Long-ago lead exposure may hasten aging’s toll

updated 9:18 a.m. ET Jan. 28, 2008

NEW YORK - 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.

Long-delayed effects
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.

John Smock / AP Andrew Todd examines results from the bone lead measurement system. Scientists can measure the amount of lead that has accumulated in the shinbone over decades to get a read on how much lead a person has been exposed to in the past.

“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.

Switch to unleaded gasoline
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.

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