WASHINGTON – Blood changes, including a steep decline in disease-fighting white cells, have been found in workers persistently exposed to low levels of benzene, a common industrial chemical known to pose a leukemia risk at high concentrations.

Researchers reported Thursday in the journal Science that workers in a Chinese shoe factory exposed to less than one part per million of benzene experienced a significant decline of white cells and found their blood-forming cells were less vigorous than normal.

U.S. occupational guidelines limit benzene exposure to one part per million, but the study found changes in the blood from lower exposure.

“We can’t say that this is associated with the future risk of disease,” said Dr. Nathaniel Rothman, one of two senior authors of the study. “But it does raise the question of what else is going on in the bone marrow” as a result of low-level exposure to benzene.

Benzene also appears to have a toxic effect on the progenitor cells that form blood cells and that the effect of benzene can be underestimated if only mature blood cells are studied, said Rothman, a researcher at the National Cancer Institute, one of the National Institutes of Health.

Benzene is one of the most frequently used chemicals in American industry. It is used as a solvent and to make plastics, resins, adhesives and synthetic fibers. Workers in shipping, automobile repair, shoe manufacture and the refining and transportation of oil and gasoline are routinely exposed to fumes from the sweet-smelling, highly combustible chemical.

In 1987, the Occupational Safety and Health Administration set the maximum allowable industrial exposure to inhaled benzene at 1 part per million in eight hours.

Rothman said it is too soon to say if that standard should be changed, but he added that the new study raises concerns about the risks of low levels of benzene and suggests that more studies are needed to ensure that American workers are protected.

“Biological events are occurring (as a result of the exposure),” said Rothman. “The question is: What are the health consequences to the workers?”