Development of occupational exposure limits in Japan.

The development of occupational exposure limits (OELs) in Japan is discussed by describing the OELs of two chemical compounds, benzene and trichloroethylene, as typical examples. As for benzene, sufficient epidemiological evidence has accumulated indicating that benzene is a human carcinogen. To establish the OEL for benzene, the OEL committee of the Japan Society for Occupational Health (JSOH) selected 9 cases of acute myeloic or monocytic leukemia out of the 14 cases of leukemia in the Pliofilm cohort, adopted the exposure estimate of Paustenbach et al. (52), and calculated the risk of benzene-induced leukemia by means of an average relative risk model. The lifetime risk of leukemia by exposure to benzene at 1 ppm for 40 years was calculated as 0.762 x 10(-3) with a 95% confidence interval between 0.621 x 10(-3) and 0.98.10(-3). The benzene level that causes one lifetime excess death from leukemia among 1,000 workers exposed to benzene for 40 years was 1.31 ppm, with a 95% confidence interval between 1.01 and 1.61 ppm. The OEL committee decided that benzene exposure should be controlled by a reference value corresponding to a lifetime risk (10(-3) or 10(-4)) of leukemia rather than by a time-weighted average (TWA) concentration. The committee has proposed that the benzene exposure level corresponding to the lifetime risk of 10(-3) is 1 ppm and that corresponding to the risk of 10(-4) is 0.1 ppm. In 1995, the International Agency for Research on Cancer (IARC) changed the carcinogenicity classification of trichloroethylene from Group 3 (not classifiable as to carcinogenicity to humans) to Group 2A (probably carcinogenic to humans). The OEL committee of the JSOH, however, reached the conclusion that since it has not been confirmed that trichloroethylene is a human carcinogen, and since carcinogenicity, if any, may be based on an epigenetic rather than genotoxic mechanism, it is not appropriate to establish the OEL of trichloroethylene presupposing that trichloroethylene is a carcinogen. The judgment of the OEL committee is that the OEL for trichloroethylene should be established on other than carcinogenicity findings, particularly on the basis of its neurological effects. In the light of accumulated evidence that a long-term exposure to trichloroethylene at 50 ppm will cause neurotoxic effects to industrial workers, the OEL committee has proposed 25 ppm (135 mg/m3) as a reference value for work environments. Finally, we propose that the general environmental air standards of benzene and trichloroethylene should be about 1/1,000 of the respective reference values for work environment.