BioEd Online

A study of Chinese factory workers has shown that exposure to the chemical benzene destroys several types of blood cell. The effects are seen even at levels below the current US legal exposure limit of 1 part per million.

Benzene, which is used as an industrial solvent but also found in cigarette smoke and vehicle exhausts, has long been linked to the blood disease leukaemia. This study shows that even low levels of the chemical can damage the blood system.

The study was carried out by Martin Smith of the University of California, Berkeley, along with colleagues based in the United States and at the Chinese Center for Disease Control and Prevention in Beijing. They spent 16 months tracking 250 workers in a shoe factory near Tianjin who had varying exposures to benzene. They compared their blood-cell counts with those of workers from a nearby clothes factory, where no benzene was used.

Workers exposed to benzene showed reduced counts of white blood cells and platelets, the researchers report in this week's Science¹. "It will be important to examine the long-term health effects in workers exposed to low levels of benzene, such as increased occurrences of serious diseases of the blood system including leukaemia," add the researchers.

Early damage

Workers exposed to the highest levels of benzene, over 10 parts per million (ppm), had around 24% fewer white blood cells than their counterparts in the clothes factory, the researchers report. And even those exposed to less than 1 ppm showed a drop of around 15%.

Cell counts were reduced in a wide range of white blood-cell types, including cells called granulocytes, lymphocytes and B cells. Because of this, the researchers suspect that benzene may be damaging the precursor cells that divide to give rise to these different cell types.

In blood samples from 29 benzene-exposed workers, those with the greatest exposure showed the biggest reduction in precursor-cell counts, suggesting that benzene does indeed affect these growing cells. Workers in the study were not found to be exposed to any other solvents, and benzene exposure outside the workplace was negligible.

The researchers also identified two genetic variants that may accelerate the damage caused by benzene. Those with certain versions of two particular metabolic enzymes, which are thought to convert benzene into more harmful compounds, suffer lower cell counts.

Active harm

People with more active versions of these enzymes are more susceptible to harm, suggests Nathaniel Rothman, a member of the study team based at the US National Cancer Institute in Bethesda, Maryland. "More than 50% of the general population seem to have a variant associated with high activity," he adds.

Despite the fact that benzene is spewed out from car exhausts, we shouldn't necessarily be worried about walking the urban streets, Rothman says. Typical benzene levels in polluted cities are a few parts per billion, far below the levels found in the Chinese shoe factory.

Nonetheless, Rothman would like other researchers to try and replicate his team's results, to confirm the health dangers and, if necessary, revisit the legal safety limits. "One study doesn't change regulations," he says. "But as more data are generated, people who deal with risk and policy will be able to use them."