Low-Level Benzene Exposure May Be Harmful

Dec. 2, 2004 -- Gasoline, auto emissions, cigarette smoke: All contain benzene, a toxin whose chronic exposure, even at relatively low doses, has been linked to leukemia. Now, research shows that airborne exposure even below U.S. occupational limits can lower levels of disease-fighting blood cells.

But an expert from the American Cancer Society says there is no reason to be alarmed.

The research appears in this week's issue of Science. It is part of a long-term study evaluating effects of benzene exposure among factory workers in China.

"For many years, people felt that these low-level exposures had no impact on human physiology, but this study shows they do," says researcher Richard B. Hayes, PhD, a senior investigator at the National Cancer Institute. "Of course, it raises questions about long-term effects, like cancer. But we did not address that in this study," he tells WebMD.

His study comes on the heels of another report, published in August, that showed a link between childhood leukemia and benzene exposure. In that study, the high-risk exposure occurred while living next door to a gas station or auto repair shop during childhood.

"There's a pretty long history of studies of benzene, including work we've done in China, that has shown benzene to be a cause of leukemia and other blood problems like aplastic anemia," Hayes tells WebMD. However, today's study is the first to look at the low-level effects of benzene exposure over a long period of time, levels below those considered the occupational safety standard level of 1 part per million (1 ppm), he notes.

Benzene and Chinese Factory Workers

In their study, researchers compared 250 benzene-exposed shoe factory workers with 140 unexposed clothes factory workers, all living in the same region near Tianjin, China. All had worked in the factories about six years. Researchers monitored their benzene exposure repeatedly for 16 months before testing each volunteer's blood and urine.

All types of white blood cells, which are disease-fighting cells, were significantly decreased in workers exposed to less than 1 part per million of benzene, he reports, as were blood particles called platelets that help clot blood.

Workers with longer work history - and therefore longer exposure -- had even lower white blood cell counts, he notes.

"These changes are not dangerous in and of themselves," Hayes tells WebMD. "But they do show that even at levels below 1 ppm, benzene affects the development of white blood cells. That alone we think is an important finding."

Researchers also looked at toxic effects on progenitor cells, a type of adult stem cell that are still developing specific functions. Hayes and his team observed "even greater decreases" in these still-developing cells. "This suggests that early progenitor cells are more sensitive than are mature cells to the toxic effects of benzene," says the report. Other studies have found a similar vulnerability among immature cells.

"It doesn't mean that change will lead necessarily to cancer," Hayes tells WebMD. "The fact that cell numbers are reduced by 10% to 20% doesn't mean that immune function is affected. This study doesn't deal with that. This shows that something is awry, but in and of itself it doesn't indicate that any damage has been done. This is a signal we need to be paying more attention to these low-level exposures."