New Findings on Benzene

The New York Times reports today on a study that evaluated decreases in white blood cell counts in Chinese workers exposed to benzene. The results were that changes in white blood cell counts were observed in workers exposed to at or below 1 ppm benzene. 1 ppm is the current Permissible Exposure Limit (PEL) for benzene.

The 1 ppm standard was one of those highly controversial compromises that fundamentally changed the way occupational exposure standards were developed. That’s a story for another time; I used to be familiar with it, but now would have to do the homework to do it justice. A summary of the issue by Peter Infante is here.

The NCI study, "Hematotoxicity in Workers Exposed to Low Levels of Benzene," was published in the December 3, 2004, issue of Science. It reports results from 250 Chinese workers exposed to benzene through the air in their workplace and compares blood cell counts to 140 non-exposed workers. The study included workplace air sampling over the course of a year, to evaluate seasonal changes in workers' exposures.

A team of investigators from the National Cancer Institute (NCI), University of California, Berkeley, the Chinese Center for Disease Control and Prevention (Chinese CDC), University of North Carolina at Chapel Hill, and the New York Blood Center and the New York Blood Center were contributors.

NCI researchers, in collaboration with the Chinese CDC, have been studying industrial workers in China since the mid-1980s. The project grew out of a Chinese national health survey which found worker exposure to benzene to be commonplace. The NCI-funded studies included 75,000 workers exposed to benzene and 35,000 unexposed workers in over 700 factories in 12 cities in China. These studies collected industrial hygiene information on the workers' exposures and surveillance information on adverse effects. These studies suggested that benzene exposure is associated with an increased risk of blood and related disorders at average benzene exposure levels of less than 10 ppm.

About the same time, Dr. Martyn Smith at U.C. Berkeley demonstrated that benzene was metabolically activated to a carcinogen by the enzyme myeloperoxidase (MPO). MPO also was later identified as a enzymatic marker for sensitivity to the adverse effects of benzene exposure. Dr. Stephen Rappaport, also at U.C., Berkeley at the time began to develop methods for biological monitoring of benzene and its urinary metabolites. In the early 1990s, the U.C. Berkeley researchers began to collaborate with NCI and Chinese CDC epidemiologists and industrial hygienists. In 1992, this interdisciplinary team carried out a study of 44 workers exposed to benzene and 44 controls in Shanghai. That study found evidence of lowered blood cell counts among workers exposed to less than 31 ppm benzene, with an average exposure of about 14 ppm in air.

Benzene has been shown to causes leukemia and lowers blood cell counts in people who are exposed to high levels of benzene at work. Historically, adverse effects to blood-forming tissues has been clearly observed with exposures higher than 100 ppm, and are less apparent at 10 ppm in air or lower. As noted by the NCI press release, further work is needed to understand the potential leukemia risks for large groups of workers exposed to low levels of benzene. It has estimated that up to 3 million workers in the U.S. may be exposed to benzene.
This isn’t going to trigger any significant action for awhile yet (especially with a Bush Administration Department of Labor). However, it does reinforce the course of prudence in reducing worker exposures to as low as feasible – in other words, just because the standard is 1 ppm isn’t really a reason to stop there.