By Janet Raloff
From San Diego, at a meeting of the Society of Toxicology
When Arlene L. Weiss and her colleagues found that urban house dust tends to contain more lead the closer it is to a frequently opened window, they reasoned that most of the heavy metal arrives from outside. Their new survey now confirms that street grit is the probable source of lead in urban homes and that flaking paint from overpasses and bridges is a major contributor.
The researchers sampled soil and street sweepings from 255 sites throughout New York City’s five boroughs. The highest lead contamination occurred directly beneath elevated train trestles, where concentrations of the metal routinely reached many thousands of parts per million (ppm). The federal limit for lead in U.S. soil is 400 ppm.
Samples of outdoor dust were much less tainted just two to three blocks away from bridges and trestles, with lead loads in the range of 200 to 500 ppm, notes Weiss, a consulting toxicologist with Environmental Medicine in Westwood, N.J. Still, her team found, even among outdoor soil samples taken where there was no apparent structural source of lead, 20 percent exceeded the federal limit.
The federal limit for lead in house dust is 40 micrograms per square foot of swabbed area. Weiss and her colleagues report in the February Environmental Research that this amount can be exceeded on surfaces near windows in New York City after only 3 weeks of dust accumulation. Frequent cleaning of interior surfaces is necessary, they argue, to limit children’s indoor exposure to the outdoor pollutant.
