Study ties lead, mental decline

Exposure long ago a factor in old age

Andrew Todd used an acrylic leg filled with human bone to calibrate the bone lead measurement system. A study suggests lead exposure can make a person's brain work as if it is older than it is. (John Smock/associated press)

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 only shows up 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 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," said 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 such as mercury and pesticides may do the same thing, he said.

Some recent research does suggest that being exposed to pesticides raises the risk of getting Parkinson's disease a decade or more later. Specialists 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 US Environmental Protection Agency, said infant mice exposed to chemicals such as 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.

Studying delayed effects in people is difficult because they 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, the average lead level in the blood of American adults fell 30 percent by 1980 and about 80 percent by 1990.

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. 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.