A lifetime of low-level exposure to lead in the environment may contribute to mental decline as people age, a study of older men suggests.

Researchers found that among the 466 men they followed, those with higher lead levels in their bone at the study’s start showed a steeper decline over time on a standard test of mental acuity.

Lead is a toxic metal that is present in the air, soil and water, though public health efforts in recent decades to reduce environmental levels - by taking lead out of gasoline and paints, for example - have cut Americans’ lead exposure.

Unborn babies and young children are especially vulnerable to the toxic effects of lead, as even low-level exposure can damage the developing brain and cause learning and behavioral problems. Adults with persistently high blood levels of lead can face health consequences as well - including high blood pressure, and damage to the kidneys, brain and nerves.

But compared with what’s known about the dangers of lead for children and people exposed to high lead levels at work, relatively little is known about the cumulative effects of “general-population levels of exposure,” according to Dr. Marc G. Weisskopf of the Harvard School of Public Health in Boston, the lead author on the new study.

To look into the question, Weisskopf and his colleagues used a special x-ray technique to measure study participants’ lead levels in bone, an indicator of cumulative lead exposure. The team also assessed the men’s performance on a standard test of memory, attention, language and other mental functions. The tests were given twice, with an average of 3.5 years in between.

The researchers found that the higher the men’s lead levels in the kneecap were at the outset, the greater the decline in their test scores over time, according to a report in the American Journal of Epidemiology.

For the men in the study, who were 67 years old on average, the biggest source of lead exposure was probably the lead that was once used in gasoline and spread into the air, Weisskopf said. Drinking water and paints used in older homes are other possible sources.

When a person is exposed to lead, part of it is laid down in the bones, where it remains for the long term, the researchers explain. “Looking at lead levels in bone gives you a window into decades of the past,” Weisskopf said.

The problem with that bone-dwelling lead, he noted, is that it can be released into the bloodstream, making it free to damage body tissue. Sequestered lead may be released during periods in which the bone is undergoing change, such as during older age and in the bone-thinning disease osteoporosis.

In this study, every increase of 20 micrograms of lead per gram of bone mineral was related to a quarter-point loss on a man’s mental test score over time - with factors such as age, education and alcohol intake taken into account.

That is a “subtle” effect, Weisskopf said, but he added that the question is whether that subtle effect on a test score reflects only a minor injury to the brain, or more significant damage. He and his colleagues are continuing to follow the study group to see whether the impact of lead exposure on mental function becomes greater over time.

Even though public health efforts have done a “great job” at reducing environmental lead levels, Weisskopf noted, “there’s still reason to maintain our concern about the effects of lead.”

SOURCE: American Journal of Epidemiology, December 2004.