Methyl tertiary-butyl ether (EHC 206, 1998)

UNITED NATIONS ENVIRONMENT PROGRAMME
INTERNATIONAL LABOUR ORGANISATION
WORLD HEALTH ORGANIZATION

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 206

METHYL TERTIARY-BUTYL ETHER

This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.

First draft prepared by Dr M. Gillner, National Chemicals
Inspectorate, Solna, Sweden, with contributions from Ms A.-S. Nihlén,
Institute for Working Life, Solna, Sweden

Published under the joint sponsorship of the United Nations
Environment Programme, the International Labour Organisation, and the
World Health Organization, and produced within the framework of the
Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization
Geneva, 1998

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WHO Library Cataloguing in Publication Data

Methyl tertiary-butyl ether.

(Environmental health criteria ; 206)

1.Methyl ethers 2.Environmental exposure
3.Occupational exposure
I.International Programme on Chemical Safety II.Series

ISBN 92 4 157206 X (NLM Classification: QD 305.E7)
ISSN 0250-863X

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR METHYL TERTIARY-BUTYL ETHER

PREAMBLE

ABBREVIATIONS

1. SUMMARY

1.1. Identity, physical and chemical properties, analytical
methods
1.2. Sources of human and environmental exposure
1.3. Environmental transport, distribution and transformation
1.4. Environmental levels and human exposure
1.5. Kinetics and metabolism
1.6. Effects on laboratory animals and in vitro systems
1.7. Effects on humans
1.8. Effects on other organisms in the laboratory and field
1.9. Evaluation of human health risks and effects on the
environment

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS

2.1. Identity
2.2. Physical and chemical properties
2.3. Conversion factors
2.4. Analytical methods
2.4.1. Procedures
2.4.1.1 Air
2.4.1.2 Soil, water and sediment
2.4.1.3 Gasoline
2.4.1.4 Biological samples

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1. Natural occurrence
3.2. Anthropogenic sources
3.2.1. Production levels and processes
3.2.2. Uses
3.2.3. Sources and releases to the environment
3.2.3.1 Industrial releases
3.2.3.2 Storage tank release
3.2.3.3 Engine emissions from on-road and off-road
vehicles and recreational boats
3.3. Other pertinent information

4. ENVIRONMENTAL BEHAVIOUR AND FATE

4.1. Transport and distribution between media
4.1.1. Air
4.1.2. Water
4.1.3. Soil
4.1.4. Multimedia
4.2. Bioconcentration
4.3. Biodegradation and transformation
4.3.1. Aerobic conditions
4.3.2. Anaerobic conditions
4.4. Abiotic degradation
4.4.1. Air
4.4.1.1 Photolysis
4.4.1.2 Hydrolysis
4.4.1.3 Photooxidation
4.4.2. Natural waters
4.4.3. MTBE half-life ranges in environmental compartments
4.5. Ozone-forming potential
4.6. Remediation

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1. Environmental levels
5.1.1. Exposure
5.1.1.1 Levels in ambient air and various
microenvironments
5.1.1.2 Dermal exposure
5.1.1.3 Estimation of total personal exposure
5.1.1.4 Other pollutants
5.2. Occupational exposure
5.2.1. Industrial operations - manufacturing and blending
5.2.2. Transportation
5.2.3. Service station attendants and garage mechanics
5.2.4. Occupational exposure limit values
5.3. Exposure via water
5.3.1. Snow and precipitation
5.3.2. Surface water
5.3.3. Groundwater
5.3.4. Drinking-water
5.4. Soil and sediment
5.5. Biota

6. KINETICS AND METABOLISM IN HUMANS AND LABORATORY ANIMALS

6.1. Human data
6.1.1. Controlled human studies
6.1.2. Exposure to oxygenated gasoline
6.2. Animal studies
6.3. In vitro studies
6.4. Physiologically based pharmacokinetic modelling

7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO SYSTEMS

7.1. Single exposure
7.2. Skin, eye and respiratory irritation; skin sensitization
7.2.1. Skin irritation
7.2.2. Eye irritation
7.2.3. Respiratory tract irritation
7.2.4. Skin sensitization
7.3. Neurotoxicity
7.4. Short-term repeated dose studies
7.4.1. Oral studies
7.4.2. Inhalation studies
7.4.3. Intraperitoneal administration
7.5. Neurotoxicity studies
7.6. Reproductive and developmental toxicity
7.6.1. Reproductive toxicity
7.6.2. Developmental toxicity
7.7. Mutagenicity and related end-points
7.8. Carcinogenicity
7.8.1. Initiation-promotion protocol
7.9. Metabolites of MTBE
7.10. Mode of action
7.10.1. Kidney tumours
7.10.2. Liver tumours

8. EFFECTS ON HUMANS

8.1. Population studies
8.2. Controlled studies
8.3. Subpopulations at special risk
8.4. Special studies
8.4.1. Organoleptic properties
8.4.2. Immunological effects

9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD

9.1. Laboratory experiments
9.1.1. Algae
9.1.2. Aquatic animal species
9.2. Field experiments

10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT

10.1. Evaluation of human health risks
10.1.1. Exposure
10.1.2. Human health effects
10.2. Evaluation of effects on the environment

11. RECOMMENDATIONS

REFERENCES

RÉSUMÉ

RESUMEN

NOTE TO READERS OF THE CRITERIA MONOGRAPHS

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This publication was made possible by grant number 5 U01 ES02617-
15 from the National Institute of Environmental Health Sciences,
National Institutes of Health, USA, and by financial support from the
European Commission.

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WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR METHYL TERTIARY-
BUTYL ETHER

Members

Dr R. B. Beems, National Institute of Public Health & the Environment,
Bilthoven, The Netherlands

Dr A. Bobra, Environment Canada, Hull, Quebec, Canada

Dr S. Borghoff, Chemical Industry Institute of Toxicology, Research
Triangle Park, North Carolina, USA

Dr J.M. Davis, National Center for Environmental Assessment, US
Environmental Protection Agency, Research Triangle Park, North
Carolina, USA (Vice-Chairman)

Dr L. Fishbein, Fairfax, Virginia, USA

Dr M. Gillner, National Chemicals Inspectorate, Solna, Sweden
(Co-Rapporteur)

Mr G. Long, Environmental Health Centre, Health Canada, Ottawa, Canada
(Co-Rapporteur)

Dr M.E. Meek, Environmental Health Centre, Health and Welfare Canada,
Ottawa, Canada (Chairman)

Dr A.A.E. Wibowo, Coronel Institute, University of Amsterdam,
Amsterdam, The Netherlands

Observers

Dr M. Constantini, Health Effects Institute, Cambridge, Massachusetts,
USA (representing the Health Effects Institute (HEI))

Dr J. Del Pup, Texaco Inc., New York, USA (representing the American
Industrial Health Council (AIHC))

Mr R. Hillier, Oil, Chemical and Atomic Workers' International Union
(OCAWIU), Lakewood, Colorado, USA (representing the International
Federation of Chemical, Energy, Mine and General Workers' Unions
(ICEM))

Dr A.K. Mallett, Arco Chemical Europe Inc., Maidenhead, United Kingdom
(representing the European Centre for Ecotoxicology and Toxicology of
Chemicals (ECETOC))

Dr M. Mehlman, Princeton, New Jersey, USA (Technical Adviser to Mr
Hillier, OCAWIU)

Dr P. Montuschi, Department of Pharmacology, Catholic University of
the Sacred Heart, Rome, Italy (representing the International Union of
Pharmacology (IUPHAR))

Dr J. Zogorski, US Department of the Interior, Rapid City, South
Dakota, USA

Secretariat

Dr E. Smith, International Programme on Chemical Safety, World Health
Organization, Geneva, Switzerland (Secretary)

Dr D. McGregor, Unit of Carcinogen Identification and Evaluation,
International Agency for Research on Cancer, World Health
Organization, Lyon, France

Assisting the Secretariat

Miss C. Grande, Air Issues Section, Health Canada, Ottawa, Canada

WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR METHYL
TERTIARY-BUTYL ETHER (MTBE)

A WHO Task Group on Environmental Health Criteria for methyl
tertiary-butyl ether met at the Conference Facility, Lord Elgin
Hotel, Ottawa, Canada from 17 to 21 April 1997. Dr E.M. Smith, IPCS,
welcomed the participants on behalf of Dr M. Mercier, Director of the
IPCS, and the three IPCS cooperating organizations (UNEP/ILO/ WHO).
The Group reviewed and revised the draft and made an evaluation of the
risks for human health and the environment from exposure to methyl
tertiary-butyl ether.

The first draft of the EHC was prepared by Dr M. Gillner,
National Chemicals Inspectorate, Solna, Sweden, with contributions
from Ms A.-S. Nihlén, Institute for Working Life, Solna, Sweden. Dr M.
Gillner and Ms Nihlén also prepared the second draft, incorporating
comments received following circulation of the first drafts to the
IPCS contact points for Environmental Health Criteria monographs.

Dr E.M. Smith and Dr P.G. Jenkins, both of the IPCS Central Unit,
were responsible for the scientific aspects of the monograph and for
the technical editing, respectively.

The efforts of all who helped in the preparation and finalization
of the monograph are gratefully acknowledged.

The financial support of the Swedish National Chemicals
Inspectorate in preparing the monograph and the Canadian Health
Protection Branch, Environmental Health Directorate, in funding the
Task Group meeting in Ottawa are gratefully acknowledged.

ABBREVIATIONS

AED atomic emission detector
ALAT alanine aminotransferase
AP alkaline phosphatase
AUC area under the curve
BCF bioconcentration factor
BTEX benzene, toluene, ethyl benzene and xylenes
BUN blood urea nitrogen
bw body weight
CHOL cholesterol
CL total plasma clearance
CNS central nervous system
CO carbon monoxide
DEN diethylnitrosamine
DIPE diisopropyl ether
DMN N-nitrosodimethylamine
EC electron capture
EROD 7-ethoxyresorufin- O-deethylase
ETBE ethyl tertiary-butyl ether
FID flame ionization detector
FOB functional observational battery
FTIR Fourier-transform infrared
GC gas-chromatography
GC-MS gas-chromatography/mass spectrometry
GC-O gas-chromatography using an oxygen-selective detector
Hb haemoglobin
HC hydrocarbon
HPLC high-performance liquid chromatography
HPRT hypoxanthine-guanine phosphoribosyl transferase
IL-1 interleukin-1
IL-4 interleukin-4
ip intraperitoneal
IR infrared
iv intravenous
K_oc adsorbtion coefficient to soil organic carbon
K_ow octanol/water partition coefficient
LC₅₀ median lethal concentration
LD₅₀ median lethal dose
LGL large granular lymphocyte
LOAEL lowest-observed-adverse-effect level
LOEL lowest-observed-effect level
LT₅₀ median lethal time
MCH mean corpuscular haemoglobin
MCHC mean corpuscular haemoglobin concentration
MCS multiple chemical sensitivities
MCV mean corpuscular volume
MTBE methyl tertiary-butyl ether
NADPH reduced nicotinamide adenine dinucleotide phosphate
NIR near infrared
NMOC non-methane organic carbon

NOAEL no-observed-adverse-effect level
NOEL no-observed-effect level
NO_x oxides of nitrogen (NO, NO₂, N₂O₄ and N₂O₃)
PID photoionization detector
ppb parts per billion
ppbv parts per billion (by volume)
ppm parts per million
PROD 7-pentoxyresorufin- O-dealkylase
RBC red blood cell
RFG reformulated gasoline
RID refractive index detector
RPLC reversed-phase liquid chromatography
sc subcutaneous
SCE sister chromatid exchange
SD standard deviation
TBA tertiary-butyl alcohol
TBF tertiary-butyl formate
TWA time-weighted average
UDS unscheduled DNA synthesis
V/F distribution volume
VOC volatile organic compound

1. SUMMARY

Methyl tertiary-butyl ether (MTBE) is one of several ethers
that may be used as fuel additives and is currently by far the
dominant one. Ethyl tertiary-butyl ether (ETBE), tertiary-amyl
methyl ether (TAME), tertiary-amyl ethyl ether (TAEE) and
diisopropyl ether (DIPE), among others, may supplement, or serve as
alternatives to MTBE for oxygenation or octane enhancement purposes
and may be found, therefore, in association with MTBE.

1.1 Identity, physical and chemical properties, analytical methods

MTBE is a volatile, colourless liquid at room temperature with a
terpene-like odour. It has low viscosity and a boiling point of
55.2°C. The freezing point is -109°C. The density is 0.7404 at 20°C.
The vapour pressure is relatively high, 33 500 Pa at 25°C. MTBE is
flammable and can form explosive mixtures with air. It is very soluble
in other ethers and alcohol. It mixes with gasoline (petrol), and is
soluble in water (42 000 g/m³ at 19.8°C). The log n-octanol/water
partition coefficient is 0.94-1.3. It is unstable in acid solution.

MTBE is analysed in all matrices generally by gas chromatography
(GC) using a range of capillary columns and detector systems that are
suited to the specific matrix. Reverse-phase liquid chromatography
(RPLC) has also been used for analysis of petrol samples.
Sorption/desorption, including purge and trap systems, and headspace
procedures have been used to prepare air, water, sediment and
biological samples for analysis.

1.2 Sources of human and environmental exposure

MTBE is not known to occur naturally in the environment.
Industrially, it is derived from the catalytic reaction of methanol
and isobutylene, and has been produced in several countries in
increasing volumes since the late 1970s. MTBE is currently among the
50 highest production volume chemicals. In 1996, the USA capacity for
production was approximately 10.6 million tonnes, and it is
anticipated that the use of MTBE will continue to increase.
Approximately 25% of gasoline in the USA is blended with MTBE. MTBE is
almost exclusively used to provide both octane enhancement and an
increase in the oxygen content of gasoline. MTBE has been added to
gasoline in concentrations up to 17% by volume.

1.3 Environmental transport, distribution and transformation

After discharge into air, MTBE will largely remain in the air,
with smaller amounts entering soil and water. In the atmosphere, MTBE
can partition into rain. However, only a small amount is removed from
the atmosphere in this manner. Atmospheric transformation by hydroxyl
radicals produces a number of products including the photochemically
stable tertiary-butyl formate (TBF) and 2-methoxy-2-methylpropanol,
which is expected to be highly reactive with hydroxyl radicals,
yielding CO₂, formaldehyde, acetone and water. When MTBE is
discharged into water, a significant amount is dissolved, with some

partitioning into air. Partitioning into biota and into sediment is
low. Biodegradability in conventional assays is limited. Generally,
biodegradability is believed to be slow in the environment. When MTBE
is released to the soil, it is transported to the air through
volatilization, to surface water through run-off and to groundwater as
a result of leaching. MTBE can persist in groundwater.

1.4 Environmental levels and human exposure

There are few data on environmental levels and human exposure.

In studies of MTBE in urban air of some cities using oxygenated
gasoline with 15% MTBE, ambient concentrations ranged from
non-detectable to 100.9 µg/m³(0.028 ppm), with several median
concentrations ranging from 0.47 to 14.4 µg/m³ (0.00013 to 0.004
ppm). Concentrations of MTBE in urban air of some cities where MTBE
was used as an octane enhancer at lower concentrations ranged from
non-detectable to 26.4 µg/m³ (0.0073 ppm).

Concentrations at ground level or near refineries ranged from 15
to 281 µg/m³. Median levels in urban air near blending facilities
were 1508 µg/m³ (0.419 ppm), with ranges of 216-35 615 µg/m³ (0.06
to 9.8 ppm).

At service stations in areas where oxygenated gasoline containing
10-15% MTBE is used, concentrations were highest in the breathing zone
during consumer refuelling (range of 300 to 136 000 µg/m³ (0.09 to 38
ppm), with levels rarely exceeding 3600 µg/m³ (10 ppm), slightly
lower at the pump island (non-detectable to 5700 µg/m³ (1.6 ppm) and
lowest at the station perimeter (non-detectable to 500 µg/m³ (0.14
ppm). Levels were generally higher at service stations without vapour
recovery systems.

Levels in the automobile cabin were 7 to 60 µg/m³ (0.002 to
0.017 ppm) during commutes and 20 to 610 µg/m³ (0.006 to 0.172 ppm)
during refuelling.

Based on limited monitoring confined almost exclusively to the
USA, MTBE has been detected in snow, stormwater, surface water
(streams, rivers, and reservoirs), groundwater and drinking-water.
Concentrations of MTBE detected in stormwater ranged from 0.2 to 8.7
µg/litre with a median of less than 1.0 µg/litre. For streams, rivers
and reservoirs, the range of detection was from 0.2 to 30 µg/litre,
and the range of medians for several studies was 0.24 to 7.75
µg/litre.

MTBE has generally not been detected in deeper groundwater or in
shallow groundwater in agricultural areas. When detected, the
concentration is less than 2.0 µg/litre. MTBE is more frequently found
in shallow groundwater (top 5-10 feet of these aquifers) in urban
areas. In this setting, the concentrations range from less than 0.2
µg/litre to 23 mg/litre, with a median value below 0.2 µg/litre.

MTBE is infrequently detected in public drinking-water systems
from groundwater. In all but 3 out of 51 systems in which it was
reported, the concentration was <20 µg/litre. There are inadequate
data to characterize the concentration of MTBE in public
drinking-water systems from surface water. MTBE has been found at high
levels (i.e. >1000 µg/litre) in a few private wells used for
drinking-water. However, it is doubtful that humans would consume
water with concentrations of MTBE greater than about 50-100 µg/litre
because of its low taste and odour threshold.

Workers with potential exposure to MTBE include those involved in
the production and distribution and use of MTBE and MTBE-containing
gasoline, including service station attendants and mechanics.

Short-term exposure (<30 min) in routine manufacturing
operations and maintenance of neat MTBE ranged from 715 to 43 000
µg/m³ (0.2 to 12 ppm), with average median values being about 3400
µg/m³ (0.95 ppm). Longer-term (30 min to 8 h) exposure ranged from
360 to 890 000 µg/m³ (0.01 ppm to 250 ppm), with median levels being
about 540 µg/m³ (0.15 ppm). For workers in blending operations,
short-term values ranged from non-detectable to 360 000 µg/m³ (100
ppm), the average median being about 5700 µg/m³ (1.6 ppm). Long-term
values ranged from non-detectable to 257 000 µg/m³ (72 ppm), the
average median being about 2000 µg/m³ (0.6 ppm).

Exposures were highest during transportation of neat MTBE and
fuel mixtures through pipelines, barges, railroad cars and trucks
(neat MTBE only), short-term values ranging from 4 to 3750 mg/m³
(0.001 to 1050 ppm) with an average median value of 140 mg/m³ (39
ppm). Long-term values ranged from 0.036 to 2540 mg/m³ (0.01 to 712
ppm), the average median value being 2.85 mg/m³ (0.8 ppm). In
distribution (i.e. loading of MTBE fuel mixtures on trucks and
delivering and unloading at service stations), short-term values
ranged from non-detectable to 225 mg/m³ (63 ppm), the average median
values being around 21 mg/m³ (6 ppm). Long-term values ranged from
0.036 to 22 mg/m³ (0.01 to 6.2 ppm), the average median value being
1.79 mg/m³ (0.5 ppm).

Median short-term exposure levels of service station attendants
ranged generally from 1.071 to 21.42 mg/m³ (0.3 to 6 ppm) and rarely
exceeded 35.7 mg/m³ (10 ppm). Median long-term exposure levels of
service station attendants averaged 1.79 mg/m³ (0.5 ppm). Median
exposures of mechanics were below detection levels for one short-term
study; the average median value for long-term exposure was
approximately 360 µg/m³ (0.1 ppm).

1.5 Kinetics and metabolism

Toxicokinetic data on MTBE in humans are mainly derived from
controlled studies in healthy adult volunteers and in a population
exposed to oxygenated gasoline. MTBE is rapidly absorbed into the
circulation following inhalation exposure. In healthy human volunteers
exposed by inhalation, kinetics of MTBE are linear up to

concentrations of 268 mg/m³ (75 ppm). Tertiary-butyl alcohol (TBA),
a metabolite of MTBE, was measured in blood and urine of exposed
humans. The peak blood levels of MTBE and TBA ranged from 17.2 to 1144
µg/litre, and 7.8 to 925 µg/litre, respectively, in humans exposed to
5.0 to 178.5 mg/m³ (1.4 to 50 ppm) MTBE. Based on a monocompartmental
model, rapid (36-90 min) and slower (19 h) components of MTBE
half-life have been identified.

In rodents, MTBE is well absorbed and distributed following oral
administration and inhalation exposure, with lower dermal absorption.
At 400 mg/kg oral and 28 800 mg/m³ (8000 ppm) inhalation exposure,
the percentage of total absorbed dose eliminated in expired air
increased with a corresponding decrease in the percentage eliminated
in urine, indicating a saturation of metabolism. TBA was not
identified in the urine of exposed rats. There is evidence of further
metabolism of TBA, based on the identification of
2-methyl-1,2-propanediol and alpha-hydroxyisobutyric acid excreted in
the urine. In vitro studies provide evidence that MTBE is
metabolized to TBA, formaldehyde and acetone.

1.6 Effects on laboratory animals and in vitro systems

In rats, the acute median oral lethal dose (LD₅₀) is
approximately 3800 mg/kg bw. The acute median lethal concentration
(LC₅₀) value for a 15-min inhalation exposure is about 141 000 mg/m³
air in mice. Signs of intoxication include CNS depression, ataxia and
laboured respiration. When the dose was non-lethal, recovery was
complete. The LD₅₀ for dermal toxicity in rabbits is >10 200 mg/kg
bw.

In a single identified study, MTBE was "moderately" irritating to
skin, causing moderate erythema and oedema following dermal
application to rabbits. It was also irritating to the eyes of rabbits,
causing mild, reversible changes. In the only identified study, MTBE
induced slight to severe respiratory irritation following exposure of
mice to 300 to 30 000 mg/m³, respectively. It did not induce skin
sensitization in studies in guinea-pigs.

Repeated exposure results primarily in increases in organ weights
and histopathological effects in the kidney of rats and the liver of
mice. Lowest reported effect levels for nephrotoxicity following
ingestion in 90-day studies are 440 mg/kg bw per day (increases in
relative kidney weight and hyaline droplet formation in Sprague-Dawley
rats). With inhalation exposure to 2880 mg/m³ (800 ppm), there were
increases in kidney weight associated at higher concentrations with a
mild increase in hyaline droplets in the proximal tubules in
Fischer-344 rats. In inhalation oncogenicity studies, at 1440 mg/m³
(400 ppm) the incidence and severity of chronic progressive
nephropathy was increased in male rats; in male mice, at this level,
there were increases in absolute liver weight (which correlated with
increases in hepatocellular hypertrophy at higher concentrations) and
an increase in relative kidney weight.

Exposure to MTBE also results in reversible central nervous
system (CNS) effects including sedation, hypoactivity, ataxia and
anaesthesia at higher concentrations and biphasic effects on motor
activity at lower concentrations. In a single 6-h inhalation exposure
study in rats, dose levels from 2880 mg/m³ (800 ppm) produced
reversible dose-related changes in motor activity in single sexes.
These effects were transient and not evident in longer-term studies.

One- and two-generation inhalation reproductive studies in rats
and four developmental studies in rats, mice and rabbits have been
identified. In these studies, specific reproductive effects were not
observed in rats at concentrations up to 28 800 mg/m³. MTBE has not
induced developmental effects at concentrations below those that were
toxic to the mothers. Decreases in uterine weight and increases in
estrogen metabolism in mice have been observed at 28 800 mg/m³.

MTBE has been adequately tested in a broad range of mutagenicity
and other genotoxicity tests. The results from these studies indicate
that MTBE is not genotoxic, although a mouse lymphoma cell tk locus
mutation assay was positive, due to the metabolism of MTBE to
formaldehyde.

Carcinogenicity studies have been conducted involving inhalation
exposure of Fischer-344 rats and CD-1 mice and gavage dosing of
Sprague-Dawley rats. In neither of the inhalation studies were methods
of statistical analysis used that adjusted for survival differences.
There were significant increases in tumour incidence in all three
studies, namely renal tubular cell tumours and Leydig cell tumours in
the male Fischer-344 rats, Leydig cell tumours in male and
leukaemias/lymphomas (combined) in female Sprague-Dawley rats, and
liver cell tumours in female CD-1 mice. The renal tubular cell tumours
and the leukaemia/lymphomas were not observed consistently, therefore,
in the different studies in rats. In addition, the sex-specific kidney
tumours were associated with sex-specific alpha2u-globulin
nephropathy, which was observed in several studies of short duration.
Increases in Leydig cell tumours occurred at the highest dose level
(1000 mg/kg bw) in the Sprague-Dawley rats, but interpretation of the
increases recorded for Fischer-344 rats was complicated by the very
high concurrent and historical control incidences. The mouse liver
tumours occurred at incidences in the control and 28 800 mg/m³ (8000
ppm exposed groups, respectively, of 2/50 and 10/50 in females and
12/49 and 16/49 in males. The increases were modest and were
accompanied by hepatocellular hypertrophy.

1.7 Effects on humans

Following the introduction of two separate fuel programmes in the
USA requiring the use of gasoline oxygenates (not necessarily MTBE),
consumers in some areas have complained about acute health symptoms
such as headache, eye and nose irritation, cough, nausea, dizziness
and disorientation. Epidemiological studies of human populations
exposed under occupational as well as non-occupational conditions, and
experimental studies of human volunteers exposed under controlled

conditions, have not been able to identify a basis for these
complaints. Although results are mixed, community studies conducted in
Alaska, New Jersey, Connecticut, and Wisconsin, USA, have provided
limited or no evidence of an association between MTBE exposure and the
prevalence of health complaints.

In controlled experimental studies on adult volunteers exposed in
inhalation chambers to MTBE at concentrations ranging from 5.0 mg/m³
(1.4 ppm) up to 270 mg/m³ (75 ppm), there were no evident effects in
terms of either subjective reports of symptoms or objective indicators
of irritation or other effects up to 180 mg/m³ (50 ppm) for as long
as 2 h. From this evidence it appears unlikely that MTBE alone induces
adverse acute health effects in the general population under common
conditions of inhalation exposure. However, the potential effects of
mixtures of gasoline and MTBE, and the manner in which most persons
are exposed to MTBE in conjunction with the use of oxygenated fuels,
have not been examined experimentally or through prospective
epidemiological methods. Moreover, the role of factors such as
awareness of MTBE, due in part to its distinctive odour, for example,
have not been investigated.

1.8 Effects on other organisms in the laboratory and field

The experimental acute toxicity (LC₅₀) of MTBE to fish,
amphibians and crustaceans is > 100 mg/litre. There are no data on
chronic or sub-lethal toxicity to aquatic species, or on toxicity to
terrestrial organisms.

1.9 Evaluation of human health risks and effects on the environment

Based on collective evidence, it appears unlikely that MTBE alone
induces adverse acute health effects in the general population under
common exposure conditions.

In studies on animals, MTBE is "moderately" acutely toxic and
induces mild skin and eye irritation but not sensitization. Repeated
exposure affects primarily the kidney of rats and the liver of mice,
with lowest reported adverse effect levels of 440 mg/kg bw per day in
rats following ingestion and 1440 mg/m³ (400 ppm) following
inhalation. MTBE has not induced adverse reproductive or developmental
effects at concentrations less than those that were toxic to the
parents.

MTBE is not genotoxic but has induced tumours in rodents
primarily at high concentrations that also induce other adverse
effects. These data are considered currently inadequate for use in
human carcinogenic risk assessment. The Task Group concluded that, in
order to provide quantitative guidance on relevant limits of exposure
and to estimate risk, acquisition of additional data in several areas
is necessary.

It does not appear that the concentrations of MTBE in ambient
water are toxic to aquatic organisms except during spills. Although
there are no data on the terrestrial toxicity of MTBE, this appears
not to be of concern since concentrations in ambient air are low and
its half-life is relatively short.

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS

2.1 Identity

Chemical formula: C₅H₁₂O

Chemical structure: CH₃
'
H₃C - C - O - CH₃
'
CH₃

Relative molecular mass: 88.15

Common name: methyl tertiary-butyl ether

IUPAC Chemical name: 2-methoxy-2-methyl propane

CAS registry number: 1634-04-4

Synonyms: 1,1-dimethylethyl methyl ether; ether
tert-butyl methyl; éther methyl
tert-butylique (French); MBE; methyl
1,1-dimethylethyl ether; methyl- t-butyl
ether; methyl tert-butyl ether;
(2-methyl-2-propyl) methyl ether;
metil-terc-butileter (Spanish);
2-methoxy-2-methylpropane; MTBE; propane,
2-methoxy-2-methyl-(CA); t-butyl methyl
ether; tert-butoxymethane; tert-butyl
methyl ether

Major trade names: 3 D Concord
Driveron
HSDB 5487
UN 2398

Constituent components of typical commercial grade:
(ARCO, 1989)

Component Weight %

MTBE 97.5
di-, tri-isobutylene, and
t-butyl alcohol 0.6
Methanol 0.2
C4 hydrocarbons 1
C5 hydrocarbons 0.4
other 0.3
water content <0.05

2.2 Physical and chemical properties

Table 1 lists the physical and chemical properties of MTBE.

2.3 Conversion factors

1 ppm = 3.57 mg/m³ at 25°C (1 atmosphere pressure)
1 mg/m³ = 0.28 ppm at 25°C (1 atmosphere pressure)

2.4 Analytical methods

Analytical methods that have been used for MTBE and for
tertiary-butanol (TBA), which is an intermediate in the aerobic
bacterial degradation of MTBE and in its mammalian metabolism, are
given for various media.

Some commonly used methods are summarized in Table 2.

2.4.1 Procedures

2.4.1.1 Air

Air samples are collected in stainless steel canisters, and the
volatile compounds concentrated in a two-stage trap to sorb the
Table 1. Physical and chemical properties of MTBE organic compounds
and to collect water. Drying is done by purging with dry N₂ at 25°C,
and the organic compounds thermally desorbed at 220°C by back-flushing
with helium. The samples can be analysed by gas chromatography/mass
spectrometry (GC-MS) using a capillary column (Kelly et al., 1993).
Harper & Fiore (1995) used a passive diffusion technique to collect
samples.

Automobile exhaust samples are collected in 3-litre bags. Diluted
emissions are concentrated in variable temperature control traps,
operating between -60°C and 180°C (DB 1 column) or between -99°C and
180°C (GS-Q megabore column). Using these twin columns, separation of
all the major components is possible (Hoekman, 1993).

2.4.1.2 Soil, water and sediment

Static headspace analysis can be used for samples of soil and
groundwater. Samples are collected in filled bottles, air is
introduced, and the bottles are shaken and equilibrated before
analysis of the gas phase.

One method is by GC-FID/PID using a megabore DB-1 capillary
column (Roe et al., 1989). Samples of groundwater can be collected
with a cone penetrometer coupled with a porous probe, and analysed by
GC using a photoionization detector (PID) (Chiang et al., 1992).

Table 1. Physical and chemical properties of MTBE

Physical state Liquid

Colour Colourless

Odour Strong, characteristic terpene-like

Freezing point (°C) -109 Windholz, 1983

Boiling point (°C) 53.6-55.2 Mackay et al., 1993
Selected value^a 55.2

Flash point (°C) -28 Budavari et al., 1996

Ignition temperature (°C) 224 Budavari et al., 1996

Spontaneous ignition temperature (°C) 460 Wibowo, 1994

Flammability Flammable/combustible

Flammability limits 1.5-8.5% in air ECETOC, 1997

Vapour pressure (Pa at 25°C) 32 659 to 33 545 Mackay et al., 1993
Selected value^a 33 500 Mackay et al., 1993

Density (g/cm³ at 20°C) 0.7404 to 0.7478 Mackay et al., 1993
Selected value^a 0.7404

Relative vapour density (air=1) 3.1 Wibowo, 1994

Log k_ow octanol/water partition coefficient 0.94 to 1.30 Mackay et al., 1993
Selected value^a 0.94

Henry's law constant at 25°C (Pa m³/mol) 59.46 to 304.96 Mackay et al., 1993
Selected value^a 70.31

Table 1. (continued)

Physical state Liquid

Dimensionless Henry's law constant (H/RT) at 25°C 0.0239 to 0.1221 Zogorski et al., 1996
Selected value^a 0.018 at 20°C

Water solubility g/m³ at 25°C 32 200 to 54 353 Mackay et al., 1993
Selected value^a 42 000 (at 19.8°C)

Solubility of MTBE in water 48 Budavari et al., 1996
(g/litre) at 25°C

Solubility of water in MTBE (g/litre) at 25°C 15 Budavari et al., 1996

Solubility in organic solvents: - very soluble in other ethers and
alcohols
- mixes with gasoline

Viscosity, g/sec. -cm 0.003 to 0.004 (calculated) Lyman et al., 1990

Other properties Unstable in acid solution pK_a = -3.70
at 23°C (measured)

Organoleptic properties

Taste 134 µg/litre (0.134 ppm) TRC, 1993

Odour
- detection threshold 0.19 mg/m³ TRC, 1993
- recognition threshold 0.29 mg/m³ (0.08 ppm) TRC, 1993

^a Criteria of selection were based on:
i) the age of the data and acknowledgement of previous conflicting or supporting values;
ii) the method of determination;
iii) the perception of the objectives of the investigators, and their need for quantitative values; and
iv) information derived from Quantitative-Structure-Property-Relationships.

Table 2. Summary of analytical procedures for MTBE

Matrix Procedure Detector Detection limit Reference

Air Sorption/desorption GC-MS 0.72-3.6 µg/m³ Kelly et al., 1993

Vehicle emission Sorption/desorption GC-FID 18-36 µg/m³ Hoekman, 1993

Water Static headspace GC-PID 10.8 µg/m³ (water) Chang et al., 1992
1.08 µg/m³ (air)

Water Purge and trap GC-MS 5 µg/litre Bianchi & Varney, 1989

Water Purge and trap GC-MS 0.52-0.090 µg/litre Munch & Eichelberger, 1992

Water Purge and trap GC-MS 0.06 µg/litre Raese et al., 1995

Sediment Purge and trap GC-MS 10-100 ng/kg Bianchi et al., 1991

Blood Purge and trap GC-MS 0.01 µg/litre Bonin et al., 1994

Gasoline Direct GC-FID 18-36 µg/m³ Johansen, 1984
(5-10 ppbv)

For samples of water and sediment, purge and trap procedures are
widely used to concentrate volatile components before analysis. For
water samples, the analytes are desorbed by open-loop stripping for
60 min at 60°C and collected on a mixture of Tenax TA and
Chromosorb-106. Desorption is then done using helium at 150°C before
analysis.

Analysis can be by GC-MS (Bianchi & Varney 1989). An expanded
procedure for volatile organic compounds developed by the US
Environmental Protection Agency (US EPA) uses a three-trap collection
system (Tenax, silica gel and charcoal) followed by GC-MS
quantification: for MTBE, a detection limit of 0.09 µg/litre was
attained using a DB-624 capillary column and a purging efficiency of
74% (Munch & Eichelberger 1992). An essentially similar procedure has
been used for estuarine sediment samples with an OV-1701 capillary
column (Bianchi et al., 1991).

MTBE in ambient groundwater has been analysed by the US
Geological Survey since 1991 using a purge and trap GC-MS method
(Raese et al., 1995). The estimated detection limit for reagent water
spiked with MTBE at 0.2 µg/litre is 0.06 µg/litre. A method for the
concurrent analysis of MTBE, TBA and tert-butyl formate (TBF) has
been developed (Church et al., 1997). The method employs direct
aqueous injection and GC-MS, and has a detection level of 0.1 µg/litre
for MTBE.

2.4.1.3 Gasoline

Samples of gasoline can be analysed directly by GC using the
following procedures. They have all shown good selectivity for
oxygenates:

- An infrared (IR) detector, using a column of Poropak Q plus
Poropak N, gave a limit of detection of 0.1% (w/v) with the
detector set at 8.3 µm (Cochrane & Hillman 1984).
- A detector system (GC-O-FID), in which oxygenates are
catalytically cracked to CO followed by reduction to methane, has
a selectivity better than 1:10⁷ (Verga et al., 1988).
- FID with a dual column system using Durawax 1 and Durabond-S
gives acceptable accuracy and repeatability at a concentration of
1% (w/w) (Levy & Yancey 1986). An alternative procedure uses
switching (Johansen 1984).
- Atomic emission detection (AED) using 777 nm near infrared (NIR)
emission and a DB-1 capillary column is a sensitive method (Diehl
et al., 1995).
- Reversed-phase liquid chromatography (RPLC) with a Hi-Chrom
"reversible column" packed with Spherisorb ODS-11 and a
refractive index detector (RID) can be used with a mobile phase
of acetonitrile:water (6:4) and back-flushing suited to the
relevant analytes (Pauls 1985). It is important that the analyte
is completely dissolved in the mobile phase.

2.4.1.4 Biological samples

Headspace or purge-and-trap concentrations of MTBE are directly
applicable to blood and urine samples. The purge and trap procedure is
coupled to quantification by GC-MS using ²H-labelled standards.
Direct GC analysis of samples is less commonly used but Schuberth
(1996), using the full headspace technique combined with capillary GC
and ion-trap detection, determined MTBE with a detection limit of
0.4-1 nmol in blood and brain tissue.

a) Blood, urine and tissues

The purge-and-trap system can be used for the analysis of blood
samples. Sorption is done with a Tenax trap and a cryogenic trap
decreasing in temperature to -150°C with desorption at 180°C. GC-MS
analysis uses a DB 624 column. This has been applied to MTBE and to
TBA using [²H₁₂] MTBE and [²H₉] TBA as the respective standards
(Bonin et al., 1994).

Headspace analysis has been used for the analysis of both MTBE
and metabolically produced TBA in a range of matrices including blood
and urine. For blood samples, GC with an SE 50 column and FID can be
used (Savolainen et al., 1985). Analysis of TBA produced from MTBE by
hepatic microsomes from rats can be made with a Carbowax B/5% Carbowax
20M packed column and FID (Brady et al., 1990). A procedure applicable
to blood and urine samples uses an SPB-1 column and FID (Streete et
al., 1992). However, this procedure appears not to have been validated
using samples contaminated with MTBE or TBA. The procedure can be
applied to tissue samples after treatment with a proteolytic enzyme
before analysis.

Analysis of MTBE (and TBA) in brain (cerebral hemispheres) and in
perirenal fat from rats dosed with MTBE was made by homogenizing the
samples in dimethyl formamide, centrifuging, and direct GC analysis of
the supernatant using a packed column with Carbowax 20M and FID
(Savolainen et al., 1985).

b) Bacterial cultures

Samples of bacterial cultures that metabolize MTBE have been
analysed for both MTBE and its metabolite TBA by direct GC analysis
using FID and a Quadrex methyl silicone capillary column (Salanitro et
al., 1994). Analysis of MTBE (and TBA) in bacterial cultures that
degraded TBA, though not MTBE, used a GC capillary column coated with
a cross-bound phase (CP-Sil 13, Chrompack) and an FID detector (Allard
et al., 1996).

¹⁴C-labelled MTBE has been used in a few investigations. In one
study dealing with aerobic biodegradation, ¹⁴CO₂ was collected after
incubation as Ba¹⁴CO₃, and the fraction incorporated into cells was
separated by filtration though 0.45 µm Millipore filters (Salanitro et
al., 1994). In another study on the accumulation of MTBE into plants,
samples were extracted with dimethylformamide for counting (Schroll et
al., 1994).

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

Natural sources of MTBE have not been reported in the scientific
literature.

3.2 Anthropogenic sources

3.2.1 Production levels and processes

MTBE is an oxygenate (oxygen-containing hydrocarbon) that is
industrially produced in several countries, including Austria,
Belgium, Canada, Finland, France, Germany, Italy, Japan, Mexico, the
Netherlands, Norway, Portugal, Sweden, Taiwan, the United Kingdom, the
USA and Venezuela.

The worldwide annual production of MTBE in 1995 was about 15
million tonnes. In the USA, in 1994 MTBE ranked 18th in terms of
production volume (6 175 000 tonnes (13.61 billion pounds)) and in
1995 there was an increase to 12th position (8 000 000 tonnes (17.62
billion pounds)) (CEN, 1996). During the years 1985-1995, production
of MTBE in the USA showed an annual increase of 25% (Storck et al.,
1996). The potential demand for MTBE is expected to increase to
284 000 barrels/day (12.2 million tonnes per year) in the year 2000.

North America is the largest consumer of MTBE, accounting for
about two-thirds of the world's annual use. In 1996 the USA was the
world's largest consumer of MTBE with a usage of 246 000 barrels/day
(10.6 million tonnes per year). Western Europe, the eastern
Mediterranean area and Asia, and Latin America used progressively
smaller amounts of MTBE in 1995. Most growth in the production
capacity of MTBE is expected to occur in the eastern Mediterranean
area, South America and the USA.

MTBE is prepared principally by reacting isobutylene (contained
in a mixed C4 stream) with methanol over an acidic ion-exchange resin
catalyst such as sulfonated styrene cross-linked with divinyl benzene
in the liquid phase and at 38-93°C and 100-200 psi. It can also be
prepared from methanol, TBA and diazomethane (Budavari et al., 1996).

3.2.2 Uses

The main use of MTBE is as an additive to gasoline. MTBE was
first added to gasoline in the late 1970s on a voluntary basis as an
octane enhancer when the phase-out of tetraethyl lead commenced, and
this use continues. MTBE is also added to gasoline in higher amounts
(up to 15% by volume) as part of national mandated air pollution
abatement programmes to reduce ambient air levels of carbon monoxide
(CO) or ozone, or both, and in reformulated gasoline (RFG) (10-11% by
volume) to reduce the emissions of benzene and other volatile
hydrocarbons. MTBE is also used in the manufacture of isobutene
(Lewis, 1993) and a minor proportion is used as a therapeutic agent

for in vivo dissolution of cholesterol gallstones in humans (Allen
et al., 1985a,b; Di Padova et al., 1986; Murray et al., 1988; Sternal
& Davis, 1992).

In the USA, oxygenated gasolines are required in two national
programmes to improve air quality (the oxygenated fuels programme and
the reformulated gasoline programme) outlined in the 1990 Clean Air
Act Amendments. MTBE is not specifically required in these programmes,
but it is the most widely used oxygenate. The winter oxygenates
programme requires gasoline sold in areas that do not meet federal air
quality standards for CO to contain no less than 2.7% oxygen by
weight, which is equal to 15% MTBE by volume. According to the
reformulated gasoline programme, large metropolitan areas with serious
ozone problems are required to use reformulated gasoline (RFG): this
is a special blend of gasoline that must contain 2% oxygen by weight
and a maximum of 1% benzene and 25% aromatic hydrocarbon by volume. To
meet this requirement, reformulated gasoline would contain 11% MTBE by
volume. About 90% of the MTBE consumed in the USA in 1996 was used in
reformulated gasoline. At the end of 1996, MTBE was used in
approximately 25% of the total gasoline pool.

During the winter driving season, 15% MTBE by volume is added to
gasoline as an oxygenate to reduce CO emissions from motor vehicles.
The extent of CO reductions depends on the fuel metering system and
emissions control technology used on the vehicle (Prakash, 1989). The
addition of oxygenates to gasoline blends generally reduces the
hydrocarbon (HC) emissions to the atmosphere. However, the levels of
exhaust nitrogen oxides (NO_x) increase when the oxygenate
concentration exceeds about 2% oxygen by weight (SNV, 1993). It also
increases the aldehyde emissions from automobile exhausts, but has not
been found to have any major influence on the chemical composition of
particulate emissions from vehicles (Watson et al., 1990). The
aldehyde (not specified) emissions are significantly reduced by
three-way catalytic converters (Prakash, 1989).

In a model analysis of changes in the concentrations of eight
volatile organic compounds (VOCs), i.e. acetaldehyde, benzene,
1,3-butadiene, ethylbenzene, formaldehyde, toluene, xylenes, and
particulate organic matter (POM), resulting from the use of
reformulated gasoline and oxyfuel containing MTBE, Spitzer (1997)
concluded that, with the exception of formaldehyde, exhaust emissions
of these VOCs would be decreased. The increased formaldehyde emissions
would, however, be offset by the reduction in the formation in the
atmosphere of formaldehyde from the other VOCs. Erdal et al. (1997)
modelled atmospheric ozone pollution reduction by the use of MTBE in
gasoline. Ozone is formed by the reaction of sunlight with NO_x and
VOCs. The use of MTBE reduces VOC and NO_x exhaust emissions and also
reduces fuel evaporation. The model estimates a reduction in peak
ambient ozone levels of 3.6-18 µg/m³ (1-5 ppb).

It is estimated that MTBE-blended gasolines account for
approximately 2% of the total unleaded gasoline in Canada (Environment
Canada, 1992). Levels of MTBE in blended gasolines range from 0.04% to
9.09% by volume, depending on the grade of gasoline, season and
geographical area. Since the use of oxygenates is not required in
Canada as part of an air abatement programme, each refiner blends in
the amounts of MTBE that it requires in order to obtain a good
gasoline end-product, depending on the batch of crude oil and the
technology used in the refinery. In 1997, the maximum concentration of
MTBE allowed in Canadian gasoline was 2.7% mass oxygen (approximately
15% by volume).

3.2.3 Sources and releases to the environment

Similar to hydrocarbon components of gasoline, fuel oxygenates
such as MTBE enter the environment during all phases of the petroleum
fuel cycle. Sources include, for example, auto emissions, evaporative
losses from gasoline stations and vehicles, storage tank releases,
pipeline leaks, other accidental spills, and refinery stack releases.
Annual estimates of MTBE mass releases to the environment from all
potential sources have not been reported in the scientific literature.
However, releases from storage tanks, vehicular emissions and
evaporative losses from gasoline stations and vehicles are perceived
to be important sources (Zogorski et al., 1996; US Interagency
Assessment, 1997).

3.2.3.1 Industrial releases

No information on industrial releases of MTBE to the environment
have been found in the scientific literature, except in the case of
the USA and Canada.

Industrial releases of MTBE in the USA have been characterized
for 1993. A total of 136 facilities released MTBE to the environment,
with an estimated total release of 1700 tonnes. Approximately 84% of
the release was by petroleum refineries, and almost all of the MTBE
was released to air (Zogorski et al., 1996).

In 1994, the total Canadian industrial release of MTBE from
refiners and manufacturers was approximately 28.2 tonnes, the bulk of
which was released into the air (98.1%) and a small amount into water
(1.9%) (Environment Canada, 1996a). The highest amounts of MTBE
released were 9.5, 9.1, 8.4 and 1.0 tonnes by industries located in
Sarnia, Burnaby, Edmonton and Saint John, respectively.

3.2.3.2 Storage tank release

Releases of gasoline containing MTBE from storage tanks may
contaminate soil and groundwater. In some cases, MTBE may enter
drinking-water supplies. In 1989 it was estimated that in the USA
there were approximately 14 000 above-ground storage tank facilities
with an estimated 70 000 tanks, of which 30-40% were used for gasoline
storage (API, 1989a). A subsequent survey of 299 storage facilities

showed that 40% had identified subsurface contamination (API, 1994).
Many sites have been identified with soil or groundwater hydrocarbon
contamination that required corrective action. The extent of MTBE
contamination at these sites is largely undocumented because
monitoring of MTBE has not been required. More stringent
release-prevention and -detection standards are now required in the
USA and, when fully implemented by December 1998, these requirements
should considerably decrease the annual volume of gasoline released to
soil and groundwater.

It is important to note that when gasoline containing MTBE enters
groundwater, high concentrations of MTBE (i.e. in excess of 1000
µg/litre) can occur. While comprehensive data on the occurrence of
MTBE in drinking-water provided from groundwater do not exist, there
have been some instances reported in the USA where drinking-water
supplies have been disrupted because of high MTBE levels. For example,
two well fields serving the city of Santa Monica, California, have
been contaminated with MTBE necessitating the purchase of alternative
water for drinking-water.

3.2.3.3 Engine emissions from on-road and off-road vehicles and
recreational boats

The use of gasoline containing MTBE in on-road and off-road
vehicles, boats and small engines will result in MTBE releases to the
environment unless recovery systems are employed. The extent of these
emissions has not been thoroughly studied, and there are few
scientific citations.

Drivas et al. (1991) estimated ambient air concentrations of
evaporative and exhaust emissions of MTBE gasoline blends during two
different situations representing worst-case concentrations: a car
idling in an open garage and a car just stopped and turned off
(hot-soak evaporative emission) in a closed garage. The predicted
maximum exhaust air concentration of MTBE was calculated to be
0.24 mg/m³ (0.07 ppm).

MTBE was not detected in samples from light-duty vehicle
emissions measured in the Caldecott Tunnel, San Francisco Bay Area, in
August 1994, when the average oxygen content of gasoline sold in the
area was 0.3% by weight (Kirchstetter et al., 1996). In October, when
the average oxygen content in MTBE gasoline was 2.0% by weight, the
concentration of MTBE in emissions was 3.3% by weight of total VOCs.

Comparison of emissions from vehicles using a standard fuel and a
reformulated fuel that contained MTBE (11% by volume) showed a
reduction in mass emission rates in the latter (Hoekman, 1992).
Although there was a decrease in the emissions of aromatics and
alkanes, the levels of alkenes and carbonyl compounds increased, and
there was considerable variation among the vehicles that were tested.
A study in California showed that increasing the concentration of MTBE
from 0.3% by weight in August to 1.6 % MTBE plus 0.4% ethanol in
October resulted in lowered emission of aromatics but increased

emissions of isobutene (86%), cisbut-2-ene (150%), formaldehyde (39%),
propionaldehyde (200%), methacrolein (50%) and butyraldehyde (40%)
(Kirchstetter et al., 1996).

Boat motors and small engines used in chain saws, other power
tools, snowmobiles, lawn mowers and garden tillers, for example, may
also release MTBE to the environment via exhaust, evaporative losses
and release of uncombusted fuel. The magnitude and significance of
these releases are not documented. In 1997 MTBE was detected in
several public water supply reservoirs that, in part, provide
drinking-water for Southern California. The predominant source of MTBE
is thought to be associated with small engines used on recreational
boats. Such engines are known to be inefficient, and release
uncombusted gasoline and emissions to water and air.

3.3 Other pertinent information

All aspects of the effectiveness of fuel oxygenates on ambient
air quality, including carbon monoxide, hydrocarbons, oxides of
nitrogen, aromatics, aldehydes and alcohols, and associated
atmospheric degradation products, have been reviewed in a number of
reports (e.g., Prakash, 1989; Environment Canada, 1993; Schuetzle et
al., 1994; HEI, 1996; Kirchstetter et al., 1996; US Interagency
Assessment, 1997).

Overall, these studies indicate that, when compared to other
gasolines, MTBE gasoline blends generally reduce CO and hydrocarbon
exhaust emissions and increase aldehyde and NO_x emissions.

4. ENVIRONMENTAL BEHAVIOUR AND FATE

4.1 Transport and distribution between media

A diagram depicting the movement of MTBE in the environment is
shown in Fig. 1.

4.1.1 Air

It can be predictable from its physicochemical properties that,
when MTBE is released into air, the greater part will exist in the
atmosphere, with small amounts entering soil and water (Mackay et al.,
1993). Based on its Henry's law constant, MTBE should partition into
atmospheric water, including rain. The concentration of MTBE in
precipitation would be in direct proportion to its concentration in
air. However, falling precipitation removes only a negligible amount
of the gas-phase compound (Zogorski et al., 1996). Therefore, chemical
degradation of MTBE should be the major removal process from the air
(Mackay et al., 1993).

4.1.2 Water

Transport and distribution of a substance between and within
media in the aquatic environment is dependent upon its solubility,
movement of the water itself, exchanges at the air-water interfaces,
adsorption to sediment and particulate matter, and bioconcentration in
aquatic organisms. The residence time in water is also dependent upon
the type of environmental conditions encountered, such as
temperatures, wind speeds, currents and ice cover (Environment Canada,
1993).

MTBE can volatilize from surface water and be removed by aeration
(Zogorski et al., 1996). According to calculations by Pankow et al.
(1996), no single volatilization half-life (t_´) will characterize
the loss process from water. In surface water, the most important
factors for the volatilization rates are the depth and velocity of the
flow. In deep and slow-moving flows, the t_´ values at both 5°C and
25°C are 85 and 78 days for calm and windy conditions, respectively.
These rates were shown to be similar to those for benzene, toluene,
ethyl benzene and xylene (BTEX) compounds. In shallow and fast-moving
flows, changing from calm to windy conditions causes a significantly
accelerated volatilization rate. Under these circumstances, MTBE
volatilizes more slowly than benzene, although it was suggested that
this is of no practical significance, as both substances volatilize
quickly in such flows. It was concluded that the t_´ values for MTBE
are highly dependent on depth and mean flow velocity. Thus, quite
large as well as very small t_´ values are possible.

Based on physicochemical properties, it can be predicted that a
release of MTBE into water would result in significant amounts being
dissolved. Most of the MTBE remains in the surface water, with some
partitioning into air and much smaller amounts into sediment and soil
(Mackay et al., 1993). The low K_ow of 0.94 suggests that partitioning
from the water to particulates and sediment is not significant. On the
basis of bioconcentration data, MTBE is not subject to bioaccumulation
or biomagnification in aquatic organisms (Environment Canada, 1993).
In the water compartment, the key removal process should be
volatilization. The amount transferred to sediment is negligible
(Mackay et al., 1993; Environment Canada, 1993).

For a gasoline containing 10% MTBE by weight, and assuming that
it does not undergo depletion of the MTBE concentration in the
gasoline due to dissolution into the water, the water solubility of
the MTBE from gasoline will be approximately 5 gm/litre at 25°C. By
comparison, the total hydrocarbon solubility for non-oxygenated fuel
is about 120 mg/litre (Poulsen et al., 1992; Zogorski et al., 1996).

The ability of MTBE to enhance the solubility in water of
monocyclic aromatic gasoline components including BTEX compounds has
been examined in models, and an increase was predicted only at
co-solvent concentrations of greater than 1% (Mihelcic, 1990). In
confirmation of this, the co-solvent effect of MTBE on the aqueous
solubility of hydrocarbons in gasoline was found to be minimal (Cline
et al., 1991). Measurements made in the laboratory in shake-flasks
showed that up to 15% MTBE was unlikely to result in enhanced
concentrations of BTEX in contaminated groundwater (Poulsen et al.,
1992). Such high concentrations of MTBE seem unlikely to be achieved
in groundwater after spillage of gasoline containing MTBE, and
although MTBE is widely distributed in shallow urban groundwater at
low concentrations in the USA, its occurrence in these samples was not
associated with correspondingly increased concentrations of BTEX
(Squillace et al., 1996).

4.1.3 Soil

Based on its physicochemical properties, it can be predicted that
when MTBE is released to the soil, it can be transported to the air
through volatilization, to surface water through run-off, and to
groundwater as a result of leaching. In the first two instances, the
release would have to be at, or near the soil surface. If the release
of MTBE occurs below the soil surface, for example from an underground
storage tank, then the most likely transport mechanism will be
leaching to groundwater. Based on its vapour pressure, volatilization
of MTBE from soil and other surfaces is expected to be significant.
Soil adsorption and mobility are based on the reported and estimated
K_oc (organic carbon sorption coefficient) values. Compounds with a
K_oc of <100 are considered to be moderately mobile. Thus MTBE, with
a Koc of 91, does not adsorb to soil particles to a great degree and
would be considered mobile. Parameters other than K_oc affecting the
leaching of MTBE to groundwater include the soil type (e.g., sandy

versus clay), the amount and frequency of rainfall, the depth of
groundwater, and the extent of degradation of the MTBE (Environment
Canada, 1993).

4.1.4 Multimedia

Several multimedia models using various emission rates and
environmental parameters have been used to predict the distribution
and concentration of MTBE in the environment (Environment Canada,
1993; Mackay et al., 1993; Hsieh & Ouimette, 1994).

4.2 Bioconcentration

Fujiwara et al. (1984) conducted studies on the bioconcentration
of MTBE in Japanese carp (Cyprinus carpio) in a flow-through system
at 25°C. The mean whole-body steady-state bioconcentration factor
(BCF) was 1.5. Further observations indicated that fish exposed for 28
days and then transferred to clean water eliminated almost all MTBE
residues within 3 days. These experimental data support the hypothesis
that MTBE has little tendency to bioaccumulate. Veith & Kosian (1983)
calculated a BCF of 2.74 (r² = 0.927) for a 28-day exposure of
fathead minnows, based on a Quantitative Structure-Activity
Relationship (QSAR).

Compounds with log K_ow values of approximately 5.0 or less do
not have significant food chain build-up. MTBE belongs to this group
(Environment Canada, 1993). Uptake from water is more important than
from food for this group of compounds.

When ¹⁴C-labelled MTBE was applied to the soil in a closed
aerated system, the concentrations of MTBE in the roots and the aerial
parts of lettuce and radish showed that transport was dominated by
foliar uptake; subsequently, translocation into the roots took place
(Schroll et al., 1994). Although neither MTBE nor its potential
metabolite TBA was detected in the plants, a considerable fraction of
the ¹⁴C label was unaccounted for and was presumed to be associated
with plant constituents.

4.3 Biodegradation and transformation

Only a limited amount of work has been accomplished on the
biodegradability of MTBE. Moreover, the studies are difficult to
compare because they have been performed under a wide variety of
conditions. Aerobic and anaerobic experiments have been conducted. For
most studies, it has been demonstrated that MTBE is difficult to
biodegrade. In contrast, BTEX is more readily biodegraded (Zogorski et
al., 1996). Half-lives for MTBE in various environmental compartments
are shown in Table 3

Table 3. Half-life ranges of MTBE in various compartments

Environmental Half-life ranges Comments Reference
compartment (h)

Air 20.7-265 Based upon measured Howard et al., 1991
photo-oxidation half-life
10-30 Mackay et al., 1993

Soil 672-4320 Estimation based upon US EPA, 1989
300-1000 aerobic biodegradation Mackay et al., 1993
half-life

Surface water 672-4320 Estimation based upon Howard et al., 1991
aerobic biodegradation
300-1000 half-life Mackay et al., 1993

Sediment 1000-3000 Mackay et al., 1993

Groundwater 1344-8640 Estimation based upon Howard et al., 1991
aerobic biodegradation
half-life
2688-17 289 Estimation based on Howard et al., 1991
anaerobic degradation
half-life

4.3.1 Aerobic conditions

Results from tests involving biodegradation of MTBE have been
variable.

Pence (1987a) used an acclimated culture containing active
sludge, soil inoculum and raw sewage. The uptake of oxygen was
measured in a mineral medium supplemented with MTBE added to the
acclimated culture at a concentration of 5 mg/litre on days 0, 7 and
11. The results showed that MTBE was poorly biodegradable under these
conditions; only 5.4% biodegradation occurred within 28 days.

No biodegradation of MTBE after 60 days was found in experiments
using aquifer soil material as inoculum; with two types of activated
sludge as inoculum, no degradation of MTBE occurred after 40 days
(Möller Jensen & Arvin, 1990).

With a standard activated sludge, and based on the oxygen uptake
rate, MTBE was biodegraded very slowly (Fujiwara et al., 1984). The
hydrocarbon components of gasoline blended with MTBE were, however,
readily degraded even though the MTBE remained.

A mixed bacterial culture was obtained by enrichment of a
hydrocarbon-contaminated soil in a basal mineral medium containing:
(i) TBA (1 g/litre) as sole carbon source or (ii) methylamine (2
g/litre) as principal carbon source supplemented with TBA. During
incubation of the first culture, the concentration of TBA fell to zero
in 20 days, but incubation of methylamine-grown cells with MTBE showed
no reduction in the concentration of MTBE after 42 days (Allard et
al., 1996). Whereas MTBE was apparently recalcitrant under the
conditions used, TBA, which is one of its putative degradation
products, was biodegradable.

In contrast to these results, a mixed bacterial culture obtained
by continuous aerobic enrichment of a sludge sample from an industrial
bioreactor was able to degrade MTBE at concentrations up to 200
mg/litre (Salanitro et al., 1994). Cell suspensions incubated with
MTBE produced TBA as a transient metabolite. MTBE labelled with ¹⁴C
in the methyl group was degraded to ¹⁴CO₂ and cellular material when
low substrate concentrations (2 mg/litre) were used, although not at a
concentration of 20 mg/litre. This experiment clearly demonstrated
oxidation of the methoxy group but left unresolved the fate of the
carbon atoms of the tertiary-butyl group.

Fifteen pure bacterial strains, with the capacity to degrade MTBE
using it as the sole carbon source, have been isolated from bioreactor
sludges and other sources. Several strains have been identified as
belonging to the genera Rhodococcus, Flavobacterium, Pseudomonas and
Oerskovia. These strains degrade up to 40% of MTBE (200 mg/ litre)
in 1-2 weeks of incubation at 22-25°C. These strains also grow on
tert-butanol, butyl formate, isopropanol, acetone and pyruvate as
sole carbon sources. Cultures of Methylobacterium, Rhodococcus and

Arthrobacter degraded MTBE within 1-2 weeks of incubation at
23-25°C. Growth on MTBE as the sole carbon source was slow compared
with growth on a nutrient-rich medium. When these compounds are mixed
with MTBE, there is a reduction in the degradation of MTBE. However,
when the microbes were initially grown on tert-butanol and then
transferred to medium containing MTBE, there was a greater degradation
of MTBE (Mo et al., 1997).

A mixed culture isolated from biological sludges has been used in
bioreactors utilizing MTBE as a sole carbon source for over a year.
The microbes were able to degrade MTBE at a concentration of 160
mg/litre after 3 days of incubation in batch experiments. Mixed
cultures have greater capacity for degradation of MTBE than pure
cultures. The addition of other ethers causes a reduction in MTBE
degradation. In soil microcosm studies, significant MTBE degradation
by mixed cultures was observed at 24°C and 10°C (Mo et al., 1997).

Howard et al. (1991) estimated, on the basis of screening tests
for aerobic biodegradation with unacclimatized aqueous systems
(Fujiwara et al., 1984), that the half-lives of MTBE in water and soil
under aerobic conditions ranged from 672 to 4320 h.

MTBE was found to be degraded by a number of propane-oxidizing
bacteria. The initial oxidation of MTBE produced nearly stoichiometric
amounts of TBA. The methoxy group of MTBE was further oxidized to
formaldehyde and finally to CO₂. At 28°C, rates of MTBE degradation
by these bacteria ranged from 3.9 to 9.2 nmol/min per mg cell protein
weight (Steffan et al., 1997).

4.3.2 Anaerobic conditions

Biodegradability of MTBE to methane under anaerobic conditions
has been determined by measuring the production of CH₄ and CO₂
during exposure of MTBE to a large population of anaerobic bacteria.
MTBE was biodegraded anaerobically only to a very limited extent
(Pence, 1987b), and an average cumulative theoretical gas production
of only 7.1% was achieve within 56 days. Anaerobic biodegradation to
methane must exceed 50% to meet the validation requirements for
demonstration of anaerobic biodegradability.

The anaerobic degradation of MTBE has been examined in different
soils (unsaturated clay, sandy loam and silty loam) collected from
various depths at three different sites (Novak et al., 1992; Yeh &
Novak, 1994). The experiments were conducted in static small-volume
anaerobic microcosms, and three different oxygen-free conditions were
simulated; with nitrate as electron acceptor (denitrification),
sulfate-reducing conditions, and anaerobic fermentation. Factors
influencing the degradation of MTBE, ETBE and TBA were determined, and
included anaerobic microbial populations, soil anions, soil moisture
content, organic content, nitrogen availability, rate of ammonium
"fixation", and soil pH. The soils were moderately acidic (pH 5.0-6.0)
with the exception of surface soils. The concentration of the added
MTBE was monitored for more than 250 days. Three parameters were

evaluated: degradation rate, lag time and time for 80% of the compound
to be degraded. No anaerobic degradation of MTBE was found in
organic-rich soils over the 250-day study period. The only situation
in which MTBE degradation occurred was in an oligotrophic soil
containing a low level of organic matter and with a pH of 5.0-6.0.
About 10% of the MTBE was lost during the first two months, although
this decrease cannot unambiguously be attributed to biodegradation.
Several conclusions may be drawn from the experiments with TBA and
ETBE:

* Whereas degradation of TBA in soil from the oligotrophic site
could be enhanced by addition of nitrate, the degradation of TBA
was inhibited by adding readily degraded ethanol.

* Biodegradation of ETBE under denitrifying conditions was
extremely sensitive to the presence of readily degraded
substrates.

These results illustrate that care should be exercised in
assessing biodegradability when several readily degraded substrates
are available, a condition that may be encountered in groundwater
contaminated with oxygenate additives.

Suflita & Mormile (1993) used sediment suspensions prepared from
samples collected from an aquifer polluted with leachate from a
municipal landfill. They assessed the formation of methane from a
range of substrates, and after at least 249 days no evidence for
anaerobic degradation of MTBE could be found. Whereas unbranched
alkanols and ketones were readily degraded, ethers in general were
resistant; in addition, oxygenates containing a tertiary or quaternary
carbon atom proved more recalcitrant than their unbranched or
moderately branched chemical analogues to anaerobic degradation.
Comparable experiments using a wider range of sediment samples
(Mormile et al., 1994) showed similar results under sulfate-reducing
or denitrifying conditions, although under methanogenic conditions a
single sample transformed MTBE into TBA. Likewise, the ethers were
unaffected by incubation with cultures of the acetogenic bacteria
Acetobacterium woodii and Eubacaterium limosum that convert
aromatic methoxy groups to acetate.

Based on the above-mentioned studies, MTBE is classed as
recalcitrant under anaerobic conditions.

Howard et al. (1991) estimated that the half-life of MTBE in
water under anaerobic conditions ranges from 2688 to 17 280 h.

4.4 Abiotic degradation

4.4.1 Air

4.4.1.1 Photolysis

Direct photolysis of MTBE is assumed to be environmentally
insignificant since it does not absorb radiation above 230 nm (Calvert
& Pitts, 1966). However, under laboratory conditions MTBE in an
oxygenated slurry system containing TiO₂ as catalyst was readily
degraded by UV light from a mercury lamp. MTBE was rapidly
photocatalytically degraded, 76% of the initial concentration being
converted to degradation products, including TBA. After 4 h MTBE was
no longer detectable (Barreto et al., 1995).

4.4.1.2 Hydrolysis

MTBE does not contain hydrolysable functional groups, and,
therefore, it is inert to environmental hydrolysis. Hydrolysis of MTBE
is assumed to be insignificant (Howard et al., 1991).

4.4.1.3 Photooxidation

MTBE is subject to photooxidation in the atmosphere. This will
occur under the influence of various mechanisms, such as the reaction
with hydroxyl radicals, water, alkoxy and peroxy radicals, oxygen
atoms, and ozone. On the basis of the rate constant of each of the
reactions and the concentrations of the reactants, the reaction with
the hydroxyl radical is considered to be the most important removal
process for MTBE in the atmosphere. Several products are generated as
a result. These include tertiary-butyl formate (TBF), the major
product, 2-methoxy-2-methyl propanol, formaldehyde, acetone, NO₂, and
the methyl radical. Molar yields of products identified from the
reaction of hydroxyl radicals with MTBE are given in Table 4). TBF is
unreactive to further photo-oxidation, while 2-methoxy-2-methyl
propanol is expected to be highly reactive with hydroxyl radicals,
yielding equimolar amounts of CO₂, formaldehyde, acetone and water.
Of these products, formaldehyde is highly reactive with the hydroxyl
radical (Wallington et al., 1988; Japar et al., 1991). Rates of
reaction of oxygenates and their decomposition products with hydroxyl
radicals are given in Table 5.

Factors influencing atmospheric lifetime, such as time of day,
sunlight intensity and temperature, also include those affecting the
availability of hydroxyl radicals. Based upon measured rate constants
for reactions with hydroxyl radicals in air (Cox & Goldstone, 1982;
Atkinson, 1985; Wallington et al., 1988, 1989; Atkinson, 1990; Japar
et al., 1990), the half-life for MTBE has been estimated to be between
20.7 and 265 h (Howard et al., 1991). Hence, MTBE is not considered to
be a greenhouse gas, nor would it contribute to the depletion of the
ozone layer (Environment Canada, 1993).

Table 4. Molar yields of products identified from the reaction of
hydroxyl radicals with MTBE

Product Molar yield^a Molar yield^b

TBF 0.68 0.76
Formaldehyde 0.48 0.37
Methyl acetate 0.14 0.17
TBA 0.062 -
Acetone 0.026 0.02

^a Smith et al., 1991.
^b Tuazon et al., 1991.

Table 5. Rates of reaction of oxygenates and their decomposition
products with hydroxyl radicals at 25°C

Compound Rate Reference
(10^-12 cm³
sec^-1 molecule^-1)

MTBE 3.2 Japar et al., 1991
ETBE 8.5 Japar et al., 1991
TBF 0.74 Smith et al., 1991
TBA 1.1 Japar et al., 1991
Formaldehyde 9.0 Atkinson & Pitts, 1978
2-methoxy-2-methyl propanala 30 Japar et al., 1991

^a Estimated from rates for other aldehydes

4.4.2 Natural waters

MTBE is not expected to adsorb significantly to bed sediments of
suspended sediments, hydrolyse, directly photolyse, or photo-oxidize
via reaction with photochemically produced radicals in water. While
MTBE is reported to be chemical unstable in acidic solutions (Budavari
et al., 1996), it is not expected to be hydrolysed in natural waters
under normal pH conditions (Lyman et al., 1990).

4.4.3 MTBE half-life ranges in environmental compartments

The half-life of a chemical in the environment depends not only
on the intrinsic properties of the chemical, but also on the nature of
the surrounding environment, such as sunlight intensity, hydroxyl
radical concentration, the nature of the microbial community and
temperature. Table 6 lists the half-life ranges in various
environmental compartments estimated by Mackay et al. (1993) and
Howard et al. (1991); these estimates are somewhat uncertain, as
implied by the order of magnitude range for some compartments.

4.5 Ozone-forming potential

Photochemical ozone-creation potentials (POCP) ranging from 20.4
to 34.6 have been determined for MTBE using a model that simulates the
formation of photochemical ozone episodes (Derwent et al., 1996). The
POCP values reflect the ability of a substance to form tropospheric
ozone as a result of its atmospheric degradation reactions. The POCP
values are calculated relative to ethylene (a chemical that is thought
to be important in such ozone formation and is given a POCP of 100).
Based on the emissions and the POCP value, MTBE (itself) is likely to
play a minor role in photochemical smog and low-level (tropospheric)
ozone formation near to sources of release.

4.6 Remediation

Examples of remedial methods that can be considered for MTBE are
air stripping, carbon absorption and soil vapour extraction. Intrinsic
bioremediation may be limited due to the variability of rates of
biodegradation of MTBE which have been previously mentioned (Zogorski
et al., 1996).

Table 6. Half-life ranges of MTBE in various compartments

Environmental Half-life ranges Comments Reference
compartment (h)

Soil 672-4320 Estimation based upon Howard et al., 1991
300-1000 aerobic biodegradation Mackay et al., 1993
half-life

Air 20.7-265 Based upon measured Howard et al., 1991
10-30 photo-oxidation half-life Mackay et al., 1993

Surface water 672-4320 Estimation based upon Howard et al., 1991
300-1000 aerobic biodegradation Mackay et al., 1993
half-life

Sediment 1000-3000 Mackay et al., 1993

Groundwater 1344-8640 Estimation based upon Howard et al., 1991
aerobic biodegradation
half-life

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

The major sources of MTBE to the general population are probably
associated with the distribution, storage and use of oxygenated
gasoline. The main source of non-occupational exposure to MTBE is
evaporative emissions from gasoline. A large portion of the population
is exposed during time spent at service stations, while driving cars,
in public parking garages, and in homes with attached garages. These
exposures generally occur through inhalation. In addition, discharges
into the soil or groundwater are a potential for contaminated water
supply and can lead to exposure when such water is drunk. Dermal
contact with MTBE may occur through accidental spills of MTBE-blended
gasoline or through the use of gasoline as a solvent. In Canada, it
has been estimated that gasolines blended with MTBE account for only
2% of the total annual gasoline consumption. MTBE is used in small
quantities by a few Canadian refiners to boost octane levels in
gasoline. A limited survey of the MTBE content of unleaded regular,
mid-range and premium gasoline across Canada in 1995 showed a range of
0 to 5.2% by volume for winter grade gasoline and 0 to 9% by volume in
summer grade gasoline. In the USA, oxygenated gasoline containing
10-15% MTBE is used in different areas and about 30% of the US
population is exposed to MTBE.

5.1 Environmental levels

5.1.1 Exposure

Ambient air and microenvironment concentrations of MTBE and other
fuel oxygenates have been measured in Canada, the USA and Finland.
When available, air data are presented below in conjunction with data
on MTBE levels in gasoline and with information on the proximity of
the samples to various point sources of MTBE.

Brown (1997) estimated average daily and average lifetime doses
of MTBE from exposure in air and drinking-water for a US population.
Concentration data and several of the population characteristics were
estimated as distributions rather than as point values. Arithmetic
mean occupational doses via air were in the range of 0.1 to
1.0 mg/kg-day, while doses from residential exposures, commuting and
refuelling were in the range of 0.0004 to 0.006 mg/kg-day. Lifetime
doses for workers were in the range of 0.01 to 0.1 mg/kg-day. The
cumulative dose distribution for the entire population of the
MTBE-using regions of the USA was estimated by combining the
distributions of doses and the numbers of people in each exposure
category. In the MTBE-using areas, arithmetic mean doses via air were
estimated to be 0.0053 and 0.00185 mg/kg-day for the chronic and
lifetime cases, respectively. It was found that 1.5% of the population
used water contaminated with MTBE leakage with an estimated geometric
mean concentration of 0.36 µg/litre and a 95th percentile
concentration of 64 µg/litre. Including ingestion, inhalation, and
dermal absorption of contaminated water, the estimated arithmetic mean
does of the population exposed via water was 1.4 × 10^-3 mg/kg-day.

5.1.1.1 Levels in ambient air and various microenvironments

a) Canada

The concentrations of MTBE in ambient air at various selected
locations in Canada have been measured as part of the National Air
Pollution Surveillance Programme in 1995 and 1996. This programme is a
joint project of the federal, provincial and municipal levels of
government. Its purpose is to monitor and assess, on a continuing
basis, the quality of the ambient air in the various regions of
Canada. The sites selected for monitoring of MTBE were based on usage
of gasoline with MTBE and/or because of nearby manufacturers of MTBE.

Pollutants from air were collected intermittently using the
canister methodology. Concentrations of MTBE was measured using the
detection principle of gas chromatography furnished with an ion trap
detector. Air samples were first passed through a cryogenic
concentration trap to gather enough analyte before injection into a GC
capillary column to allow compound speciation and quantification.
Approximately 200 ml of the canister sample was concentrated. A
cryogenic trap held at -150°C was used to concentrate the air sample.
Once the sample was concentrated, the trap was heated to 150°C and the
sample was back-flushed onto the column. MTBE and other hydrocarbons
were separated using a fused silica capillary column. The GC oven was
programmed to remain at 60°C for 3 min, then increased to 280°C at a
rate of 8°C/min. Calibration standards were prepared using the static
dilution technique. The detection limits were 0.05 to 0.1 µg/m³.

Table 7 lists the ambient concentration of MTBE in air at various
locations in Canada from 1995 to 1996.

Table 8 shows some MTBE atmospheric concentrations at the fence
line of a petroleum refinery at St John, New Brunswick, Canada, during
a period when there were complaints of odour. The same collection and
analytical methodology was used. The maximum concentration is not
considered representative of the area.

b) USA

In many urban areas in the USA having elevated levels of ozone or
CO, oxygenates such as MTBE are regulated for use in gasoline at
concentrations of 2.0% and 2.7% oxygen by weight (called reformulated
and oxygenated gasoline, respectively). These concentrations are
achieved by adding MTBE at 11% and 15% by volume, respectively. In
other areas, MTBE is used as an octane enhancer in premium gasoline at
concentrations up to 9% by volume, but usually at much lower
concentrations. It is important to note that MTBE is the predominant
oxygenate currently in use in these gasoline mixtures, followed by
ethanol (approximately 65% and 35% of the oxyfuels sold contain MTBE
and ethanol, respectively). Oxygenates used to a minor extent include
ETBE, TAME and DIPE (HEI, 1996). In 1994, oxygenates were added to
more than one-third of the gasoline market in the USA.

Table 7. Concentrations of MTBE in ambient air in Canada (1995-1996)
(Environment Canada, 1996b)

City^a Industrial site(s) and distance(s) Sample date MTBE
to monitoring site (where applicable) concentration
(µg/m³)^b

Edmonton(1)^c Two petroleum refineries - 1 km. 20/7/95 7.21
Acetic acid plant - 2.5 km
26/7/95 11.39
1/8/95 0.81
7/8/95 2.93
8/8/95 5.50
12/9/95 2.49
27/5/96 3.35
Edmonton(2)^d N/A 26/7/95 < DL
1/8/95 < DL
1/9/95 < DL
6/9/95 < DL
24/9/95 < DL
30/9/95 < DL
27/5/96 < DL
Halifax^d N/A 3/4/96 < DL
15/4/96 0.13
21/4/96 0.15
Montreal(1)^c Two refineries (BTX, petroleum) 21/8/95 1.54
- 1.6, 2.5 km
21/8/95 0.59
12/9/95 1.06
16/3/96 < DL
15/5/96 0.42
21/5/96 0.28
27/5/96 0.23
Montreal(2)^d N/A 16/3/95 0.15
16/5/96 0.18
21/5/96 0.22
27/5/96 0.37
Montreal(3)^e N/A 10/3/96 0.16
16/3/96 0.28
9/5/96 0.95
Montreal(4)^f N/A 9/5/96 0.22
15/5/96 < DL
21/5/96 0.70
St. John^c Petroleum refinery - 3 km 9/5/96 1.02
15/5/96 3.73
Stouffville^f N/A 6/10/95 0.19
18/10/95 0.35
Toronto(1)^d N/A 29/8/95 < DL
2/9/95 < DL
2/9/95 0.07

Table 7. (continued)

City^a Industrial site(s) and distance(s) Sample date MTBE
to monitoring site (where applicable) concentration
(µg/m³)^b

Toronto(2)^d N/A 17/8/95 0.03
29/8/95 < DL
2/9/95 < DL
Vancouver(1)^f N/A 23/8/95 0.27
29/8/95 0.89
29/8/95 0.16
30/8/95 0.33
Vancouver(2)^f N/A 22/3/95 0.14
Vancouver(3)^c Two gasoline processing and 1/8/95 2.13
storage plants - 0.5, 3 km
13/8/95 1.82
25/8/95 3.35
2/9/95 1.78
2/9/95 26.43
21/2/96 0.31
10/3/96 1.10
16/3/96 0.48