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Oral
Borneff et al. (1968) exposed male and female NMRI mice to 1 mg K 2 CrO 4 per day for two years. A
significantly increased incidence of stomach carcinomas were noted in female mice; a significantly
increased incidence of benign stomach tumors (papillomas and hyperkeratomas) were noted in both
male and female mice.
Methodology
Inhalation
US EPA (1984) used both multistage linearized “competing risks” and “crude” procedures to estimate
human cancer risks associated with chromium inhalation exposure from the data set of Mancuso (1975).
The resulting cancer unit risk (1.2 E-2 (µg/m 3 ) -1 ) (US EPA, 1998) was a maximum likelihood estimate
(MLE) derived from the “competing risks” procedure, calculated on the basis of total chromium
exposure. It was also assumed that the smoking habits of chromate workers were similar to those of
the U.S. white male population. CDHS (1985) adopted the US EPA linear nonthreshold procedure to
estimate low-dose chromium inhalation cancer risk. A multistage linearized “crude” procedure was
used to derive a cancer unit risk of 1.5 E-1 (µg/m 3 ) -1 , which was the 95% upper confidence limit for the
estimate of the relative risk in the Mancuso (1975) study. The cancer mortality in Mancuso (1975) was
assumed to be due to Cr(VI), which was further assumed to be no less than one-seventh of total
chromium. This contrasts with the unit risk developed by US EPA (1984), which was calculated on the
basis of total chromium exposure.
Oral
An oral cancer potency factor for chromium(VI) was derived from the benign and malignant mouse
stomach tumor incidence data reported by Borneff et al. (1968). From the average weight of the
treated mice, 31 grams, the dose of 1 mg/day of K 2 CrO 4 (0.26 mg/day Cr) was calculated to be 8.39
mg/kg-day, and from the incidence of benign and malignant stomach tumors combined, the oral cancer
potency of Cr(VI) was calculated using a linearized multistage procedure to be 3.17 × 10 -2
(mg/kg-day) -l (CDHS, 1991). Surface-area scaling was used to extrapolate the animal cancer potency
to human cancer potency. The scaling factor was calculated by taking the ratio, human body weight
divided by animal body weight, raised to the one-third power. This extrapolation factor, (70 kg/0.031
kg) 1/3 , was calculated to be 13.1. Multiplying the above potency estimate, 3.17 ×10 -2 (mg/kg-day) -1 ,
made from the mouse study, by the extrapolation factor gives the estimate, 0.42 (mg/kg-day) -1 , for the
carcinogenic potency of Cr(VI) ingested by humans.
V. REFERENCES
California Department of Health Services (CDHS) 1985. Health Assessment for Chromium.
Epidemiological Studies and Surveillance Section, Berkeley, CA.
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