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Lead Exposure and Other Pollutants Linked To Mental Decline
NEW YORK (AP) - Could it be that the “natural” mental decline that
afflicts many older people is related to how much lead they absorbed decades
before?
That’s the provocative idea emerging from some recent studies, part
of a broader area of new research that suggests some pollutants can cause harm
that shows up only years after someone is exposed.
The new work suggests long-ago lead exposure can make an aging person’s
brain work as if it’s five years older than it really is. If that’s
verified by more research, it means that sharp cuts in environmental lead levels
more than 20 years ago didn’t stop its widespread effects.
“
We’re trying to offer a caution that a portion of what has been called
normal aging might in fact be due to ubiquitous environmental exposures like
lead,” says Dr. Brian Schwartz of Johns Hopkins University.
“
The fact that it’s happening with lead is the first proof of principle
that it’s possible,” said Schwartz, a leader in the study of lead’s
delayed effects. Other pollutants like mercury and pesticides may do the same
thing, he said.
In fact, some recent research does suggest that being exposed to pesticides
raises the risk of getting Parkinson’s disease a decade or more later.
Experts say such studies in mercury are lacking.
The notion of long-delayed effects is familiar; tobacco and asbestos, for
example, can lead to cancer. But in recent years, scientists are coming to
appreciate
that exposure to other pollutants in early life also may promote disease much
later on.
“
It’s an emerging area” for research, said Dr. Philip Landrigan
of the Mount Sinai School of Medicine in New York. It certainly makes sense
that
if a substance destroys brain cells in early life, the brain may cope by drawing
on its reserve capacity until it loses still more cells with aging, he said.
Only then would symptoms like forgetfulness or tremors appear.
Linda Birnbaum, director of experimental toxicology at the U.S. Environmental
Protection Agency, said infant mice exposed to chemicals like PCBs show only
very subtle effects in young adulthood. But more dramatic harm in areas like
movement and learning appears when they reach old age.
Animal studies also show clear evidence that being exposed to harmful substances
in the womb can harm health later on, she said. For example, rodents that encounter
PCBs or dioxins before birth are more susceptible to cancer once they grow
up.
Studying delayed effects in people is difficult because they generally must
be followed for a long time. Research with lead is easier because scientists
can
measure the amount that has accumulated in the shinbone over decades and get
a read on how much lead a person has been exposed to in the past.
Lead in the blood, by contrast, reflects recent exposure. Virtually all Americans
have lead in their blood, but the amounts are far lower today than in the past.
The big reason for the drop: the phasing out of lead in gasoline from 1976
to 1991. Because of that and accompanying measures, the average lead level
in the
blood of American adults fell 30 percent by 1980 and about 80 percent by 1990.
That’s a major success story for environmentalists. But work by Schwartz
and Dr. Howard Hu of the University of Michigan suggests that the long-term
effects of the high-lead era are still being felt.
In 2006, Schwartz and his colleagues published a study of about 1,000 Baltimore
residents. They were ages 50 to 70, old enough to have absorbed plenty of lead
before it disappeared from gasoline. They probably got their peak doses in
the 1960s and 1970s, Schwartz said, mostly by inhaling air pollution from vehicle
exhaust and from other sources in the environment.
The researchers estimated each person’s lifetime dose by scanning their
shinbones for lead. Then they gave each one a battery of mental ability tests.
In brief, the scientists found that the higher the lifetime lead dose, the
poorer the performance across a wide variety of mental functions, like verbal
and visual
memory and language ability. From low to high dose, the difference in mental
functioning was about the equivalent of aging by two to six years.
“
We think that’s a large effect,” Schwartz said.
Hu and his colleagues took a slightly different approach in a 2004 study
of 466 men with an average age of 67. Those men took a mental ability test
twice, about four years apart on average. Those with the highest bone lead
levels showed more decline between exams than those with smaller
levels, with the effect of the lead equal to about five years of aging.
Nobody is claiming that lead is the sole cause of age-related mental decline,
but it appears to be one of several factors involved, Hu stressed.
If so, it would join such possible influences as high blood pressure, diabetes,
stroke, emotional stress and maybe education level, said Bradley Wise of the
National Institute on Aging. Nobody knows exactly what causes mental decline
with age, he said.
Although the studies by Hu and Schwartz suggest lead is involved, Wise and
others say they don’t prove the link.
“
I think many things impact how we age, but I think right now it’s maybe
premature to be giving lead a huge role in our age-related cognitive decline,” said
Dr. Margit L. Bleecker, director of the Center for Occupational and Environmental
Neurology in Baltimore. Still, she called the lead hypothesis “a very interesting
idea” deserving more study.
Others were more impressed.
“
The new evidence from these studies should concern people” said epidemiologist
Andrew Rowland of the University of New Mexico. “These two research groups
are finding adverse effects on the aging brain at low levels of lead exposure.
More work needs to be done, but these studies are raising important questions.”
In any case, scientists still face some basic mysteries about the delayed
effects of lead. For example, when does it actually harm the brain? Does
a high level
in the shinbone merely identify those who were the most harmed by chronic
exposure decades ago? Or does lead in the bone continue to do its dirty work
over a
lifetime, leaching into the bloodstream and continuously hammering the brain?
Just how lead impairs brainpower is still a mystery. And so is the question
of whether anything can be done to help people who have absorbed a lot of
lead over
a lifetime.
A medical procedure called chelation can remove lead from the body, but it
wouldn’t
help in this case, said experts.
For younger people, prevention is a clearer strategy, Hu said. He called
for tougher federal standards on lead exposure in the workplace.
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