There's no doubt that benzene, a widely used industrial chemical, can be harmful. But a new study raises concerns about even very low levels of exposure: Workers in Chinese shoe factories who inhaled less than 1 part per million (ppm) of benzene--an exposure considered safe under U.S. occupational guidelines--had fewer white blood cells than did unexposed workers.
Benzene is ubiquitous. People are commonly exposed from secondhand cigarette smoke, gasoline vapors, and air pollution, although typically only on the order of parts per billion. In industrial settings, where benzene is used as a solvent and in chemical manufacturing, a higher risk of leukemia led the United States in 1987 to regulate the maximum allowable workplace exposure at 1 ppm of benzene averaged over 8 hours.
To determine whether blood cells are affected at even smaller exposures, an international group of researchers compared 250 workers exposed to benzene-laden glues in two shoe factories in China to 140 unexposed workers who sew clothes in other Chinese factories. The researchers carefully gauged benzene exposure by taking urine samples and testing air samples in the factories, as well as at each worker's home. After 16 months, they took blood samples.
As expected, workers exposed to benzene at levels of 1 ppm and higher had fewer white blood cells, such as granulocytes and B cells, than did unexposed workers. But this also held true for the 109 workers exposed to less than 1 ppm benzene, even after controlling for smoking and other potential confounding factors. These workers had on average 15% to 18% fewer granulocytes and B cells than unexposed workers--raising concerns about effects on bone marrow, notes study co-author Qing Lan of U.S. National Cancer Institute (NCI) in Bethesda, Maryland. Additional experiments suggested that exposure to less than 10 ppm impaired proliferation of the progenitor cells that give rise to blood cells in some workers, they report in the 3 December issue of Science.
"It really breaks new ground on the potential effects of low levels" of benzene, says toxicologist Bernard Goldstein of the University of Pittsburgh's School of Public Health. The findings may lead to demands for lowering the benzene exposure standard, says geneticist Gilbert Omenn of the University of Michigan Medical School in Ann Arbor: "This paper should cause a stir in occupational and environmental health circles."