Benzene can damage human blood cells even when inhaled at levels below the U.S. workplace exposure limit of 1 ppm over eight hours, evidence from researchers in China and the U.S. suggests.

"The implications of this report are far-reaching," says Robert Snyder, professor of toxicology at Rutgers University, New Brunswick. Not only does it encourage a rethinking of the current occupational standard, it also suggests that environmental exposure from sources such as cigarette smoke, gasoline, and car exhaust "will have to be taken more seriously."

The findings were reported in Science [306, 1774 (2004)] by an international team including Qing Lan and Nathaniel Rothman at the National Cancer Institute and Martyn T. Smith at the University of California, Berkeley. They compared blood and urine samples from 250 shoe factory workers in China, who routinely inhale benzene from shoe glue, with samples from 140 workers at nearby clothes manufacturers who are not exposed to benzene.

The team found that levels of most types of white blood cells are depressed in workers exposed to average benzene levels as low as 0.57 ppm. That benzene affects nearly all blood cell types suggests the involvement of bone marrow progenitor cells. The study also shows that early progenitor cells are more sensitive to benzene than mature blood cells.

In addition, the research team found that individual workers' susceptibility to blood cell damage from benzene varies with their ability to metabolize benzene. This result supports earlier work showing that benzene's metabolites--quinones and free radicals--are the culprits.