Even at levels below the US occupational standard of 1 part per million (ppm), benzene has toxic effects on nearly all types of blood cells, according to a new report.

Few studies have looked at low levels of exposure to benzene, Dr. Martyn T. Smith, at the University of California, Berkeley, and colleagues note in the journal Science.

Smith’s group compared 250 benzene-exposed shoe workers in China and 140 matched but unexposed “control” subjects. Benzene and toluene exposure was monitored for up to 16 months before blood samples were taken.

A total of 109 participants had exposure levels of less than 1 ppm, 110 were exposed to levels of 1 to 10 ppm, and 31 of the subjects had exposure greater than 10 ppm.

Platelets and all types of white blood cells were significantly decreased, even at levels less than 1 ppm, compared to controls, the authors report.

To exclude the effect of other potential exposures, the team identified 30 subjects exposed to less than 1 ppm benzene with negligible exposure to other solvents, and still they saw similar reductions in blood cells.

“Our findings show that biologic effects are occurring in this exposure range, which raises a question about what other kinds of events are occurring, particularly in the bone marrow in these individuals,” co-author Dr. Nathaniel Rothman, at the National Cancer Institute in Bethesda, Maryland, told Reuters Health.

“They also raise a question about what kinds of long-term health effects are occurring in these individuals,” he added.

SOURCE: Science, December 3, 2004.