Analysis of OSHA's short-term-exposure limit for benzene

A review of the data cited by OSHA in its final standard for exposure to benzene provides no clear scientific basis for a short-term-exposure limit (STEL). While leukemia and bone marrow toxicity were related to cumulative exposures of benzene received by workers, no evidence was presented that the rate of exposure at a given cumulative exposure contributed to the effects. Likewise, animal experiments suggested that exposures of several hours duration at a given level of benzene induced more bone-marrow toxicity when administered 3 rather than 5 days/week but did not indicate that the rate of exposure over shorter time scales played any role. The toxicokinetics of benzene in humans were also studied to determine whether nonlinear dose-rate effects would be likely to result from peak exposures associated with an exposure dose of 8 ppm-hr, which is allowed under the permissible exposure limit. This led to three conclusions. First, the concentration of benzene in the bone marrow should be sufficiently damped that the impact of a peak exposure should be minimal. Second, the peak concentration of benzene in the liver should be within the capacity of the cytochrome P450 system to maintain first-order metabolism. And finally, the maximum blood concentration of metabolites should be well below levels which have been shown to induce toxic effects in vitro. Taken together, the toxicokinetic relationships and the absence of clear experimental dose-rate effects suggest that the current STEL for benzene is unwarranted, assuming that 8-hr average exposures are kept below 1 ppm. While the argument can be made, on the basis of health considerations, that the existing 8-hr limit for benzene is too high, the rate of exposure during short periods appears to be irrelevant. Thus, we recommend that health professionals focus upon long-term exposures to benzene received by large numbers of workers rather than devote scarce resources to evaluate transient air levels.