A 90-Day Chloroform Inhalation Study in Female and Male B6C3F1 Mice: Implications for Cancer Risk Assessment

High doses of chloroform induced liver cancer in male and femaleB6C3F₁ mice when administered by gavage, kidney cancer in maleOsborne-Mendel rats when given by gavage or in the drinkingwater, and kidney cancer in male BDF₁ mice when administeredby inhalation. The weight of evidence indicates that chloroformis acting through a nongenotoxic-cytotoxic mode of action. Thepresent study was designed to investigate the dose-responserelationships for chloroform-induced lesions and regenerativecell proliferation in B6C3F₁ mice as the basis for formulationof a biologically based risk assessment for inhaled chloroform.Different groups of female and male B6C3F₁ mice were exposedto atmospheric concentrations of 0, 0.3, 2, 10, 30, and 90 ppmchloroform 6 hr/day, 7 days/week for exposure periods of 4 daysor 3, 6, or 13 consecutive weeks. Some additional exposure groupswere exposed for 5 days/week for 13 weeks or were exposed for6 weeks and then examined at 13 weeks. Bromodeoxyuridine wasadministered via osmotic pumps implanted 3.5 days prior to necropsy,and the labeling index (LI, percentage of nuclei in S-phase)was evaluated iminunohistochemically from histological sections.Complete necropsy and microscopic evaluation revealed treatment-induceddose- and time-dependent lesions only in the livers and nasalpassages of the female and male mice and in the kidneys of themale mice. Large, sustained increases in the liver LI were seenin the 90-ppm groups at all time points. The female mice weremost sensitive, with a no-observed-adverse-effect level (NOAEL)for induced hepatic cell proliferation of 10 ppm. The hepaticLI in the 5 days/week groups were about half of those seen inthe 7 days/week groups and had returned to the normal baselinein the 6-week recovery groups. Induced renal histologic changesand regenerative cell proliferation were seen in the male miceat 30 and 90 ppm with 7 days/week exposures and also at 10 ppmwith the 5 days/week regimen. Nasal lesions were transient andconfined to mice exposed to 10, 30, or 90 ppm for 4 days. Ina previous cancer bioassay, a gavage dose of 477 mg/kg/day produced a 95% liver tumor incidence in female B6C3F₁ mice. Thisgavage dose is equivalent to a daily 6 hr/day inhalation exposureof approximately 80 ppm, based on the observed induced increasesin the LI as an internal dosimeter. The United States EnviromnentalProtection Agency currently uses the linearized multistage modelapplied to the mouse liver tumor data from the chloroform gavagestudy to estimate a virtually safe dose (VSD) as a one in amillion increased lifetime risk of cancer. The resulting valueis an airborne exposure concentration of 0.000008 ppm. Assumingthat chloroform-induced female mouse liver cancer is secondaryto events associated with necrosis and regenerative cell proliferation,then no increases in liver cancer in female mice would be predictedat the NOAEL of 10 ppm or below based on the results reportedhere. Applying an uncertainty factor of 1000 yields an estimateof a VSD at 0.01 ppm. This estimate relies on inhalation dataand is more consistent with the mode of action of chloroform.