Despite an ongoing national effort to limit exposure to lead, most adults in the United States have accumulated a substantial amount of this noxious metal in their bones. A new Harvard study ties this lurking danger to an increased risk of cataracts, the leading cause of age-related blindness in the world.
The risk is particularly high for older people who breathed lots of lead-polluted air before nonleaded gas became widespread. Now, the largest source of lead in this country comes from older houses.
“Generalized low-lead exposure, along with pockets of higher exposure, remain commonplace,” says Debra Schaumberg, an assistant professor of medicine at the Harvard Medical School. “In the U.S. more than 80 percent of homes built before 1980 are contaminated by lead-based paint and/or leaded water pipes.”
Schaumberg, along with colleagues at Brigham and Women’s Hospital, the Harvard School of Public Health, and Schepens Eye Research Institute, all in Boston, tracked 795 older men for nine years. They compared lead levels in their shinbones and kneecaps with the results of regular eye examinations. Those with the highest levels of the metal had about three times greater risk of getting cataracts than those with the lowest levels.
“These are the first data collected to suggest that accumulated lead exposure experienced by adults in the U.S. may be an important, unrecognized factor for cataract,” Schaumberg and her colleagues note in the Dec. 9 issue of the Journal of the American Medical Association.
Although the research was conducted with men, the scientists believe that an equal risk exists for women. “We think the same basic biological mechanism is at play, so there’s no reason to expect that there would be any difference in the findings for women,” Schaumberg notes. “We are planning to study the lead situation in women.”
Lead in your eye
Evidence from other studies indicates that lead accumulated in bones contributes to a number of chronic problems including high blood pressure, decreased kidney function, and problems with memory and learning. Also, more lead has been measured in the eye lenses of people with cataracts than with clear lenses. To carry things further, Schaumberg and her team recruited 795 men 60 years and older from another ongoing study of healthy males.
Every three to five years these men underwent extensive physical examinations that included eye evaluations. Between 1991 and 1999, the amount of lead in their shinbones (tibias) and kneecaps was also measured. Analysis of this information clearly shows that those with the most lead accumulated in their shinbones showed the highest risk of cataract.
Lead in their kneecaps showed a like result. Shin lead is considered more significant because lead stays longer, making it a better long-term marker for lead exposure. The lead works its way from bones, via the blood stream, to the lenses. Here it interacts with crystalline proteins in a way that causes a loss of lens transparency.
Centuries ago, according to the American Medical Association, people believed the fanciful idea that whiteness behind the pupil consisted of a waterfall, or cataract, descending from the brain. Thus, the name. In fact, the white is due to changes in delicate protein fibers in the lens, and these are what the lead affects.
Keep the lead out
According to the researchers, cataracts account for 40 percent of all blindness worldwide. In the United States, costs of cataract surgery make up the largest single item in the Medicare budget. In many developing countries, where lead exposure is higher than it is here, cataracts loom as an important unrecognized public health problem.
For their own defense, Schaumberg advises individuals to check water that comes out of home faucets for lead contamination. Those involved with repairs and renovations in homes with lead paint should make special efforts to keep their exposure low. And so should those engaged in pastimes such as working with stained glass windows and target practice in indoor firing ranges.
“Low,” Schaumberg points out, “means even lower than current federal occupational standards. Men in our study who developed cataracts had lead levels substantially below these standards.”
Her best advice: Get your blood lead level measured and try to keep it below 10 micrograms per deciliter, the maximum currently recommended for children.
Howard Hu, professor of occupational and environmental medicine at the Harvard School of Public Heath and a participant in the research, thinks federal standards should be lowered to below this level. “Our study adds to other evidence that current federal regulations for protecting adult workers from lead toxicity are out of date,” he says. “Those regulations allow adults to have lead burdens three to four times higher than those we saw that posed risk of cataract.”