The results of a new study suggest that even low levels of exposure to the chemical benzene have measurable adverse health effects. Researchers writing today in Science report that Chinese factory workers breathing benzene at levels permissible by U.S. standards have fewer white blood cells and platelets than unexposed workers do.

Experts have known for years that high levels of benzene reduce white blood cell counts and cause leukemia in people. Benzene is found in gasoline and tobacco and is used in many chemical manufacturing processes. U.S. guidelines allow workers to breathe air with as much as one part per million (1 ppm) benzene, averaged over an eight-hour workday.

To look for effects of such low-level exposures, scientists from institutions including the National Cancer Institute (NCI), the University of California at Berkeley and the Chinese Center for Disease Control and Prevention teamed up to assess 250 shoe factory workers in China, who are routinely exposed to various levels of benzene. By carefully measuring individual laborers' exposure to benzene and other chemicals, the researchers showed that the 109 workers exposed below the 1 ppm level still had white blood cell counts almost 15 percent lower than similar workers who were not exposed. The reduction was larger for individuals subjected to more than 10 ppm of benzene.

Nathaniel Rothman of NCI, one of the senior authors of the study, says that although these reductions are modest, they raise the question of what other kinds of changes are taking place in the blood-forming regions of the bone marrow. The investigators found even larger reductions in the so-called progenitor cells circulating in the blood of a smaller number of laborers chosen for further study. In the bone marrow, these cells normally differentiate into various blood cell types. If low-level benzene exposure also affects the stem cells that give rise to the progenitor cells, it would help explain the link to between benzene and leukemia. Based on observed cancer rates at high exposures, studies have estimated the increased cancer risk from lifetime exposure at the 1 ppm level to be between 0.7 and 2.5 percent, according to the U.S. Environmental Protection Agency.