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Benzene Safety Level Questioned By Study

Exposure may harm blood cells even at U.S. limits

Dec 2, 2004 | Houston Chronicle

The federal safety level for benzene, a leukemia-causing air toxin found throughout Houston and routinely inhaled by petrochemical plant workers, is too high to protect workers from blood cell problems, according to a study published Thursday in the journal Science.

The study looked at 250 shoe-factory workers in China exposed to benzene fumes from glue and found that people exposed to levels below 1 part per million the U.S. occupational standard experienced a significant decline in their white blood cell counts. Blood-forming cells performed less vigorously than normal, raising concern about harm to the bone marrow where those cells are found.

"We're finding evidence that biological changes in the blood system are occurring. It raises a question about the long-term risks for leukemia," said Dr. Nathaniel Rothman, a senior investigator at the National Cancer Institute who co-authored the study with NCI molecular epidemiologist Qing Li and others. "A direct link hasn't been established. Our work is showing something is going on, and if something is going on, we need to understand it."

The study is of special interest in the Houston area, which is well-known for its poor air quality as well as for being one of the world's largest centers of petrochemical production, employing thousands of people in the industry.

At high levels of occupational exposure, benzene causes leukemia, but its disease-causing potential at lower levels is unknown. 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.

Refiners compliance cited

Experts said it is too soon to know whether the standard should be changed.

OSHA's Bill Perry 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 had not yet been reviewed and "evaluation will take some time."

Occupational health professionals say the largest refineries, such as Exxon, Dow and Shell, keep their benzene levels well below the U.S. standard. Even if the U.S. standard were cut in half, those companies could probably meet that limit without difficulty, "because they're probably there already," said Jonathan Ward, director of environmental toxicology at the University of Texas Medical Branch at Galveston.

"The Fortune 500 companies have invested a lot of money in occupational safety and health, and understand it influences the bottom line," said Dr. Arch Carson, an assistant professor of occupational medicine at the University of Texas School of Public Health. "But the majority of our American work force works in companies that employ less than 50 people. It's those companies that have a harder time assessing the problem and controlling it."

It is unclear what percentage of the petrochemical work force is employed at small plants, said Gary Adams, president of Chemical Market Associates, a Houston-based consulting firm.

390 workers monitored

In the study, researchers compared 250 benzene-exposed shoe workers with 140 unexposed people who were otherwise similar in terms of age and sex and who worked in nearby clothing factories.

For up to 16 months, the researchers monitored workers for benzene exposure through personal air monitors and urine samples taken after workers got off their shifts. In blood samples, they found a steep decrease in disease-fighting lymphocytes, T-cells, platelets and other cell types.

Ward, who has conducted similar research on the chemical 1,3-butadiene, said the cell changes are a reaction to the toxicity of benzene. Benzene also causes damage to blood cell DNA, and with enough damage, a person can develop leukemia, a potentially fatal blood cancer.

Ward said the strength of the China study is that it looked at a fairly large group of people and closely measured their exposure to the chemical.

"Obviously having such a clear-cut finding in individuals to exposures below the current limit will probably give rise to a careful review of the exposure limit," Ward said. "I would think that this might push us in the direction of" lowering the limit.

Neighborhood levels

Carson disagreed and said that most occupational health professionals consider current standards adequate for the vast majority of workers.

Workers in the oil industry, shipping, auto repair and shoe-making are exposed to benzene, and general public is exposed through cigarette smoke, gasoline and auto emissions. Levels measured in Houston neighborhoods are far lower than the occupational limit, and are measured in parts per billion.

Houston ambient levels range from 2 to 4 parts per billion, and any health impact has not been observed by scientists.

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