"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. 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.

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 because lead was phased out of gasoline from 1976 to 1991.

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.

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.

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.