury was a naturally occurring element in Earth's atmosphere long

before coal-fired generators, medical-waste incinerators, and

chlor-alkali plants put more there. Some mercury escapes into the air

when volcanoes erupt and mountains erode. It stands to reason that

mercury has been accumulating in the flesh of fish, shellfish, and

marine mammals since humankind began eating them—which is most likely

why humans have a protein called metallothione to help detoxify

mercury and other heavy metals.

 

 

 

But human activities have caused the mercury content of the atmosphere

to rise by 1.5 percent a year, according to the U.S. Geological

Survey, and the problem is global. Roughly half of the mercury

deposited on U.S. soils and streams comes across the Pacific from

Asia. Last year a United Nations report found that the toxin can

travel thousands of miles in the atmosphere to contaminate pristine

and uninhabited areas, such as the Arctic. Still, the United States

has so far balked at attempts by the United Nations Environment

Programme to draw up a binding protocol to reduce mercury pollution

worldwide.

 

 

 

In the 1990s the United States made considerable progress in curbing

emissions from incinerators for medical and municipal waste. Yet the

number of states issuing local fishing advisories went from 27 to 48

in the last decade. Due to heightened concern, advisories for mercury

are increasing faster than for any other pollutant.

 

 

 

The EPA is in the final stages of formalizing a rule that would limit

emissions from coal-fired utilities, which produce 42 percent of the

nation's domestic mercury pollution. The agency's standing proposal

has been for a 70 percent reduction in mercury emissions by 2018. But

environmentalists argue that the Clean Air Act calls for a 90 percent

reduction by 2008. In 1992 the Natural Resources Defense Council sued

the EPA for not maintaining the act's standards, and in 1994 the

parties reached a settlement. Under the terms of the agreement, the

agency is required to issue a cleanup rule this month.

 

 

 

 

Visit www.cfsan.fda.gov/~dms/admehg3.html for the EPA's

recommendations regarding seafood consumption.

 

The EPA's Toxics Release Inventory offers easy access to the most

current information on a range of toxins released in this country:

www.epa.gov/tri.

 

Global Mercury Assessment. A United Nations Report:

www.chem.unep.ch/mercury.

 

 

< www.epa.gov= " " tri<= " " a= " " href= " http://www.epa.gov/tri " />

Environmental Defense, an advocacy group, has prepared a list to help

consumers decide which and how much fish is safe to eat:

www.oceansalive.org.

header_litres

 

 

Toxicological Effects of Methylmercury. National Research Council, 2000.

 

Evidence of Harm. David Kirby. St. Martin's Press, 2005. An

investigation of the medical controversy over the use of mercury in

vaccines.