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Lead exposure on the job tied to brain damage

Posted on: Friday, 2 June 2006, 13:28 CDT

By Anne Harding

NEW YORK (Reuters Health) - The cumulative exposure to lead can cause degeneration of the brain's white matter, which may explain the previously reported progressive decline in brain function in these individuals, the results of a study of organic lead workers shows.

The brain's white matter contains nerve fibers, with many of these fibers, or "axons," surrounded by substance called myelin, the source of the whitish appearance. Myelin acts as an insulator and it increases the speed of transmission of all nerve signals.

The more a worker was exposed to lead on the job, as measured by the amount of the metal found in the bone, the worse the brain damage was many years later, Dr. Walter Stewart of Geisinger Health System in Danville, Pennsylvania and colleagues report in the journal Neurology.

"It's one of the first studies that shows that an exposure in the distant past can affect the brain and cause what we call progressive changes," Stewart told Reuters Health in an interview.

Stewart and his team had previously found a link between past lead exposure and progressive decline in mental function. To investigate whether lead exposure might also be associated with structural changes in the brain, the researchers conducted MRI brain scanning in 532 men who had worked manufacturing lead gasoline additives at a New Jersey chemical plant. On average, the last lead exposure the men had experienced on the job was 18 years previously.

The greater a worker's lead exposure, the researchers found, the more severe and extensive were the white matter abnormalities in the brain. Greater lead exposure was also associated with a smaller brain volume, while specific regions of the brain also appeared to have shrunk after greater lead exposure.

There are a number of possible explanations for the findings, Stewart and his team note. Lead exposure could accelerate normal aging-related changes in the brain.

Exposure to the metal also has been tied to high blood pressure, which has been linked to brain deterioration, including white matter abnormalities.

And finally, lead itself could have a direct harmful effect on brain cells. Stewart said he and his colleagues are hoping to separate out these potential causes in future research.

It is difficult to extrapolate from the current findings to the general population, Stewart said, because these men were exposed to both organic lead and inorganic forms of lead. Most of the subjects, who lived through the 1950s, 1960s or 1970s, were also exposed to lead in gasoline emissions or paint.

"There's a common belief, although nobody ever proved it, that inorganic lead does not affect the adult brain," Stewart said. However, he added, "We have evidence to suggest that inorganic lead is probably toxic to adult humans" as well as to children.

In an accompanying editorial, Drs. Andrew S. Rowland of the University of New Mexico in Albuquerque and Robert C. McKinstry of the Washington University School of Medicine in St. Louis call the current study "a major contribution" to the knowledge of the long-term effects of lead.

SOURCE: Neurology, May 23, 2006.

Source: REUTERS

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