United States researchers suggest long-ago lead exposure can make an aging person's brain work as if it's five years older than it really is.

"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, a leader in the study of lead's 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.

"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.

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 is 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 per cent by 1980 and about 80 per cent by 1990.

But work by Schwartz and Dr. Howard Hu of the University of Michigan suggests 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.

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.

Various factors

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.

"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.

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?

"I think that both things are happening," Schwartz said, though he suspects most of the damage occurred in the past, during years of higher exposure. Hu's suspicions are similar.

Prevention strategy

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.

Schwartz noted that most Americans younger than 30 have gotten much less lead from the environment than the men in his study did. And Hu hopes that the lead effect will peter out in the future.

However, Hu points out that there's still lead in the environment, and exposure remains especially high in many developing countries. And citing evidence that lead can cross the placenta, he says women who grew up in the 1970s might dose their fetuses with the metal.

"Kids who grew up in the 21st century have a lot less to worry about" than their elders, Hu said. But "it's hard for me to be totally optimistic the current generation is completely scot-free."