EPA /635/R-20/370Fc
www.epa.gov/iris
Toxicological Review of tert-Butyl Alcohol (tert-Butanol)
[CASRN 75-65-0]
August 2021
Integrated Risk Information System
Center for Public Health and Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC

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EXECUTIVE SUMMARY
Summation of Occurrence and Health Effects
tert-Butanol is produced by humans for multiple purposes, such as a solvent for
paints, a denaturant for ethanol and several other alcohols, an agent for dehydrating,
and in the manufacture of flotation agents, fruit essences, and perfumes, tert-Butanol
also is a primary metabolite of methyl tert-butyl ether (MTBE] and ethyl tert-butyl
ether (ETBE). Exposure to tert-butanol occurs primarily through breathing air
containing tert-butanol vapors and consuming contaminated water or foods.
Exposure can also occur through direct skin contact.
Animal studies demonstrate that chronic oral exposure to tert-butanol is associated
with kidney and thyroid effects. No chronic inhalation exposure studies have been
conducted. Evidence is suggestive of carcinogenic potential for tert-butanol, based
on thyroid tumors in male and female mice and renal tumors in male rats.
EFFECTS OTHER THAN CANCER OBSERVED FOLLOWING ORAL EXPOSURE
Kidney effects are a potential human hazard of oral exposure to tert-butanol. Kidney
toxicity was observed in males and females in two strains of rats. Kidney weights were increased in
male and female rats after 13 weeks or 15 months of treatment Histopathological examination in
male and female rats showed increased incidence or severity of nephropathy after 13 weeks of oral
exposure, increases in severity of nephropathy after 2 years of oral exposure, and increased
transitional epithelial hyperplasia after 2 years of oral exposure. Additionally, increased
suppurative inflammation was noted in females after 2 years of oral exposure. In one strain of
mice, the only kidney effect observed was an increase in kidney weight (absolute or relative) in
female mice after 13 weeks, but no treatment-related histopathological lesions were reported in the
kidneys of male or female mice at 13 weeks or 2 years. A mode of action (MOA) analysis
determined that tert-butanol exposure induces a male rat-specific alpha 2u-globulin-associated
nephropathy, tert-Butanol, however, is a weak inducer of alpha 2u-globulin nephropathy, which is
not the sole process contributing to renal tubule nephropathy. Chronic progressive nephropathy
(CPN) might also be involved in some noncancer effects, but the data are complicated by alpha
2u-globulin nephropathy in males. Effects attributable to alpha 2u-globulin nephropathy in males
were not considered for kidney hazard identification. Females are not affected by alpha 2u-globulin
nephropathy, so changes in kidney weights, transitional epithelial hyperplasia, suppurative
inflammation, and severity and incidence of nephropathy in female rats are considered to result
from tert-butanol exposure and are appropriate for identifying a hazard to the kidney.
At this time, evidence of selective developmental toxicity, neurodevelopmental toxicity, and
reproductive system toxicity following tert-butanol exposure is inadequate with minimal effects
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observed at otherwise toxic dose levels. The available information also is inadequate to draw
conclusions regarding liver and urinary bladder toxicity because of lack of consistency and lack of
progression, respectively.
ORAL REFERENCE DOSE (RFD) FOR EFFECTS OTHER THAN CANCER
Kidney toxicity, represented by increases in severity of nephropathy in female rats, was
chosen as the basis for the overall RfD (see Table ES-1). The kidney effects observed in female rats
in the chronic study by NTP (19951 were used to derive the RfD. Increased severity of nephropathy
was selected as the critical effect because it was observed in female rats consistently, it is an
indicator of kidney toxicity, and it was induced in a dose-responsive manner. While dose-response
modeling was technically feasible, there was uncertainty related to BMR type and values to use for
this type of endpoint; accordingly, the point of departure (POD) was derived from the
lowest-observed-adverse-effect level (LOAEL) of 43 mg/kg-day (NTP. 1995).
The overall RfD was calculated by dividing the POD for increases in severity of nephropathy
by a composite uncertainty factor (UF) of 100 to account for the extrapolation from animals to
humans (3), derivation from a LOAEL (3), and for interindividual differences in human
susceptibility (10).
Table ES-1. Organ/system-specific oral reference doses (RfDs) and overall
RfD for tert-butanol
Hazard
Basis
Point of
departure3
(mg/kg-d)
UF
Chronic RfD
(mg/kg-d)
Study
exposure
description
Confidence
Kidney
Increases in
severity of
nephropathy
43.2
100
4 x 10"1
Chronic
Medium
Overall RfD
Kidney
43.2
100
4 x 10"1
Chronic
Medium
aHuman equivalent dose PODs were calculated using body weight to the % power (BW3/4) scaling (U.S. EPA, 2011)
EFFECTS OTHER THAN CANCER OBSERVED FOLLOWING INHALATION EXPOSURE
Kidney effects are a potential human hazard of inhalation exposure to tert-butanol.
Although no effects were observed in mice, kidney weights were increased in male and female rats
following 13 weeks of inhalation exposure. In addition, the severity of nephropathy increased in
male rats. No human studies are available to evaluate the effects of inhalation exposure. As
discussed above for oral effects, endpoints in males specifically related to alpha 2u-globulin
nephropathy were not considered for kidney hazard identification. Changes in kidney weights and
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severity of nephropathy in females, however, are considered a result of tert-butanol exposure and
are appropriate for identifying a hazard to the kidney.
INHALATION REFERENCE CONCENTRATION (RFC) FOR EFFECTS OTHER THAN CANCER
Kidney toxicity, represented by increases in severity of nephropathy, was chosen as the
basis for the RfC (see Table ES-2). Although endpoints from a route-specific study were considered,
the availability of a physiologically based pharmacokinetic (PBPK) model for tert-butanol in rats
(Borghoffetal.. 20161 allowed for more specific and sensitive equivalent inhalation PODs derived
from a route-to-route (RTR) extrapolation from the PODs of the oral NTP T19951 study. The POD
adjusted for the human equivalent concentration (HEC) was 491 mg/m3 based on increases in
severity of nephropathy.
As discussed in Section 2.2.2, it is recognized that there is uncertainty in RTR extrapolation
because actual risk may not correlate exactly with the internal dose metric used for the
extrapolation (in this case, average blood concentration of tert-butanol). EPA is not aware of a
quantitative analysis of such uncertainty; it would involve comparing cross-route extrapolation to
toxicity data for a number of chemicals and endpoints sufficient to characterize the accuracy of the
approach. Such an analysis is beyond the scope of this assessment However, it is EPA's judgment
that this uncertainty is less than the uncertainty of the alternative, which would be to base the RfC
on the subchronic toxicity data. In particular, toxicity to the kidney requires that tert-butanol be
systemically distributed in the blood, hence the toxicity must be correlated with some measure of
blood concentration. The uncertainty in the extrapolation occurs because the metric used might
not accurately predict the effect, versus other possible metrics such as peak concentration.
The RfC was calculated by dividing the POD by a composite UF of 100 to account for
toxicodynamic differences between animals and humans (3), derivation from a LOAEL (3), and
interindividual differences in human susceptibility (10).
Table ES-2. Organ/system-specific inhalation reference concentrations (RfCs)
and overall RfC for tert-butanol
Hazard
Basis
Point of
departure3
(mg/m3)
UF
Chronic RfC
(mg/m3)
Study
exposure
description
Confidence
Kidney
Increases in severity
of nephropathy
491
100
5 x 10°
Chronic
Medium
Overall RfC
Kidney
491
100
5 x 10°
Chronic
Medium
BMDL = benchmark dose lower confidence limit.
Continuous inhalation HEC that leads to the same average blood concentration of tert-butanol as drinking water
exposure to the rat at the BMDL.
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EVIDENCE OF HUMAN CARCINOGENICITY
Under EPA Cancer Guidelines fU.S. EPA. 2005al. there is suggestive evidence of carcinogenic
potential for tert-butanol. tert-Butanol induced kidney tumors in male (but not female) rats and
thyroid tumors (primarily benign) in male and female mice following long-term administration in
drinking water (NTP. 1995). The potential for carcinogenicity applies to all routes of human
exposure.
QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM ORAL EXPOSURE
In accordance with EPA's guidance on alpha 2u-globulin fU.S. EPA. 1991bl. rat kidney
tumors are unsuitable for quantitative analysis because not enough data are available to determine
the relative contribution of alpha 2u-globulin nephropathy and other processes to the overall
kidney tumor response. A quantitative estimate of carcinogenic potential from oral exposure to
tert-butanol was based on the increased incidence of thyroid follicular cell adenomas in female
B6C3F1 mice and thyroid follicular cell adenomas and carcinomas in male B6C3F1 mice (NTP.
19951. The study included histological examinations for tumors in many different tissues,
contained three exposure levels and controls, contained adequate numbers of animals per dose
group (~50/sex/group), treated animals for up to 2 years, and included detailed reporting of
methods and results.
Although tert- butanol was considered to have only suggestive evidence of carcinogenic
potential, the National Toxicology Program (NTP) study was well conducted and suitable for
quantitative analysis. Slope factors were derived for thyroid tumors in female or male mice. The
modeled tert-butanol POD was scaled to human equivalent doses (HEDs) according to EPA
guidance by converting the benchmark dose lower confidence limit corresponding to 10% extra
risk (BMDLio) on the basis of body weight scaling to the % power (BW3/4) fU.S. EPA. 2011. 2005b).
Using linear extrapolation from the BMDLio, a human equivalent oral slope factor was derived
(slope factor = 0.1/BMDLio). The resulting oral slope factor is 5 x 10"4 per mg/kg-day.
QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM INHALATION EXPOSURE
No chronic inhalation studies of exposure to tert-butanol are available. Although the mouse
thyroid tumors served as the basis for the oral slope factor, RTR extrapolation is not possible for
these thyroid effects in mice because the only PBPK model available is for rats. Therefore, no
quantitative estimate of carcinogenic risk could be determined for inhalation exposure.
SUSCEPTIBLE POPULATIONS AND LIFESTAGES FOR CANCER AND NONCANCER OUTCOMES
Information is inadequate to identify any populations or lifestages that might be especially
susceptible to tert-butanol.
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KEY ISSUES ADDRESSED IN ASSESSMENT
This document assesses the human relevance of the kidney effects observed in male and
female rats, particularly as the effects relate to alpha 2u-globulin nephropathy and the exacerbation
of chronic progressive nephropathy. EPA 1991 and International Agency for Research on Cancer
(IARC) 1999 frameworks were used to evaluate whether tert-butanol caused alpha
2u-globulin-associated nephropathy (Capen etal.. 1999: U.S. EPA. 1991a). The presence of alpha
2u-globulin in the hyaline droplets was confirmed in male rats by alpha 2u-globulin
immunohistochemical staining. Linear mineralization and tubular hyperplasia were reported in
male rats, although only in the chronic study. Other subsequent steps in the pathological sequence,
including necrosis, exfoliation, and granular casts, either were absent or inconsistently observed
across subchronic or chronic studies. None of these effects occurred in female rats or in either sex
of mice, although these endpoints were less frequently evaluated in these models. Evidence implies
that an alpha 2u-globulin MOA is operative, although it is relatively weak in response to
tert-butanol and is not solely responsible for the renal tubule nephropathy observed in male rats.
Chronic progressive nephropathy (CPN) also plays a role in exacerbating nephropathy in both male
and female rats. While the etiology of CPN is unknownfNIEHS. 2019: Hard and Khan. 2004: Peter et
al.. 19861 and it has no known analog in the aging human kidney fNIEHS. 2019: Hard etal.. 20091. it
cannot be ruled out that a chemical which exacerbates CPN in rats could also exacerbate disease
processes in the human kidney (e.g. chronic kidney disease, diabetic nephropathy,
glomerulonephritis, interstitial nephritis, etc.) (NIEHS. 20191 Several other effects in the kidney
unrelated to alpha 2u-globulin were observed in female rats, including suppurative inflammation,
transitional epithelial hyperplasia, and increased kidney weights (NTP. 1997.1995). These specific
effects observed in female rats, not confounded by alpha 2u-globulin-related processes, are
considered the result of tert-butanol exposure, and therefore, relevant to humans.
Concerning cancer, alpha 2u-globulin accumulation is indicated as relatively weak in
response to tert-butanol exposure and not the sole mechanism responsible for the renal tubule
carcinogenicity observed in male rats. CPN and other effects induced by both alpha 2u-globulin
processes and tert-butanol play a role in renal tubule nephropathy, and the evidence indicates that
CPN augments the renal tubule tumor induction associated with tert-butanol exposure in male rats.
Poor dose-response relationships between alpha 2u-globulin processes and renal tumors in male
rats and a lack of renal tumors in female rats despite increased CPN severity, however, suggest that
other, unknown processes contribute to renal tumor development Based on this analysis of
available MOA data, these renal tumors are considered relevant to humans.
In addition, an increase in the incidence of thyroid follicular cell adenomas was observed in
male and female mice in a 2-year drinking water study (NTP. 1995). Thyroid follicular cell
hyperplasia was considered a preneoplastic effect associated with the thyroid tumors, and the
incidences of follicular cell hyperplasias were elevated in both male and female B6C3F1 mice
following exposure. U.S. EPA f!998al describes the procedures the Agency uses in evaluating
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chemicals that are animal thyroid carcinogens. The available evidence base is inadequate for
concluding that an antithyroid MOA is operating in mouse thyroid follicular cell tumorigenesis. No
other MOAs for thyroid tumors were identified, and the mouse thyroid tumors are considered
relevant to humans fU.S. EPA. 1998al.
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