Lead toxicity: bones tell the real story.

Lead toxicity: Bones tell the real story

Researchers from four collaborating institutions say they have data demonstrating for the first time that blood measurements may significantly underestimate the lead stored in a child's body. Moreover, these data suggest that blood-lead concentrations currently deemed "acceptable" for young children by the Centers for Disease Control "offer little or no margin of safety," says study director John F. Rosen of Albert Einstein College of Medicine in New York City.

Rosen's data come from an instrument his team developed, which beams low-energy (10.5-keV) X-rays at the shin bone. This radiation excites electrons in the "L" shell -- the second electron shell -- of bone-bound lead. As excited electrons relax, they fluoresce, emitting X-rays at a different wavelength. These emissions offer a noninvasive gauge of lead in the skeleton, where up to 95 percent of this metal is stored.

The researchers present their bone-lead measurements of 59 symptomless children, aged 1 to 6 years, in the January PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. All children had blood-lead levels between 22 and 55 micrograms per deciliter ([mu]g/dl), with an average of 34 [mu]/dl. These findings showed that even some children with blood-lead levels in the so-called acceptable range -- 22 to 26 [mu]g/dl -- carried lead levels of more than 15 [mu]g per gram of bone. This is roughly three times what has been reported typical of healthy children, Rosen notes. More worrisome, he found that those children with blood-lead values of about 39 [mu]g/dl carried an average 37 [mu]g/g of lead in their bones -- a concentration exceeding the 19 to 27 [mu]g/g usually found in adults after decades of cumulative exposure, and comparable to levels in lead workers.

The standard technique for measuring lead in bone is painful and cumbersome, and requires collecting every ounce of urine over an 8-hour period. X-ray fluorescence is simpler and painless, Rosen says. It takes only 16.5 minutes and, in combination with blood-lead assays, yields comparably reliable diagnoses of children requiring chelation therapy to remove dangerous lead excesses, he adds.

"I'm very impressed with the enormous potential of the technique for assessing lifetime exposure to lead," says Bruce Fowler, the University of Maryland-Baltimore toxicologist who chairs a new National Academy of Sciences committee to evaluate effects of low-level lead. However, he adds, there is still concern over the safety of X-ray fluorescence and the quality of data it offers. Both issues, he notes, are slated for thorough review next month at an international meeting in Columbia, Md., sponsored by the National Institute of Environmental Health Sciences.