home edit2 whole TSD November 2002 PDF format - OEHHA

METHYL TERT-BUTYL ETHER (MTBE)
CAS No: 1634-04-4
I. PHYSICAL AND CHEMICAL PROPERTIES (From HSDB, 2001)
Molecular weight 88.15
Boiling point
55.2°C
Melting point
-108.6°C
Vapor pressure 250 mm Hg at 25°C
Air concentration conversion 1 ppm = 3.6 mg/m 3
II.
HEALTH ASSESSMENT VALUES
Unit Risk Factor: 2.6 E-7 (µg/m 3 ) -1
Slope Factor: 1.8 E-3 (mg/kg-day) -1
[Cancer slope factor (CSF) derived by OEHHA (1999a) from the geometric mean of the
potency estimates obtained for male rat kidney adenomas and carcinomas (Chun et al. 1992),
male rat Leydig interstitial cell tumors and female rat leukemia and lymphomas (Belpoggi et al.
1995, 1998) using potency values derived from the lower 95% confidence limit on the 10%
tumor dose (LED 10 ) with pharmacokinetic adjustments; inhalation unit risk factor derived from
the CSF by OEHHA (1999b).]
III.
CARCINOGENIC EFFECTS
Human Studies
No studies regarding the carcinogenic effects of human exposure to MTBE were found in an earlier
search by ATSDR (1996) or more recently by OEHHA (1999a).
Animal Studies
Oral
Male and female Sprague-Dawley rats (60/sex/group) were exposed to MTBE by gavage at doses of
0, 250 or 1,000 mg/kg body weight/day, four days/week for 104 weeks (Belpoggi et al., 1995, 1997,
1998). Animals were maintained until natural death; the last animal died at 174 weeks of age. A doserelated
increase in the combined incidence of lymphomas and leukemia was observed in female rats
(Table 1). The authors reported that the increase was highly significant (p < 0.01) in the high-dose
group and marginally significant in the low-dose group, when analyzed using a log-ranked test. When
analyzed using the Fisher exact test, the combined incidence of lymphomas and leukemia in high-dose
females was significantly different from controls at the p = 0.001 level. Historical control incidence rates
in this laboratory for lymphomas and leukemias (combined) was < 10% in female Sprague-Dawley rats
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