Benzene hematotoxicity and leukemogenesis.

Benzene is ubiquitous and accepted as a human carcinogen by regulatory agencies. Proposed regulations assume without proof that the carcinogenic response to benzene exposure is "one hit" implying a linear with no threshold. There is no solid experimental proof for this concept. This research involves exposure of CBA/Ca male mice to benzene vapor in varying concentrations. Exposure to 300 ppm 6 hrs/day, 5 days/week, for 16 weeks is highly leukemogenic. Exposure for the same time to 100 ppm is also leukemogenic. Concentrations from 25 ppm to 400 ppm 6 hrs/day, 5 days/week, for 10 exposures produce an increasing lymphopenia. Exposure to 100 ppm for the same exposure time produces anemia, decrease in stem cell content of marrow, and marrow cellularity. Further dose-effect studies are required to test the "one hit hypothesis" and to determine whether the same integral dose of benzene administered over variable exposure has the same or different biological responses. It is of concern that biologic effects are observed at 25 ppm only 2.5 times the present permissible time-weighted average exposure during a working day and research by others (see Discussion) has demonstrated an effect (noncarcinogenic) at 10 ppm.