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.
Common industrial chemical
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 ppm 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?”
In the Science study, researchers analyzed blood samples from 240 workers who were routinely exposed to benzene-laced glue in a Chinese shoe factory. They compared the results with blood drawn from 140 unexposed workers in a Chinese clothing plant.
The researchers also tested the air in the factories and took urine samples to determine precisely the intensity of benzene exposure in the two workplaces.
They found 109 shoe factory workers who were exposed to less than 1 ppm of benzene had an average of 15 percent to 18 percent fewer white blood cells than did workers in the other plant.
Researchers also discovered that the blood progenitor cells in the shoe factory workers were less able to grow and reproduce than were such cells taken from the clothing workers.
'Evaluation will take some time'
Dr. Gilbert Omenn of the University of Michigan Medical School said in Science that the study “should cause a stir in the occupational and environmental health circles.”
Bill Perry, director of the Office of Chemical Hazards at the Occupational Safety and Health Administration, said the current benzene standard was developed “after more than a decade of extensive research and a comprehensive regulatory process.”
He said the study in Science has not yet been reviewed and “evaluation will take some time.”
Officials from the Environmental Protection Agency did not respond to requests for comment.
The study is a subset of a larger benzene study, now under way in China, that was prompted by concerns among scientists there that the chemical might increase leukemia among workers.
Dr. Martyn T. Smith of the University of California, Berkeley, was the second senior author. Other co-authors included those from the Chinese Center for Disease Control and Prevention; the University of North Carolina, Chapel Hill; the New York Blood Center, White Plains, and the Peking Union Medical College in Beijing.