2. Danger signs
3. Mercury mystery
4. Safety: in eye of
A man tends his nets at Mahe, in the Seychelles
Islands. An ongoing study says he's not poisoning his family with
his catch. Photo: NOAA
studies on these remote islands have shown the danger -- or is that
the safety? -- of low-level mercury ingestion.
Farming Atlantic salmon, Faroe Islands,
Denmark, 1991. Photo: NASA
The science of mercury exposure rests on three studies of the association
between mercury levels during pregnancy and the performance of children
on psychological tests, in three remote reaches of the ocean:
Seychelles (Indian Ocean):
A study begun in 1989 looked for an association between mercury
exposure and behavior, motor skills, memory and thinking in 779
children born to mothers who averaged a dozen fish meals a week.
At the latest follow-up, around age 9, (see "Prenatal Methylmercury
Exposure..." in the bibliography), higher
mercury exposure was associated with two test results. Boys, but
not girls, were slower at one movement test, but only when using
their less capable hand. Boys and girls exposed to more mercury
were rated as less hyperactive by their teachers. The authors concluded,
"These data do not support the hypothesis that there is a neurodevelopmental
risk from prenatal methylmercury exposure resulting solely from
ocean fish consumption."
New Zealand (South Pacific): A study of 237 children
found no association between test performance and mercury. But when
data from one child, whose mother had by far the highest mercury
level, was ignored, scores on 6 of 26 tests did link mercury levels
to lower performance. We don't know which analysis is more credible,
but then, neither do the study authors, who concluded, "We do not
believe it is possible to say based on current data, whether results
including or eliminating this child are nearer to the truth" (see
"Influence of Prenatal Mercury..." in the bibliography).
Faeroes Islands (North Atlantic): Scientists
from Denmark studied a traditional population that ate some fish,
and occasionally feasted on mercury-contaminated whales. By age
7, 614 children with the most complete mercury-exposure data had
lower scores in 8 of 16 tests of language, memory and attention
(see "Neurotoxic Risk..." in the bibliography).
The results, unfortunately, did not add up.
The Faeroes study found brain problems, but the New Zealand study
was equivocal, and the Seychelles study exonerated mercury.
Everybody we talked to agreed that the three
studies were good science. But how could similar, high-quality
studies produce such conflicting results?
Genetics: Perhaps different genes gave these three groups a differing
ability to compensate for mercury, or to dump it from their bodies.
Speaking about the contrasting results from the Seychelles and the
Faeroes, H. Vasken Aposhian, professor of pharmacology and molecular
biology at the University of Arizona, says, "The explanation could
be this is two genetically radically different groups of people.
It's very possible that there are polymorphisms [genetic differences]
in these groups that caused them to differ in how they react to
the toxicity of methylmercury." Aposhian, who served on the National
Research Council mercury review panel that reported in 2000, says,
"Without realizing it, there could be a damn good genetic reason
why both results could be right."
Technique: Perhaps the technique for measuring
mercury exposure affected the results, says Gary Myers, a professor
of pediatric neurology at the University of Rochester Medical Center,
who works on the Seychelles study. Myers notes that the Faeroes
study measured mercury in umbilical-cord blood, while "every other
study that has ever been done has used hair as a biomarker... There
are reasons to think that hair is better ... it's thought that organic
mercury enters the hair the same way as it enters the brain, through
the blood-brain barrier."
Diet: The diet in the Faeroes
is dull and Scandinavian, observes Myers, while people in the Seychelles
eat more vegies, fruits and spices, all sources of anti-oxidants
and other healthy chemicals. "If you have good enough nutrition,
that may overcome the mercury toxicity," he suggest. "Or maybe there's
an interaction with a trace element. Selenium is known to lower
the toxicity of mercury. There are number of possible explanations."
again: Could the source of the mercury affect the results?
While people in the Seychelles average one or two doses of fish-'n-mercury
on the average day, in the Faeroes, most of it comes from occasional
feasts on pilot whales. "It's pulsed, once a month," says Gochfeld,
the New Jersey mercury expert. "Even for the same amount of mercury,
the fact that you are getting it all at a single dose may have greater
consequence than if you get an equal amount, in fish, day after
day. Maybe you can't clear it out." (This explanation, however,
is rejected by Phillip Grandjean, the Danish scientist who lead
the Faeroes study. In the publication cited above, he said that
removing the children with the most erratic exposure did not substantially
change the results.)
Multiple poisons: The blubber of pilot whales
contains lots of the persistent pollutant polychlorinated biphenyl
(PCB). Could a witches' brew of PCBs and mercury explain why similar
mercury exposure was dangerous in the Faeroes but not in the Seychelles?
Perhaps, says Gochfeld. "PCB exposure is a ticklish issue. there
are people like me who believe that methylmercury and PCB interact."
However, as he notes, when the Faeroes study statistically eliminated
the effects of PCBs, mercury still seemed damaging.
Recent work at the University of Illinois,
however, indicates that PCBs and mercury are a dangerous duo. In
a study probing the effects of multiple chemicals on Wisconsin people
who eat a lot of fish, young rats slipped and fell more if their
mothers had been exposed to PCBs and methylmercury, rather than
mercury alone. "Our findings suggest that we should seriously consider
the possible impact of additive toxic effects on human health,"
said study leader Susan Schantz, a professor of veterinary biosciences at the
University of Illinois at Urbana-Champaign.
So how do you come up with a safe
standard for environmental mercury?