xvEPA
TOXICOLOGICAL REVIEW
of
METHYL METHACRYLATE
(CAS No. 80-62-6)
In Support of Summary Information on the
Integrated Risk Information System (IRIS)
January 1998
U.S. Environmental Protection Agency
Washington, DC
-------
TABLE OF CONTENTS
DISCLAIMER i
FOREWORD ii
CONTRIBUTORS AND REVIEWERS iii
1. INTRODUCTION 1
2. CHEMICAL AND PHYSICAL INFORMATION RELEVANT TO ASSESSMENTS . 2
3. TOXICOKINETICS/TOXICODYNAMICS RELEVANT TO ASSESSMENTS 4
3.1. ABSORPTION 4
3.2. DISTRIBUTION 4
3.3. METABOLISM 6
3.4. EXCRETION 8
4. HAZARD IDENTIFICATION
4.1. STUDIES IN HUMANS 10
4.1.1. Human Noncancer Studies 10
4.1.2. Human Cancer Studies 15
4.2. PRECHRONIC, CHRONIC, AND CANCER BIO AS SAYS IN ANIMALS 16
4.2.1. Acute Inhalation Studies 16
4.2.2. Subchronic and Chronic Inhalation Studies 19
4.2.3. Acute Oral Studies 26
4.2.4. Subchronic and Chronic Oral Studies 27
4.3. REPRODUCTIVE AND DEVELOPMENTAL STUDIES 28
4.4. OTHER STUDIES RELATED TO NONCANCER OR CANCER
EFFECTS FROM CHRONIC EXPOSURE TO METHYL METHACRYLATE 30
4.4.1. Genotoxicity 30
4.4.2. Allergies/Sensitization 33
4.4.3. Dermal and Ocular Effects 34
4.5. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER EFFECTS
AND MODE OF ACTION 34
4.6. WEIGHT-OF-EVIDENCE EVALUATION AND CANCER
CHARACTERIZATION 37
4.6.1. Genotoxicity and Animal Evidence 37
4.6.2. Human Evidence 40
4.6.3. Structure-Activity Relationships 41
4.6.4. Summary 42
-------
TABLE OF CONTENTS (Continued)
4.7. SUSCEPTIBLE POPULATIONS 41
4.7.1. Possible Childhood Susceptibility 41
4.7.2. Possible Gender Differences 42
5. DOSE-RESPONSE ASSESSMENTS 42
5.1. ORAL REFERENCE DOSE (RfD) 42
5.1.1. Choice of Principal Study and Critical Effect 42
5.1.2. Method of Analysis 42
5.1.3. Chronic RfD Derivation 42
5.2. INHALATION REFERENCE CONCENTRATION 43
5.2.1. Choice of Principal Study and Critical Effect 43
5.2.2. Method of Analysis 44
5.2.3. Chronic RfC Derivation 46
5.3. CANCER ASSESSMENT 46
6. MAJOR CONCLUSIONS IN CHARACTERIZATION OF
HAZARD AND DOSE-RESPONSE 46
6.1. HAZARD IDENTIFICATION 46
6.2. DOSE-RESPONSE ASSESSMENT 48
7. REFERENCES 51
8. APPENDICES 62
Appendix A: RfC Benchmark Concentration Analyses of Data From Lomax (1995) .... 62
Appendix B: Summary of and Response to External Peer Review Comments 64
-------
DISCLAIMER
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
-------
FOREWORD
The purpose of this review is to provide scientific support and rationale for hazard
identification and dose-response assessments for both cancer and noncancer effects (the oral
reference dose and the inhalation reference concentration) from chronic exposure to methyl
methacrylate. It is not intended to be a comprehensive treatise on the chemical or toxicological
nature of methyl methacrylate.
In Section 6, EPA has characterized its overall confidence in the quantitative and
qualitative aspects of hazard and dose-response (U.S. EPA, 1995a). Matters considered in this
characterization include knowledge gaps, uncertainties, quality of data, and scientific
controversies. This characterization is presented in an effort to make apparent the limitations of
the individual assessments and to aid and guide the risk assessor in the ensuing steps of the risk
assessment process.
For other general information about this assessment or other questions relating to the
Integrated Risk Information System (IRIS), the reader is referred to EPA's Risk Information
Hotline at 513-569-7254.
-------
CONTRIBUTORS AND REVIEWERS
Chemical Manager/Author
Jeffrey S. Gift, Ph.D.
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Research Triangle Park, NC
Reviewers
This document and summary information on IRIS have received peer review both by EPA
scientists and by independent scientists external to EPA (U.S. EPA, 1994a). Subsequent to
external review and incorporation of comments, this assessment has undergone an Agencywide
review process whereby the IRIS Program Director has achieved a consensus approval with
EPA's Program Offices (the Offices for Air and Radiation; Planning and Evaluation; Prevention,
Pesticides, and Toxic Substances; Research and Development; Solid Waste and Emergency
Response; and Water) and Regional Offices.
Internal EPA Reviewers
Mark M. Greenberg, M.S.
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Research Triangle Park, NC
Daniel J. Guth, Ph.D.
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Research Triangle Park, NC
Annie M. Jarabek
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Research Triangle Park, NC
Yin-Tak Woo, Ph.D.
Office of Prevention, Pesticides, and Toxic Substances
U.S. Environmental Protection Agency
Washington, DC
in
-------
CONTRIBUTORS AND REVIEWERS (continued)
External Peer Reviewers
Michael T. Kleinman, Ph.D.
University of California, Irvine
Department of Community and Environmental Medicine
College of Medicine
Irvine, CA
Larry G. Lomax, Ph.D.
Pathology Associates International Corporation
National Center for Toxicological Research
Jefferson, AR
James E. McLaughlin, Ph.D.
Rohm & Haas Company
Toxicology Department
Spring House, PA
Summaries of the external peer reviewers' comments and the disposition of their
recommendations are in Appendix B.
IV
-------
1. INTRODUCTION
This document presents the derivation of the noncancer dose-response assessments for
oral exposure (the oral reference dose or RfD) and for inhalation exposure (the inhalation
reference concentration or RfC) and the cancer hazard and dose-response assessments for methyl
methacrylate (MMA).
The RfD and RfC are meant to provide information on long-term toxic effects other than
carcinogen!city. The RfD is based on the assumption that thresholds exist for certain toxic
effects, such as cellular necrosis, but may not exist for other toxic effects, such as some
carcinogenic responses. The RfD is expressed in units of mg/kg/day. In general, the RfD is an
estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the
human population (including sensitive subgroups) that is likely to be without an appreciable risk
of deleterious effects during a lifetime. The inhalation RfC is analogous to the oral RfD. The
inhalation RfC considers toxic effects for both the respiratory system (portal of entry) and for
effects peripheral to the respiratory system (extrarespiratory or systemic effects). It is expressed
in units of mg/m3.
The carcinogen!city assessment is meant to provide information on three aspects of the
carcinogenic risk assessment for the agent in question: the U.S. Environmental Protection
Agency (EPA) classification, quantitative estimates of risk from oral exposure, and inhalation
exposure. The classification reflects a weight-of-evidence judgment of the likelihood that the
agent is a human carcinogen and the conditions under which the carcinogenic effects may be
expressed. Quantitative risk estimates are presented in three ways. The slope factor is the result
of application of a low-dose extrapolation procedure and is presented as the risk per mg/kg/day.
The unit risk is the quantitative estimate in terms of either risk per |ig/L of drinking water or risk
per |ig/m3 of air breathed. The third form in which risk is presented is drinking water or air
concentration, providing cancer risks of 1 in 10,000, 1 in 100,000, or 1 in 1,000,000.
Development of these hazard identifications and dose-response assessments for MMA has
followed the general guidelines for risk assessments as set forth by the National Research Council
(1983). Other EPA guidelines that were used in the development of this assessment include Risk
Assessment Guidelines of 1986 (U.S. EPA, 1987a), 1996 (proposed) Guidelines for Carcinogen
Risk Assessment (U.S. EPA, 1996a), Guidelines for Developmental Toxicity Risk Assessment
(U.S. EPA, 1991b), (proposed) Interim Policy for Particle Size and Limit Concentration Issues in
Inhalation Toxicity (U.S. EPA, 1994b), (proposed) Guidelines for Neurotoxicity Risk Assessment
(U.S. EPA, 1995b), Methods for Derivation of Inhalation Reference Concentrations and
Application of Inhalation Dosimetry (U.S. EPA, 1994c), Guidelines for Reproductive Toxicity
Risk Assessment (U.S. EPA, 1996b) Recommendations for and Documentation of Biological
Values for Use in Risk Assessment (U.S. EPA, 1988a), and Use of the Benchmark Dose
Approach in Health Risk Assessment (U.S. EPA, 1995c).
Literature search strategies employed for this compound were based on the Chemical
Abstracts Service Registry Number (CASRN) and at least one common name. As a minimum, the
following databases were searched: RTECS, HSDB, TSCATS, CCRIS, GENETOX, EMIC,
-------
EMICBACK, DART, ETICBACK, TOXLINE, CANCERLINE, and MEDLINE and MEDLINE
backfiles.. Information in previous reviews (U.S.EPA, 1988b, 1991c; ECETOC, 1995) and any
pertinent information submitted by the public to the Integrated Risk Information System (IRIS)
submission desk was also considered in the development of this document.
2. CHEMICAL AND PHYSICAL INFORMATION RELEVANT TO ASSESSMENTS
The structural formula of MMA is shown here:
CH3
CH2=C-C-O-CH3
II
o
Figure 1. Structure of methyl methacrylate.
The empirical formula for MMA is C5H8O2, and the CAS registry number is 80-62-6. The
synonyms for MMA listed in MEDLARS (1986), are as follows: metakrylan metylu;
methacrylate de methyle; methacrylic acid methyl ester (SCI); methacrylsaeuremethyl ester;
methyl alpha-methacrylate; methyl methacrylate; methyl methacrylate monomer, inhibited; methyl
methacrylate monomer, uninhibited; methyl methylacrylate; methyl 2-methyl-2-propenoate; methyl
2-methylpropenoate; methyl-methacrylat; methylmethacrylaat; metil metacrilato; MME; NA 1247;
NCI-C50680; Pegalan; RCRA waste number U162; UN 1247; 2-propenoic acid, 2-methyl-methyl
ester.
Some physical and chemical properties of MMA are shown in Table 1. MMA undergoes
reactions that are typical of its constituent functional groups. The sites of chemical reactivity are
the terminal vinylic carbon, the double bond, the allylic methyl group, the ester moiety, and the
functional group in the nonmethacrylate moiety (Nemec and Kirch, 1978). MMA takes part in
Diels-Alder reactions. When combined with 1,3-butadiene, a cyclic co-oligomerization occurs,
catalyzed by nickel complexes in the presence of phosphites and triethylaluminum (Nemec and
Kirch, 1978). The addition of MMA to nitric acid containing various concentrations of dinitrogen
trioxide results in the substitution of an allylic hydrogen with nitro or nitroso groups (Nemec and
Kirch, 1978).
MMA is a colorless flammable liquid with a strong acrid odor. It readily polymerizes on
exposure to light, heat, oxygen, ionizing radiation, or catalysts (EPA, 1991b; IARC, 1979; NTP,
1986). It is used primarily to make a variety of resins and plastics, and is most often polymerized
to polymethyl methacrylate, which is used to make acrylic sheets, acrylic moldings, and extrusion
powders. MMA is also copolymerized with other acrylates and used to make surface coating
resins, lacquers, and emulsion polymers. The chemical is used in medicine and dentistry to make
-------
prosthetic devices and as a ceramic filler or cement (EPA, 1991b; IARC, 1979; NTP, 1986).
Because MMA is relatively volatile (vapor pressure of 40 mmHg at 25°C) and widely used,
significant occupational exposure to the chemical can be expected to occur. The potentially
exposed population includes manufacturers of MMA and its polymers, as well as doctors, nurses,
dentists, and dental technicians.
Table 1. Physical and chemical properties of methyl methacrylate
Parameter
Molecular wt.
Melting pt. (°C)
Boiling pt. (°C)
Specific gravity
at 20°C
Heat of polymerization
(cal/g)
Vapor pressure
(mmHg at 25 °C)
Refractive index
(nD2)
Autoignition
temperature (°C)
Solubility
Data
100.11
-48
100-101
0.945
-138
40
1.4142
421
Slightly soluble in water
Reference
Nemec and Kirch, 1978
Nemec and Kirch, 1978;
Weastetal., 1988
Nemec and Kirch, 1978;
Weastetal., 1988
Weiss, 1980
Weiss, 1980
Sandmeyer and Kirwin, 1981
IARC, 1994; Weastetal.,
1988
Hawley, 1981
Hawley, 1981
Soluble in alcohol, ether, acetone, Windholz et al., 1983; Weast
methyl ethyl ketone, tetrahydrofuran, et al., 1988
hydrofuran, esters, aromatic and
chlorinated hydrocarbons
3. TOXICOKINETICS/TOXICODYNAMICS RELEVANT TO ASSESSMENTS
3.1. Absorption
MMA is readily absorbed into the blood via the lungs, gastrointestinal tract, and skin.
Absorption through the respiratory tract is indicated by the lethal effects seen in several animal
inhalation studies (Section 4.2.1). MMA was found in the blood of Sprague-Dawley rats
following inhalation exposure to 96.7 ppm (395.9 mg/m3) for 1, 2, 3, or 4 h (Raje et al., 1985).
The concentration of MMA in blood did not vary significantly with exposure duration. The mean
concentration in the blood for the four exposure periods was 11.14 mg/100 mL blood. Although
-------
some parent compound is initially present in the blood, MMA is metabolized rapidly and
completely within 10 days (see Table 2).
Morris and Frederick (1995) measured uptake of MMA in the surgically isolated upper
respiratory tract of male F344 rats under various flow conditions. Under cyclic flow conditions,
18%, 20%, and 16% was deposited at inspired MMA concentrations of 25, 100, and 500 ppm,
respectively. Under unidirectional flow conditions, deposition of MMA was 3% less on the
average.
The experiments of Bratt and Hathway (1977) show that MMA is rapidly absorbed from
the gastrointestinal tract of rats. Adult male Wistar rats were treated with 5.7 mg/kg 14C-MMA
by gavage. Up to 65% of the dose was expired from the lungs in 2 h, which shows the rapidity of
the absorption. Recovery of radiolabel in the urine and feces accounted for only 7.4% of the
administered dose, thereby indicating nearly complete absorption from the gastrointestinal tract
(see Table 2). In addition, significant levels of methacrylic acid ( > O.SmM), a product of MMA
degradation, were found in rat serum 5 min after a single dose of 8 mmol MMA/kg body weight
(Bereznowski, 1995).
MMA can be absorbed through the skin, causing death of the animals (Autian, 1975; see
also Table 10). Verkkala et al. (1983) showed that MMA is absorbed through the skin. In their
study, male Wistar rats exposed to MMA liquid on 12 cm2 of tail skin absorbed 0.78 ± 0.20 g
MMA during a 3-h occlusive exposure.
3.2. Distribution
The only studies that provide definitive information regarding the distribution of MMA in
a mammalian system following inhalation, oral, or intravenous exposures are those of Raj e et al.
(1985), Bratt and Hathway (1977), and Wenzel et al. (1973). Once absorbed, MMA is largely
metabolized to methacrylic acid and eventually to CO2 via the TCA cycle. Very little is retained
in the body as unchanged MMA after 10 days (Bratt and Hathway, 1977). That which is not
metabolized to CO2 and exhaled or excreted in the urine or feces is primarily retained in the liver
and adipose tissue, though Raje et al. (1985) report finding small amounts of MMA in the brain
and lungs immediately following 1-, 2-, 3-, and 4-h exposures.
-------
Table 2. Excretion and retention of radioactivity in rats after administration
of methyl[14C]methacrylatea
Route of Dose
administration (mg/kg) Urine
Recovery of 14C (% of Dose)"
Exhaled gases
Feces
14CO2
Unchanged Carcass
methyl [14C] plus
methacrylate skin Total
By stomach tube 5.7 4.7
2.7
88.0
0.1
4.1
99.6
Intravenous
5.7
6.6
1.7
84.0
0.7
99.6
Torm of labeled compound was methyl[l,3- 14C]-propylene-2-carboxylate.
b!0 days after administration.
Source: Bratt and Hathway (1977).
Raje et al. (1985) exposed Sprague-Dawley rats to 96.7 ppm (395.9 mg/m3) MMA vapor
for 1, 2, 3, or 4 h and the blood, brain, and lungs from each rat were analyzed for MMA content.
The MMA content of each tissue type did not vary significantly with exposure duration, so the
results were averaged to calculate the mean concentration in each tissue for the four exposure
periods. Blood showed the highest concentration of MMA of the three tissues (11.4 mg/100 mL
blood), followed by brain (25.24 |ig/g) and lung (20.6 |ig/g). This experiment indicates that
saturation of the tissues with MMA occurs, but the data were not sufficient to establish the time
to saturation for the exposure concentration tested.
In the experiments of Bratt and Hathway (1977) mentioned above, it was found that
10 days after oral or i.v. dosing of rats with 14C-MMA, only 4.1%-6.6% remained in the carcass,
most of which was found only in the liver and adipose tissue. Approximately 3.3% of an
intraperitoneally administered dose of MMA was retained in the carcass of female Wistar rats
(Croutetal., 1982).
Wenzel et al. (1973) administered radiolabeled MMA intravenously to Wistar rats.
Autoradiographic analyses showed that high activities occurred in the blood and kidneys; low
activities were detected in the liver and bone marrow at 5 min after injection. No activity was
detected in the brain or spinal cord at this time point. At 2 h post injection, the total activity had
decreased and detected activity had shifted to compact bone. From 4 to 8 h after administration,
the activity was found only in bones, liver, intestine, and salivary glands.
3.3. Metabolism
Labeled methylmalonic acid, methacrylic acid, succinic acid, methylmalonic semialdehyde,
and p-hydroxyisobutyric acid have been identified in the urine of male Wistar rats administered
-------
14C-MMA by either gavage or intravenous injection (Bratt and Hathaway, 1977; Crout et al.,
1982) and methylmalonic acid was also detected in the urine of a human volunteer following
ingestion of the chemical (Crout et al., 1982). The authors propose that administered MMA is
metabolized in the same way as the small amounts of methacrylate that are formed in the course
of valine metabolism. According to the proposed pathway, MMA ([1] in Figure 2) is
enzymatically converted to methacrylic acid (2) and is esterified to its CoA ester (4), which is a
normal catabolite of valine (3). The CoA derivative is then hydroxylated to p-hydroxyisobutyric
acid, oxidized and esterified by CoA to methylmalonyl OCoA (5), and enters the citric acid cycle
as succinyl CoA (6).
CH2 CH2
II H20 ||
CH3CCOOCH3 >• CH3CCOOCH +CH3OH
(D (2)
I?
CH3 CH2
I II
CH3CHCH(NH2)COOH >- CH3CCOSCoA
(3) (4)
T
CH3CHOOH >• CH3CH2COSCoA
+CO2
citric acid
cycle
CH3CHOSCoA +» HOOCCH2CH2COSCoA
I (6)
COOH
(5)
Figure 2. Proposed pathway for MMA metabolism.
Source: Crout et al. (1992).
Further evidence in support of this pathway for MMA metabolism comes from in vitro
studies with rat and human blood. It has been shown that MMA is hydrolyzed by both human and
rat serum enzymes to methacrylic acid and presumably to methanol in vitro (Bereznowski, 1995;
Corkill et al., 1976) and in vivo (Morris and Frederick, 1995; Bereznowski, 1995; Crout et al.,
1979). Bereznowski (1995) reported that the rate of methacrylic acid production in vitro by rat
blood serum was approximately threefold higher than the rate in human blood serum. Both
Bereznowski (1995) and Corkill et al. (1976) found that the rate of disappearance of MMA from
blood showed a first-order dependence on MMA concentration and suggested that a simple
enzymatic reaction is involved. The half-life at 37°C was 20-40 min (Corkill et al., 1976). Miller
et al. (1981) suggested that the rapid disappearance of MMA from the blood could be due to the
-------
binding of this compound with nonprotein sulfhydryl compounds in red blood cells rather than
hydrolysis. However, in a series of in vitro experiments, Bereznowski (1995) showed that the
accumulation of methacrylic acid in rat and human blood serum exposed to MMA results in a
typical enzymatic substrate-saturation curve, which is reduced by inhibitors of nonspecific
carboxylesterase and negated when the serum was boiled (to denature the serum proteins). Thus,
the most likely explanation for MMA's rate of disappearance in blood is serum esterase-catalyzed
hydrolysis to methacrylic acid and methanol. The kinetics of conversion when methacrylic acid
was isolated from human serum incubated with MMA (Corkill et al., 1976; Bereznowski, 1995)
indicate that this pathway was the major, and possibly the only, initial step in the metabolism of
MMA in blood.
Morris and Frederick (1995) showed that MMA is also metabolized to methacrylic acid by
enzymes in the upper respiratory tract of rats following inhalation exposure. They measured
uptake of MMA and ethyl acrylate (EA) in the surgically isolated upper respiratory tract of male
F344 rats under various vapor flow conditions and over a wide range of inspired concentrations.
To examine the potential influences of carboxylesterase metabolism, uptake was measured in non-
pretreated rats and in rats pretreated with the carboxylesterase inhibitor bis-nitrophenylphosphate
(BNPP). BNPP pretreatment reduced upper respiratory tract (URT) uptake for both EA and
MMA by approximately one-third, indicating that a large fraction of these vapors is metabolized
by carboxylesterase in the nose. In addition, MMA exposure at 566 ppm and absolute deposition
rates of 35-42 jig/min (unidirectional flow) resulted in 20% lowering of nasal nonprotein
sulfhydryl content (NPSH). EA only required an 88 ppm exposure at a 7-15 g/min (unidirectional
flow) absolute deposition rate to cause the same drop in NPSH levels, suggesting that EA is 3-5
times more reactive with nasal SH groups than is MMA. These results are consistent with in vitro
reaction rates of these vapors with glutathione (Tanii and Hashimoto, 1982). In contrast to their
parent esters, acrylic or methacrylic acid exposures were not found to deplete nasal NPSH even at
delivered dose rates greater than those for the parent esters. While the reported metabolism of
MMA to methacrylic acid in the nose, and evidence for similar olfactory damage caused by the
acid versus the ester (Miller et al., 1981; 1985) strongly suggest an acid metabolite-dependent
mechanism for olfactory toxicity, it is important to note that the esters are directly reactive. The
depletion of NPSH or covalent binding to macromolecules may play an important role in initiating
olfactory toxicity as well (Frederick et al., 1994).
Studies using both =*-naphthyl butyrate (Bogdanffy et al., 1987) and MMA (Greene, 1996)
as substrates for determining carboxyesterase activity consistently demonstrate a higher level of
activity for this enzyme in olfactory tissue of humans and rodents than in other nasal tissue. The
hydrolysis of MMA to methacrylic acid is strongly inhibited by the addition of low concentrations
of =*-naphthyl butyrate in rat and human nasal tissue (Greene, 1996), suggesting that MMA and «-
naphthyl butyrate are metabolized by the same carboxyesterase enzyme. In rats and mice,
olfactory homogenates of p-nitrophenyl butyrate (Bogdanffy et al., 1987) and dibasic esters
(Bogdanffy et al., 1991) metabolize these esters 7-10 times more efficiently than respiratory tissue
homogenates. Greene (1996) showed that olfactory tissue metabolized MMA at rates threefold
higher than respiratory tissue in rats and humans, and 12-fold higher than respiratory tissue in
hamsters.
-------
Other studies have shown that the rate of carboxyesterase activity in olfactory tissue is
significantly lower in humans than in F344 rats (Mattes and Mattes, 1992; Greene, 1996) and
Syrian hamsters (Greene, 1996). Greene (1996) has reported that maximum rates (Vmax) of MMA
metabolism in rat and hamster olfactory tissue were comparable, and human olfactory tissue rates
were at least 13-fold lower. Mattes and Mattes (1992) examined carboxylesterase
activity in total F344 rat nasal epithelium and human nasal polyps using the substrate =*-naphthyl
butyrate. They found substantially higher enzyme activity in the rat nasal extracts than in human
nasal polyps (see summary of Kmax and Vmax values from these studies in Table 3). The Michaelis
constant (Km) was approximately the same in rat and human nasal extract and was significantly
less than that reported for rat nasal carboxylesterase activity using dibasic esters as a substrate.
Using MMA as a substrate, Greene (1996) found that carboxyesterase activity in human liver
tissue was 500-fold higher than in human olfactory tissue.
Intraperitonal injection of MMA into female Wistar rats caused no significant difference in
urinary excretion of thioethers (mercapturic acids) compared with controls unless the animals
were pretreated with the carboxylesterase inhibitor, tri-o-tolyl phosphate (Delbressine et al.,
1981). The authors suggest that MMA can be detoxified by addition of glutathione to the
ethylene group, as well as by hydrolysis by carboxylesterase.
In short, metabolism of MMA has been studied in vitro and in vivo in both rodents and
humans. Several studies have confirmed the initial hydrolysis of MMA to methacrylic acid and
methanol, one study indicates that the rate of hydrolysis is slower in human than in rat blood, and
studies suggest that the rate of metabolism by carboxylesterase is substantially higher in rat nasal
tissue than in human nasal tissue, including olfactory tissue. Available evidence suggests that
MMA is enzymatically converted to methacrylic acid and is esterified to CoA, which is
hydroxylated to p-hydroxyisobutyric acid, oxidized and esterified by CoA to methylmalonyl CoA,
and enters the citric acid cycle as succinyl CoA. Methacrylic acid, methyl malonic acid, ethyl
malonic acid, b-hydroxyisobutyric acid, and mercapturic acid have been identified as urinary
metabolites of the rat, and methyl malonic acid has been shown to be a urinary metabolite of
humans.
3.4 Excretion
Most of an orally or parenterally administered dose of 14C-labeled MMA is excreted as
CO2 (Bratt and Hathway, 1977; Crout et al., 1982). Wistar rats given MMA orally,
-------
Table 3. Kinetic constants for carboxyesterase activity in human, rat, and
hamster nasal tissue
Species
Human
Human
Human
Human
Rat
Rat
Rat
Rat
Hamster
Hamster
Hamster
Tissue Kma
Nasal (polyps) 60.8 ± 7.90
Nasal
(olfactory)
Nasal
(respiratory)
Liver
Nasal (total) 50.0 ± 2.9
Nasal 140
(olfactory)
Nasal 150
(respiratory)
Liver 100
Nasal 280
(olfactory)
Nasal 400
(respiratory)
Liver 200
V b
T max
145.5 ± 11.3
0.48 ± 0.42d'e
2.7d
0.15±0.13d'e
494.0
5007 ± 377
12d,e
18. Od
3.5d
-------
the administered dose was excreted by the lungs as unmetabolized MMA. Urinary excretion
accounted for about 4.7%-14.5% of the administered radioactivity (Bratt and Hathway, 1977;
Crout et al., 1982). Metabolites detected in the urine following oral or intravenous dosing with
radiolabeled MMA included methacrylic acid, succinic acid, methylmalonic acid, methylmalonic
semialdehyde, p-hydroxyisobutyric acid, and an unidentified 14C-labeled acid. No parent
compound was detected in the urine. These results indicate that the small amount of inhaled
MMA that initially reaches the blood as reported by Raje et al. (1985) would be almost
completely metabolized within a few days.
Wistar rats administered MMA by intraperitoneal injection excreted increased amounts of
thioethers in their urine when carboxylesterase activity was inhibited by pretreatment with tri-o-
tolyl phosphate (Delbressine et al., 1981). 7V-acetyl-,S'-(2-carboxypropyl)cysteine was identified in
the urine of pretreated rats (Delbressine et al., 1981).
By examining workers following occupational inhalation exposures, Mizunuma et al.
(1993) determined that about 1.5% of inhaled MMA is excreted in the urine as methanol. Even
though urine detection of methanol shows some promise for biologic monitoring of MMA
exposure, the lowest MMA exposure concentration at which exposed subjects could be
distinguished from nonexposed subjects was reported to be about 20 ppm MMA.
4. Hazard Identification
4.1 Studies in Humans
This section is a review of human studies relevant to the derivation of health benchmarks.
An overall synthesis of this information and its relation to the potential for MMA to cause cancer
and noncancer effects are presented in Sections 4.5 and 4.6, respectively.
4.1.1. Human Noncancer Studies
There have been several reported cases of reactions in individuals exposed to MMA for
short periods during mixing of the monomer and polymer. Typical nervous system symptoms
included headache, lethargy, lightheadedness, and a sensation of heaviness in the arms and legs
(Lozewicz et al., 1985; Donaghy et al., 1991; Scolnick and Collins, 1986). No measured
exposure concentrations were reported for these cases. In one case, the exposure concentration
was estimated to be about 0.4 to 1.5 ppm (1.6 to 6.1 mg/m3) (Scolnick and Collins, 1986).
Respiratory symptoms reported in humans include chest tightness, dyspnea, coughing, and
wheezing (Lozewicz et al., 1985; Pickering et al., 1986; Savonius et al., 1993), and reduced peak
expiratory flow (Savonius et al., 1993). Exposure measurements were not taken, but Pickering et
al. (1986) estimated environmental concentrations of MMA ranged from 0 to 374 ppm (0 to
1,531.3 mg/m3) during the poly-MMA mixing procedures employed. Cases of contact dermatitis
have also been reported (Scolnick and Collins, 1986; Guill and Odom, 1978). These cases may
represent individuals who are sensitive to the neurologic, dermal, and respiratory effects of
exposure to MMA. However, in some cases high exposures (not quantified) persisted for several
years.
10
-------
A decrease in olfactory function was not detected among 175 MMA-exposed workers
relative to 88 nonexposed controls in a cross-sectional study at an acrylic sheet production facility
in Germany (Muttray et al., 1997). Time-weighted average MMA exposures were characterized
as being up to 50 ppm over the past 6 years and up to 100 ppm prior to that. Duration of
exposures as determined from work histories ranged from 1 to 33 years with a mean exposure
duration of 9.6 (±7.1) years. The authors indicated that workers were not exposed to other
confounding substances. Only one out of 175 exposed workers (0.6%) had an olfactory disorder
as determined by the Rhino-Test,® which was described as being sufficiently sensitive to detect
"clinically relevant hyposmia." Five out of 88 controls (5.7%) exhibited a diminished sense of
smell. All five cases of smell disorder were attributed to either trauma or nasal septoplasty
surgery. The authors stated that "low-level hyposmia" could not be ruled out with this test.
Olfactory function was investigated in 731 workers from a facility involved in the
manufacture of acrylates and methacrylates (Schwartz et al., 1989; University of Pennsylvania,
1988). Participants were administered the University of Pennsylvania Smell Identification Test
(UPSIT) and a questionnaire to collect background information on age, gender, health status,
smoking history, work information, and other necessary data. The study was conducted in a
manner that examined the effects of both current exposure (cross-sectional study) and cumulative
exposure (nested case-control study) to acrylates and methacrylates on olfactory function. For
the cross-sectional study, workers were classified into four exposure groups: those having no
significant chemical exposures; those exposed to chemicals other than acrylic acid, methacrylic
acid, acrylates, or methacrylates; those having low exposure to these chemicals only (low-
acrylate/methacrylate group); and those having high exposure to these chemicals only (high-
acrylate/methacrylate group). In the case-control study, classification into one of the two
acrylate/methacrylate exposure categories, duration of employment at the plant, and cumulative
exposure to acrylate/methacrylate were considered individually for their possible association with
olfactory function. There were no significant differences in scores on the UPSIT among the four
exposure categories in the cross-sectional study. However, the case-control study revealed
significantly (p < 0.05) elevated crude exposure odds ratios for olfactory function loss for all
workers and for workers who never smoked (2.0 and 6.0 for these two groups, respectively).
After controlling for multiple confounding factors (chemical exposure, smoking status, ethnic
group, medications, age, history of smell dysfunction, history of medical problems, level of
education, gender, and work shift tested), the odds ratios were 2.8 and 13.5 for all workers and
never-smokers, respectively. There was also evidence of a concentration-dependent relationship
between olfactory dysfunction and cumulative exposure scores. While an exposure-related effect
is suggested in this study, actual exposure concentrations were not reported, MMA exposure was
not examined separately from exposure to other acrylates/methacrylates, and it is not possible to
make a specific assessment of the effects of exposure to MMA on olfactory function. The reason
effects were reported in this study and not the Muttray et al. (1997) study could be due to a
higher coexposure to other acrylates, the use of a more sensitive diagnosis method, or a
combination of both explanations.
A study was conducted in December 1992 to identify the prevalence of occupational
asthma attributable to MMA exposure in ICF s acrylic factories in Darwen, Lancashire, UK
(Pickering et al., 1993). Approximately 400 (90%) of the exposed workforce were administered
lung function tests to determine forced expiratory volume in 1 sec (FEVj) and forced vital
11
-------
capacity, as well as a questionnaire. Each individual's level of exposure was estimated to be none,
low, medium, or high based on knowledge of their current work area, previous work areas, length
of time in the industry, and their self-reported exposure to short-term but high levels of MMA.
Work-related respiratory symptoms were reported by 5/125 (4%), 16/198 (8%), and 6/61 (10%),
and work-related nasal symptoms were reported by 6/125 (5%), 14/198 (7%), and 4/61 (7%) for
workers classified in the low, medium, and high exposure groups, respectively. Although these
data are suggestive of a dose-response, the trends did not reach statistical significance at the 5%
level according to three-way and paired chi-squared tests. Number of packets of cigarettes
smoked times number of years was the only parameter associated with loss of lung function.
Exposure classification had no statistically significant effect on lung function once smoking habits
were accounted for. Workers reported eye irritation and nasal irritation following 51% and 37%,
respectively, of the incidences of transient high exposures.
Two studies in the literature have addressed the issue of whether occupational exposure to
MMA has the potential to cause chromosome aberrations and sister chromatid exchange (SCE).
Marez et al. (1991) scored SCE in lymphocytes from 31 workers occupationally exposed to
MMA (mean exposures 1 to 22 ppm) and a control of 31 men whose mean age and smoking
habits were similar. The number of SCE in exposed workers (7.85 ± 2.66) was similar to the
control group (7.49 ± 2.33). However, the rate of SCE was significantly higher (10.0 ± 1.65) for
a group that had been exposed to MMA at peak concentrations ranging from 114 to 400 ppm.
Seiji et al. (1994) evaluated chromosome aberration rates and SEC frequencies in peripheral
lymphocytes of 38 male workers exposed to MMA vapors at concentrations of 0.9 to 71.9 ppm.
Consistent with the results of Marez et al. (1991), comparison of the exposed group with a
concurrent nonexposed group revealed no evidence of mutagenicity at these relatively low
exposure levels.
Marez et al. (1992) reported an increased incidence of cardiac arrhythmias and paroxysmal
unspecific repolarization changes (large T waves in the ECG) among 22 workers exposed to
MMA concentrations (8-h average) of either 18.5 or 21.6 ppm over 18 controls. The effects
among exposed workers were found to be equally distributed over the day, did not correlate with
exposure, and were few in number relative to known data concerning the normal heart. However,
the authors could not exclude a role of MMA in the significant increases in these effects over
controls. The relevance of this effect and its relation to MMA exposure are unclear.
Marez et al. (1993) investigated the pulmonary effects of MMA in a group of 40
occupationally exposed workers compared with a group of 45 controls. The exposed groups
worked in two French factories with reported air concentrations (8-h averages) of 18.5 and 21.6
ppm MMA. Peak exposures were not reported, but the ranges were 9 to 32 ppm and 11.9 to
38.5 ppm, respectively. Eight of the 40 workers had between 5 and 10 years exposure to MMA
and 32 workers had been exposed for more than 10 years. The study made use of a questionnaire
and spirometry measurements of FVC, FEVj, and maximum expiratory flow volume (MEFV) and
expiratory flow volume when 50% of the forced vital capacity remained to be exhaled (MEFV50).
MEFV50 (p=0.04) and MEFV50/MEFV (p=0.01) showed significant reductions in the exposed
group as compared with the controls following an 8-h work shift, indicating possible mild airway
obstruction. An increased incidence of chronic cough was also reported for the exposed group
over controls. No data were given regarding the exposure concentrations on the day pulmonary
12
-------
function tests were performed, and these effects, including the reduced average MEFV50, may
have been due to acute airway irritation caused by peak exposures.
Workers (n=441) with potential exposure to MMA during 1976-1983 were given a health
examination to assess possible toxic effects of the chemical (Lang et al., 1986). Duration of
employment ranged from 3 months to about 30 years. Exposure concentrations ranged from 11.3
to 203.2 mg/m3 depending on the location and year of sampling. Workers were divided into
control, low-, and high-exposure groups, but the cutoff for these divisions was not given. Certain
effects were increased in the high-exposure group during some years but not others (e.g.,
increased laryngitis, pulse rate, palpitations, dyspnea, fever, neurasthenia). Quantitative use of
this study in a dose-response assessment is not possible because of concomitant exposure to other
chemicals and the general lack of consideration of other possible confounding factors.
A retrospective study was conducted on 134 workers at 5 plants manufacturing
polymethyl methacrylate sheets (Cromer and Kronoveter, 1976). Of these workers, 91 were
exposed to MMA and 43 had no known exposure to the chemical. Effects of both acute (pre- vs.
post-shift) and chronic (of unspecified duration) exposure to MMA were investigated. The
authors did not report duration of exposure or employment history for any of the workers. The
evaluation for chronic effects included questions administered via an extensive questionnaire
(including questions on smoking habits, occupational and medical history, and respiratory, renal,
hepatic, gastrointestinal, dermatologic, and neurologic symptomatology), as well as pulse and
blood pressure measurements, pulmonary function tests, hematology, urinalyses, and blood
chemistry. TWA concentrations were measured over an 8-h work shift for each exposed worker
by personal samplers. Two samples were taken per worker per work shift and averaged. Air
samples were also taken during work shifts (approximately 3 h per sample) in the buildings in
which the controls worked. The exposed population was divided into three exposure groups
according to average exposure: 25-50 ppm, 5-25 ppm, and less than 5 ppm per day.
Concentrations of MMA in control areas were less than 0.3 ppm for all samples except one (this
one was 0.8 ppm).
There were no significant differences in symptomatology, pulse rate, or blood pressure
between the exposed and unexposed workers during the course of a normal workday (i.e.,
without peak accidental exposures). For chronically exposed workers, the only significant
differences in symptomatology were for cough in the under 5 ppm group (p=0.029) and for
expectoration in the 5-25 ppm group (p=0.006). However, the percentages of smokers in the
under 5 ppm (62%) and 5 to 25 ppm (70%) were markedly higher than for the control group
(39%). A greater number of workers in the 25-50 ppm and 5-25 ppm groups reported skin and
allergy problems and nervous system symptomatology than the control group, but the difference
was not statistically significant. No differences in pulmonary function parameters were observed.
No significant differences in urinalysis results were found. There were no control values for this
effect, however, only a "not currently exposed" group. There were several significant differences
in various blood chemistry parameters between various groups and the control group, but none of
these could be clearly related to exposure. The authors suggested that variations in triglycerides
in all exposure categories, calcium, phosphorus, and serum glucose in the "not currently exposed"
group, and serum glucose in the 25-50 ppm group warranted further investigation. The results of
this study are not considered useful for a quantitative assessment because of the lack of
13
-------
adjustment for smoking and the lack of information regarding worker employment history and
duration of exposure.
Four hundred and fifty-four male workers, who were exposed to styrene and MMA in
their work environment, were compared with a control population of workers with no known
exposure to these compounds (Jedrychowdki, 1982). The control population was known to be
exposed to several other chemicals, including methanol, phenol, and/or carbon monoxide. The
mean concentration of MMA in workplace air was 11.06 mg/m3 (range of 0.2-382.2 mg/m3).
Surveyed workers of both populations were also divided into subgroups of nonsmokers
(consisting of ex-smokers and those who never smoked) and smokers. Chronic bronchitis and/or
asthmatic symptoms were slightly lower in the exposed group than in the control group, even
when smoking and nonsmoking subgroups were analyzed separately, but this difference was not
significant. There was a significant difference (p < 0.05) in the frequency of lung obstruction
between the groups, with the exposed group having an occurrence rate of more than twice the
control group (45.4% vs. 18%). Smokers in the control group had a higher incidence of lung
obstruction than nonsmokers (20.9% vs. 13.6%), but there was very little difference between
smokers and nonsmokers in the exposed group (46.5% vs. 42.7%). There was also a significant
difference in lung function, as determined by the ratio of the observed forced expiratory volume at
1 min to the expected volume (FEV1(obs)/FEV1(exp)), between exposed and control group workers
and between smokers and nonsmokers. Exposed workers and smokers had lower
FEV1(obs)/FEV1(exp) values than controls and nonsmokers. Calculations of the relative risk of
developing obstructive lung syndrome (using unexposed nonsmokers as the unit risk) showed the
risk was highest for the group of exposed current smokers (5.5 compared with 4.7 for exposed
nonsmokers and 1.7 for nonexposed smokers). Problems with this study are that concomitant
exposure to other chemicals, particularly styrene, occurred in both the exposed and control
groups, and there was no attempt to correlate exposure levels of MMA with observed effects.
The observed lung obstruction could be due to styrene, which has been linked in other studies to
throat irritation (Harkonen, 1978) and lung obstruction (Chmielewski and Renke, 1975; Lorimer
etal., 1976).
Thirty-five workers from three different laboratories manufacturing dental prostheses
using MMA responded to questionnaires administered by mail (Money et al., 1987). Skin
exposure occurs as the dough used to make the prostheses is worked with bare hands. In
addition, concentrations of MMA in the air and personal exposure concentrations in the
laboratories were monitored. Of the 35 workers, 19 reported skin rashes (or similar problems),
17 reported tingling in hands, 15 suffered eye irritation during use, 12 had regular headaches, and
12 had whitening of fingers in cold. Other conditions reported (incidences not reported) were
irritation to the lining of the nose, increased tingling in fingers on immersion in hot water, and dry
fingertips after contact with MMA. Air concentrations in the three labs measured ranged from
0.2 to 6 ppm in one lab with good ventilation and hygiene conditions, and from 24 to 102 ppm in
a lab set up in a garage that had no ventilation and poor hygiene conditions. No attempt was
made to correlate measured concentrations with reported symptoms and no control group was
monitored.
In a survey of the respiratory health of 780 workers exposed to MMA, information was
collected on the individuals via a questionnaire, forced expiratory volume was measured, and a
14
-------
posteroanterior chest x-ray was done on each participant (Monroe et al., 1981). Sex, smoking,
work history, and length of service at the plant were considered in the data analyses. When
confounding factors were controlled for, the only factor associated with MMA exposure was a
decrease in FEVj in never-smokers who worked with the chemical (p < 0.005). The authors state
that this could be an artifact of the small sample size in this category (n=17).
A cohort of 2,671 men from two American Cyanamid plants was observed from 1951 to
1983 to determine any differences in mortality between those exposed to MMA (1,561 workers)
and those having no exposure (1,110 workers) (Collins et al., 1989). Cumulative exposure
(calculated as the product of the number of days in the job and estimated average exposure in
ppm divided by 365) to MMA was estimated for each person based on information on plant
operation, and smoking status was considered in the analyses. Estimated mean cumulative
exposure levels ranged from 0.13 to 1.00 ppm over the study period. Results from standard
mortality ratios (SRMs) calculated for the cohort showed no increase in mortality from all causes,
or from any specific cause, between the exposed and unexposed plant workers, or between
exposed workers and the U.S. population.
4.1.2. Human Cancer Studies
Several cancer mortality studies have been conducted on workers exposed to MMA (and
other acrylates). Walker et al. (1991) have summarized a series of studies performed by Rohm
and Haas Company at their Bristol and Knoxville acrylic manufacturing sites. These studies
involved a total cohort of 13,863 workers, including 3,934 white males employed at the Bristol
plant between 1933 and 1945 (the so-called Early Bristol cohort; 2,904 were hired between 1941
and 1945), 6,548 white males hired at the Bristol plant between 1946 and 1986 (Later Bristol
cohort), and 3,381 white males employed at the Knoxville plant between 1943 and 1982.
Exposure to ethyl acrylate and/or MMA was based on job history and a job-specific exposure
scale. Ethyl acrylate exposure was not determined separate from MMA exposure at any of the
plants. An excess of colon cancer in workers exposed to ethyl acrylate and/or MMA when
compared to local rates was reported in the Early Bristol cohort, reported by Walker et al. (1991)
as shown in Table 4.
Table 4. Deaths from colon cancer in Early Bristol cohort
Achieved dose"
None (not exposed)
0-4 units
5-9 units
10-14 units
;> 15 units
Observed deaths
12
13
6
1
11
Expected deaths
9.66
9.39
5.17
2.24
4.58
Fitted rate ratiob
1.24
1.39
1.16
0.45
2.4
aDoses of ethyl acrylate/MMA at least 20 years since first achieving dose; employment > 10 months.
bFitted mortality ratio of cohort mortality rate and the combined Bucks County and Burlington County white male
mortality rate for the same age and calendar period.
15
-------
Source: Walker etal. (1991).
Cancer of the rectum was also elevated in the Early Bristol cohort (10 deaths observed;
5.23 expected) but these data are considered less significant and robust than the colon cancer data
(Walker, 1991; ECETOC, 1995). In the Later Bristol cohort, a deficit of colorectal cancer, based
on U.S. mortality rates, was observed (SMR for colon cancer of 0.91; SMR for rectal cancer of
zero). The Knoxville cohort also showed a deficit of colorectal cancer, with SMRs for colon and
rectal cancer in the whole cohort (including those not exposed) of 0.96 (20 observed versus 20.74
expected) and 0.16 (1 observed versus 6.34 expected), respectively. At 20 years after exposure,
the SMR for colon cancer in the Knoxville cohort was 1.52 for all exposure categories. However,
there were deficits at the higher exposure levels and an excess at the lowest level.
In the Early Bristol cohort, mortality from all causes, all malignant neoplasms, and cancer
and noncancer respiratory diseases were all lower than expected. Similarly, all but mortality from
all malignant neoplasms (SMR=1.02) and mortality from cancer of the respiratory system
(SMR=1.05) were reduced in the Later Bristol cohort. The Knoxville cohort showed marginal
increases in mortality from all causes (SMR=1.06), all malignant neoplasms (SMR=1.13), and
cancer (SMR=1.44) and noncancer (SMR=1.10) respiratory disease.
As described above in Section 4.1.1, Collins et al. (1989) observed no increase in
mortality from all causes, or from any specific cause, between the exposed and unexposed
American Cyanamid plant workers, or between exposed workers and the U.S. population.
Mortality from malignant neoplasms was similar to the U.S. population (SMR=1.04) and
unexposed men in the same plant (SMR=1.01). Cancers of both digestive organs and peritoneum
(SMR=0.74) and large intestine (SMR=0.39) were less than expected, with 1 colon cancer death
compared to 2.6 expected (SMR=0.39), and no rectal cancer deaths. In the exposed population
there was a small excess of both cancer (15 observed deaths vs. 12.5 expected; SMR=1.20) and
noncancer (4 observed deaths vs. 1.9 expected; SMR=2.16) respiratory disease.
4.2. Prechronic, Chronic, and Cancer Bioassays in Animals
This section is a review of laboratory animal studies relevant to the derivation of health
benchmarks for MMA. An overall synthesis of this information and its relation to the potential for
MMA to cause noncancer and cancer effects is presented in Sections 4.5 and 4.6, respectively.
Certain studies that were considered inadequately documented for the purposes of this assessment
or that used irrelevant dosing regimens may not have been discussed in this section. A more
complete listing/discussion of all types of repeat or single-exposure studies can be found in other,
more detailed reviews (ECETOC, 1995; U.S. EPA, 1991).
4.2.1. Acute Inhalation Studies
Lomax et al. (1994) performed a short-term exposure study using acrylic acid, which has
bearing on the relevance of the RfC derivation practice of adjusting from intermittent to
continuous dosing regimens for MMA (see Section 5.1.3). Conversion of MMA to methacrylic
acid is believed to be an important factor in the appearance of olfactory changes in the nasal
passages of rodents exposed to MMA (Lomax et al., 1997; U.S. EPA, 1991b). Similar
16
-------
observations have been made for the corresponding metabolite of ethyl acrylate, acrylic acid (AA)
(Miller et al., 1985). While there are some differences between methyl-substituted and un-
substituted acrylics at low exposure levels, such as the rate of glutathione binding for methacrylic
acid versus acrylic acid (Stott and McKenna, 1985; Delbrissine et al., 1981; Frederick et al.,
1994), the toxicologic endpoint for both compounds is the same, and the Lomax et al. (1994)
acrylic acid study is deemed relevant to the derivation of the MMA RfC. Lomax et al. (1994)
exposed groups of 15 B6C3F1 female mice to 0, 5, or 25 ppm AA in a whole-body inhalation
apparatus for 14 consecutive days (two weeks) for 4.4, 6, or 22 h/day. Upon termination of
exposure, the nasal cavity was collected from 10 animals per exposure group. The remaining 5
animals per group were maintained under standard animal husbandry conditions without exposure
for 6 weeks prior to histopathologic analysis. They found that the three dose groups having
similar concentration x time products (5 ppm x 22 h; 25 ppm x 4.4 h; 25 ppm x 6 h) all had very
similar incidence and severity of lesions in the nasal cavity following 14 days of exposure. They
also note that while nasal cavity effects induced by AA are fully reversible for these exposure
groups, effects from exposure to 25 ppm AA for 22 h/day were both more severe than would be
expected from a linear C x T relationship and induced lasting respiratory metaplasia.
Another inhalation study with bearing on the issue of whether to apply a duration
adjustment factor to MMA intermittent exposure levels is a recent study by Pinto (1997) in which
groups of 45 female F344 rats (five animals per time point) were exposed whole body for 6 h/day
to 0, 110, or 400 ppm MMA for 1, 2, 5, 10, or 28 consecutive days. Minimal degeneration/
necrosis was noted at the 110-ppm exposure level following 1 and 2 days of exposure.
"Regenerative changes" at the 110-ppm exposure level were noted following 5 and 10 days of
exposure. These regenerative changes were characterized as disorganized epithelium of varying
height "with loss of regular arrangement of cells, focal loss of apical cytoplasm and pseudoglands
within the epithelium." Also noted at this exposure concentration and duration was a
"depletion/loss of PAS staining affinity in the Bowman's glands, minimal hypertrophy of the
Bowman's glands/ducts, and exudate within the nasal passages." No lesions were noted in rats
exposed to 110 ppm for 28 days, nor in rats exposed to 110 ppm for 28 days followed by 28- day
and 13-week recovery periods. The reported regeneration at the 5th and 10th days of 110-ppm
exposure and lack of lesions at subsequent time points may be somewhat misleading. Other
studies have shown that some functional loss occurs in rats despite apparent complete
regeneration of olfactory epithelium (Wong et al., 1997; Youngentob, 1997). In addition, there is
question as to whether humans have this ability to regenerate olfactory epithelium and recover
even partial olfaction (Yamagishi and Nakano, 1992). Finally, Lomax et al. (1997) have reported
degenerative changes in olfactory epithelium following chronic exposure to 100 ppm MMA. As
noted by Pinto (1997), the apparent lack of olfactory lesions following 28 days of exposure to 110
ppm MMA suggests that degenerative olfactory epithelial changes, attributable to a direct irritant
effect of MMA, develop over an extended period of time. It could also mean that degenerative
changes were present in this study at 28 days in an as yet undetectable form. In any case, the
extent to which rats and humans tend to recover with continuous exposure to MMA is still under
investigation, and duration of exposure cannot be ruled out as a contributing factor toward the
development/progression of adverse olfactory effects of MMA.
Repetitive exposures of F344/N rats and B6C3FJ mice (5 of each sex and species) for 6
h/day, 5 days/week to air containing MMA at concentrations of 0, 75, 125, 250, 500, 1,000,
17
-------
2,000, 3,000, or 5,000 ppm (0, 307, 512, 1,024, 2,047, 4,094, 8,189, 12,283, or 20,472 mg/m3)
for 10 or 11 days resulted in the death of all animals at the highest concentration (NTP, 1986).
Two of 5 female rats exposed to 3,000 ppm died. In the groups of male mice, 2/5, 1/5, 3/5, and
4/5 exposed to concentrations of 500, 1,000, 2,000, and 3,000 ppm, respectively, died before the
end of the study. Mean body weights for rats exposed to 2,000 or 3,000 ppm were 10%-19%
lower than those of controls. Ruffled fur was the only clinical sign observed in rats that appeared
to be related to exposure. In mice, dyspnea as well as redness and swelling of the nasal region
were attributed to exposure. Short-term exposure of rats and mice to concentrations of MMA
ranging from 1,191 to 16,000 ppm (4,876.5 to 65,512 mg/m3) for 4 h produced central nervous
system and respiratory effects at all exposure levels and death at the highest exposure levels.
Hypoactivity, dyspnea, and anesthesia were observed in exposed animals.
Female ICR white mice were exposed to an average concentration of 1,520 ppm (6,224
mg/m3) for 4 h/day, (two 2-h exposure periods/day, separated by 1 h of no exposure), 5
days/week, for 2 weeks (McLaughlin et al. 1979). Weight loss was greater in treated animals
versus controls. No treatment-related pathological changes were observed in the tissues
examined.
Exposure of 10 male Sprague-Dawley rats to 1,000 ppm (4,094 mg/m3) for 56 h over a 7-
day period produced statistically significant variations in blood chemistry and lung damage (Tansy
et al., 1980b). Albumin, glucose, blood urea nitrogen, serum glutamate-oxaloacetate
transaminase, serum glutamate-pyruvate transaminase, and albumin/glucose ratio were
significantly lower (p < 0.05) in exposed animals than in controls. Frank lung damage consisted
of adherence of visceral pleura to parietal pleura, fibrosis, lung edema, and parenchyma changes
suggestive of emphysema, and was greater in exposed rats than in the sham-treated controls
(quantitative data not presented). The same experimenters exposed groups of male Swiss
Webster mice intermittently to 0, 100, or 400 ppm (0, 409, or 1,638 mg/m3) MMA vapor for a
total of 160 h for each group. Sleeping times, as determined by time to return of righting reflex
following intraperitoneal injection of sodium pentobarbital, were significantly decreased with
increasing dose in the high-exposure group (p < 0.05). This decrease in sleeping time could
represent an induction of enzymes capable of metabolizing pentobarbital. However, at
approximately 100-fold higher exposure concentrations, acute (14 min) exposures to MMA can
cause an increase in pentobarbitol sleep time (Lawrence and Autian, 1972), indicating that
metabolic rates and processes can vary considerably with different dosing regimens.
Male Sprague-Dawley rats were exposed to 400 ppm (1,638 mg/m3) MMA for periods of
60 min interspersed with 30-min periods of no exposure (Innes and Tansy, 1981). Decreases in
the neuronal firing rate from cells located in the lateral hypothalamus and the ventral hippocampus
were observed in exposed but not control animals. No statistical analyses were done and the
relevance of this observation to MMA toxicity in humans is not known.
The brains and lungs of male Sprague-Dawley rats (4 per group) were examined
microscopically following exposure to 96.7 ppm (395.9 mg/m3) MMA for 1, 2, 3, or 4 h (Raje et
al., 1985). No lesions were observed in the brains of the rats. Examination of the lungs of
animals exposed for 2 h or longer revealed interalveolar congestion and hemorrhage, pulmonary
18
-------
vasodilation, and edema. This may be indicative of the irritating effect of MMA on pulmonary
and alveolar capillaries; however, no controls were used and the group size was small.
Several studies of the acute effects of high MMA exposures, including LC50 studies, have
been performed (Tansy et al., 1980, as reported in Oberly and Tansy, 1985; Spealman et al., 1945;
Deichmann, 1941). LC50 estimates range from 7,093 ppm for 4-h exposures to rats to 13,200
ppm for 3-h exposures to mice. Other effects from acute (approximately 1,000 to 10,000 ppm)
exposures to rats, mice, guinea pigs, and dogs included irritation of the eyes, nose, and respiratory
tract, including labored breathing, hemoglobinuria, loss of reflex activity, coma, liver
degeneration, and tubular degeneration in the kidney (dogs only).
4.2.2. Subchronic and Chronic Inhalation Studies
This section is a review of subchronic and chronic inhalation laboratory animal studies
relevant to the derivation of health benchmarks for MMA. An overall synthesis of this
information and its relation to the potential for MMA to cause noncancer and cancer effects is
presented in Sections 4.5 and 4.6, respectively. Sections 4.5 and 4.6 also contain summary tables
of key subchronic and chronic laboratory animal studies (Table 8) and carcinogen bioassays
(Table 10).
Eighteen male beagle dogs were exposed to 0, 100, or 400 ppm (0, 409, or 1,638 mg/m3)
MMA in air for 6 h/day, 5 days/week (duration adjusted to 0, 73, or 292.5 mg/m3), for 3 mo
(Drees et al., 1979). Response of 36 variables (including systolic and diastolic blood pressure,
EKG, heart and respiratory rates, hematology, pathology, clinical chemistry, and urinalyses) to
exposure were monitored. No significant differences were found between exposed and
unexposed animals for any of the parameters monitored.
Male Sprague-Dawley rats (50 per group) were exposed to 0 or 116 ppm (0 or 475
mg/m3) MMA vapor for 8 h/day, 5 days/week (duration adjusted to 0 or 113 mg/m3), for either 3
or 6 mo (Tansy et al., 1976). The authors noted that the experimental animals looked shaggy and
did not groom themselves during exposure periods, and that the sham-treated controls generally
had a better appearance than experimental animals. Animals exposed for 3 mo were observed to
have an absence of visceral and subcutaneous fat deposits. Animals exposed for 6 mo were
deficient in subcutaneous fat compared with control animals. Whole-body weight was decreased
(p < 0.05) following both 3 and 6 mo of exposure, but the decrease was less than 10% relative to
that of control animals. Lung weight and spleen weight were significantly decreased (p < 0.05)
following 3 mo of exposure, but not following 6 mo of exposure. The weights of the epididymal
fat pads and the left popliteal fat pad were decreased after 6 months exposure, but the decrease
was significant only for the popliteal fat pad (p < 0.05). Mean serum alkaline phosphatase was
significantly increased (p < 0.05) in both the 3-mo and 6-mo exposure groups. In the 6-mo
exposure group, inorganic phosphate was significantly higher (p < 0.05), and total serum protein,
cholesterol, blood urea nitrogen, serum glutamate-oxaloacetate transaminase, and
calcium/phosphate ratio were significantly lower (p < 0.05) than those of the control group. The
intestinal transit time was significantly lower (p < 0.05), and the length of the small intestine was
significantly greater (p < 0.05) than control animals.
19
-------
Sprague-Dawley rats (23 per group) were exposed to 0 or 116 ppm (0 or 475 mg/m3) for
7 h/day, 5 days/week, for 3 mo (duration adjusted to 99 mg/m3) (Tansy et al., 1980a). A subset
of 9 animals from each group underwent special metabolic performance studies that determined
food and water consumption, total fluid output, and fecal number and weight. Blood analyses,
measurements of terminal body weight and organ weights, and routine histological examination of
selected tissues (heart, lungs, kidneys, small bowel, and liver) were performed at the end of the 3-
mo exposure period. There were no significant differences between the body weights or weights
of the adrenals, epididymal fat pads, or left popliteal fat pad between exposed and control groups.
Total bilirubin was significantly lower (p < 0.05) and cholesterol was significantly higher (p <
0.05) in experimental animals. Metabolic performance data were divided into two categories, 5-
day mean values covering the exposure days (when only data could be collected only during the
17-h periods of no exposure) and 48-h weekend values. The only significant difference (p < 0.05)
between exposed and control groups was an increase in average weekday fecal excretion during
weeks 7, 10, and 11.
Adult male Sprague-Dawley rats were exposed by inhalation to 116 ppm (475 mg/m3)
MMA for periods of either 3 or 6 mo (Tansy, 1979a; Tansy et al., 1980b). Histologic
examination of the tracheas of rats exposed for both time periods revealed epithilia denuded of
cilia and reduction of the cellular covering of microvilli. The data in these papers were so poorly
presented that they cannot be used to draw any meaningful conclusions regarding the effects of
MMA.
Groups of F344/N rats and B6C3FJ mice (10 of each sex and species per group) were
exposed by inhalation to concentrations of 0, 500, 1,000, 2,000, 3,000, or 5,000 ppm (0, 2,047,
4,094, 8,189, 12,283, or 20,472 mg/m3 ) for 6 h/day, 5 days/week (duration adjusted to 0, 365.5,
731, 1,462, 2,193, or 3,656 mg/m3) for 14 weeks (NTP 1986). Animals were monitored daily,
body weights were taken weekly, and necropsy and/or histologic examinations were performed on
all animals. Histologic exams were done on animals in the 1,000, 3,000, and 5,000 ppm groups
and on all animals dying before the end of the study. Controls were also examined histologically.
All rats in the 5,000 ppm group died. Nine of 10 and 3 of 10 females died at 3,000 and
2,000 ppm, respectively. One of 10 male rats died from each group exposed to 2,000 and 3,000
ppm. Final mean body weights were 20% and 25% less for surviving males and females,
respectively, in the 3,000 ppm exposure group. At 2,000 ppm, these numbers were 7% and 11%
for males and females, respectively. Initial compound-related clinical signs included listlessness,
serous ocular discharge, nasal discharge, and prostration. Significant inflammation of the nasal
cavity associated with necrosis and loss of olfactory epithelium was observed at 3,000 ppm and
above in male rats and at 2,000 ppm and above in female rats (increased in incidence and severity
with dose). Other compound-related pathologic changes included follicular atrophy of the spleen
(4/10 males) and bone marrow atrophy (8/10 males in the high-dose group). Malacia and gliosis
of the brain in surviving females that increased in incidence and severity with dose, and cerebellar
congestion and hemorrhage of the cerebellar peduncles in early-death females at the two highest
doses were observed. No NOAEL was identified for rats in this subchronic study.
In male mice, 2/10, 4/10, and 8/10 died at 2,000, 3,000, and 5,000 ppm, respectively. In
female mice, 1/10 and 8/10 died at 2,000 and 5,000 ppm, respectively. Final mean body weight in
20
-------
males was 13%-27% lower in exposed groups compared with controls, and final mean body
weight in females was 6%-18% less than that of controls. In both sexes, final mean body weight
and body weight gain decreased with increasing concentration at levels of 500 ppm and greater.
Nasal epithelium metaplasia was observed in mice exposed to 500 ppm or greater. Kidney lesions
(cortical necrosis, cortical tubular degeneration, and/or focal mineralization) were observed in
1/10, 3/10, and 5/10 males at concentrations of 2,000, 3,000, and 5,000 ppm, respectively. Nasal
turbinate inflammation with necrosis and loss of olfactory epithelium was observed in 4/10, 5/10,
and 8/10 males and 5/10, 4/10, and 8/10 females at concentrations of 2,000, 3,000, and 5,000
ppm, respectively. Three of 10 male mice had extensive necrosis of the liver at 5,000 ppm. No
NOAEL was identified for mice in this subchronic study.
Male F344/N rats (50 per group) and B6C3FJ mice (50 of each sex per group) were
exposed to concentrations of 0, 499, or 984 ppm (0, 2,043, or 4,029 mg/m3) for 6 h/day, 5
days/week (duration adjusted to 0, 365, or 719 mg/m3), for 102 weeks (Chan et al., 1988; NTP,
1986). Female rats (50 per group) were exposed to concentrations of 0, 249, or 499 ppm (0,
1,020, or 2,043 mg/m3; duration adjusted to 0, 182, or 365 mg/m3) following the same exposure
regimen as for mice and male rats. All animals were observed twice daily and clinical
examinations were conducted once per week. Body weights were taken once per week for the
first 13 weeks and once per month for the remainder of the study. Necropsy and histologic exams
were performed on all animals. Tissues examined included gross lesions and tissue masses,
regional lymph nodes, mandibular lymph node, sternebrae including marrow, thyroid gland,
parathyroids, small intestine, rectum, colon, liver, mammary gland, prostate/testes/epididymis or
ovaries/uterus, lungs and mainstem bronchi, nasal cavity and turbinates, skin, heart, esophagus,
stomach, salivary gland, brain, thymus, trachea, pancreas, spleen, kidneys, adrenal glands, urinary
bladder, pituitary gland, preputial or clitorial gland, and tracheobronchial lymph nodes. There was
no significant difference in survival of rats or mice at the concentrations tested.
Mean body weights were 5%-10% lower than those of controls in male rats in the highest
exposure group after week 81, and 6%-l 1% lower in female rats in the highest exposure group
after week 73. Mean body weights in male and female mice were lower than in controls
throughout most of the study period (7%-19% lower after week 21 in the low-exposure group,
5%-18% lower after week 13 in the high-exposure group for males; 5%-16% lower in the low-
dose group after week 34, 4%-17% lower after week 34 in the high-dose group for females).
Inflammation of the nasal cavity was observed in both male and female rats at both
exposure concentrations, at a frequency that was significantly (p < 0.05) greater than that
observed in the control group. Both serous (37/50 low-dose males, 44/50 high-dose males, 17/50
low-dose females, 32/50 high-dose females) and suppurative (21/50 low-dose males, 30/50 high-
dose males, 12/50 low-dose and high-dose females) inflammation occurred (the incidence of
suppurative inflammation in females was not statistically significant). Incidences of olfactory
epithelium degeneration (atrophy and metaplasia) were also significantly (p < 0.01) elevated in all
groups (39/50 low-dose males, 42/50 high-dose males, 39/50 low-dose females, 44/50 high-dose
females). LOAELs of 249 and 499 ppm for extrathoracic effects were identified for female and
male rats, respectively. Alveolar macrophages were increased significantly (p < 0.05) in male rats
(20/49 and 16/50 in the low- and high-exposure groups, respectively) and focal or multifocal
fibrosis of the lung was significantly (p < 0.05) higher in high-exposure females (7/50). A
21
-------
LOAEL of 499 ppm was identified for pulmonary effects in male rats, and a NOAEL of 249 ppm
was identified for pulmonary effects in female rats. Other observed effects included a moderate
increase in mononuclear cell leukemia in exposed female rats and dose-related decreases in tumors
of the pituitary gland and neoplasms of the preputial gland in exposed male rats.
Mice exposed to MMA also had an increase of nasal lesions relative to control animals.
These included significant (p < 0.01) increases in the incidences of acute and chronic inflammation
(37/50 low-dose males, 42/50 high-dose males, 42/49 low-dose females, 45/50 high-dose
females), epithelial hyperplasia (44/50 low-dose males, 46/50 high-dose males, 43/49 low-dose
females, 47/50 high-dose females), cytoplasmic inclusions in the nasal mucosa (46/50 low-dose
and high-dose males, 44/49 low-dose females, 46/50 high-dose females), and degeneration of the
olfactory sensory epithelium (48/50 low-dose and high-dose males, 44/49 low-dose females,
47/50 high-dose females). A LOAEL of 500 ppm for both male and female mice was identified.
Lung interstitial inflammation was significantly (p < 0.05) more frequent in male mice in the
highest exposure group (8/50 compared to 1/50 and 0/50 in the control and low-exposure groups,
respectively). This corresponds to a NOAEL of 499 ppm. Concentration-related decreases in the
incidences of hepatocellular tumors in mice of both sexes, in the frequency of alveolar/bronchial
tumors in male mice, and in the frequency of pituitary gland neoplasms in female mice were also
observed.
F344 rats (70 of each sex per group) were exposed to mean concentrations of 0, 25,
99.79, or 396.07 ppm (0, 102.4, 408.6, 1621.7 mg/m3) for 6 h/day, 5 days/week (duration
adjusted to 0, 18.3, 73, 289.6 mg/m3) for 2 years (Hazelton Laboratories, 1979a). Parameters
monitored included mortality and other clinical signs of toxic effects, body weights, organ weights
(brain, kidneys, lungs, spleen, thyroids, adrenals, and testes/ovaries), ophthamology, hematology,
clinical chemistry, urinalysis, and gross and histopathology (only on animals from the control and
high-exposure groups at weeks 13 and 52). Interim studies were conducted on selected animals
from each group at weeks 13, 52, and 104, and from the control and high-level group at weeks 26
and 78. There were no significant differences in mortality between the exposed and nonexposed
groups. Body weights in the females exposed to the highest concentration were generally
significantly lower than controls after week 52, and the authors considered this to be related to
exposure. There was some evidence of decreased weight gain in animals exposed to the two
higher doses, but this was not analyzed in the study (the mean body weights for the control
groups were about 15 to 25 grams lower at the start of the experiment than those of the high-
exposure group). No differences in hematological parameters between control and exposed
groups could be clearly correlated with exposure. There were significant (p < 0.05) increases in
relative lung, liver, kidney, and ovary weights in females sacrificed at week 13, and significant (p
< 0.05) decreases in relative thyroid and adrenal weights of rats of both sexes sacrificed at week
52. There were no adverse pathological findings that could be clearly associated with exposure to
MMA vapor. No consistent trend with exposure was revealed, but microscopic examination of
nasal tissues revealed minimal to slight focal rhinitis in 4/10 females exposed to 396.07 ppm
(compared to 1 male and 1 female in the control group), and an inflammatory exudate was
observed in 3 of the 4 exposed females. At 52 weeks, livers of 9/10 males and 6/10 females
exposed to 396.07 ppm showed minimal nonsuppurative pericholangitis (compared with 5/10
control males and 2/10 control females). An increased incidence in lesions of mild rhinitis was
observed in the nasal turbinates of exposed animals at week 104. These consisted of serous and
22
-------
purulent exudates, pleocellular infiltrates, distended submucosal glands, focal
squamousmetaplasia, and inflammatory polyps. Because the increased incidence was found in all
exposure groups and did not appear to be concentration dependent, these lesions may not have
been treatment related.
At the request of EPA, the U.S. Methacrylate Producers Association (MPA)
commissioned a reexamination of the nasal tissue block and a rereview of the histopathology of
the rat nasal tissues from the Hazelton (1979a) study (Lomax, 1992; Lomax et al., 1997). This
reevaluation was requested because the initial study did not involve examination of the nasal
tissues of the low- and mid-exposure groups. In addition, because of MMA's propensity to cause
effects in the olfactory epithelium as demonstrated in other studies (NTP, 1986), this reanalysis
included examination of nasal tissue blocks in accordance with contemporary techniques with
prescribed levels of sectioning (Young, 1980). Tables 5 and 6 show the effects of MMA on both
olfactory and respiratory epithelium and the various exposure levels. Chronic exposure to MMA
did not appear to affect squamous epithelium at any exposure level. Effects in the respiratory
epithelium were observed primarily at the 400 ppm exposure level and were described as
hyperplasia of submucosal glands and/or goblet cells in the anterior regions of the nasal cavity,
especially around the dorsal meati and along the nasal septa. Inflammation of the mucosa and/or
submucosa was also observed. Changes to respiratory epithelium were bilateral and slight to
moderate in severity. Rats exposed to 100 or 400 ppm MMA had concentration-dependent
histopathological changes to the olfactory portion of the dorsal meatus in the anterior portions of
the nasal cavity. Microscopic changes were primarily observed in the olfactory region lining the
dorsal meatus in the anterior region of the nasal cavity. These changes were characterized by
degeneration and atrophy of the neurogenic epithelium and submucosal glands lining the dorsal
meatus, basal cell hyperplasia, replacement of olfactory epithelium with ciliate (respiratory-like)
epithelium, and inflammation of mucosa and submucosa. These changes were generally bilateral
in distribution and the severity of the lesions varied from minimal to slight at 100 ppm to slight to
moderate at 400 ppm. One male rat from the 400 ppm exposure group showed severe olfactory
degenerative effects (Lomax, 1992). One male rat from each of the 100 and 400 ppm exposure
groups had a small solitary polyploid mass attached to the lateral wall of one side of the anterior
nasal cavity. These masses were morphologically similar, consisting of differentiated
23
-------
VI
4*
0
ON O
•is
, .2 .2 .3 .3 .3 S
^ y y t« t« w ~
e e
& &
mb
a a
"^ >S
H P3 C/3 C/3
15
M (^
£ 8
a b
24
-------
Table 6. Severity grade of selected nasal cavity microscopic tissue changes from
rats exposed to MMA for 2 years
Males
Females
Concentration (ppm)
Olfactory epithelium
Degeneration/atrophy, dorsal
meatus, unilateral or bilateral
Basal cell hyperplasia, unilateral or
bilateral
Replaced by ciliated epithelium,
unilateral or bilateral
Inflammation, mucosa/submucosa,
unilateral or bilateral
Respiratory epithelium
Hyperplasia, submucosal
gland/goblet cell, unilateral or
bilateral
Inflammation, mucosa/submucosa,
unilateral or bilateral
Severity grade
Minimal
Slight
Moderate
Severe
Minimal
Slight
Moderate
Slight
Moderate
Slight
Moderate
Severe
Slight
Moderate
Slight
Moderate
0
0
0
0
0
5
0
0
0
0
0
0
0
0
1
3
1
25
0
0
0
0
3
0
0
0
0
0
0
0
0
0
0
0
100
7
33
2
0
5
27
1
2
0
16
1
0
1
0
2
0
400 0
0
11
26
1
0
14
19
12
3
21
7
1
13
12
20
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
0
25
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
100
0
24
0
0
0
18
0
7
0
5
0
0
0
1
0
0
400
0
10
29
0
0
23
8
16
5
17
8
0
6
3
8
1
Sources: Lomax (1992); Lomax et al. (1997).
25
-------
seudoglandular structures arising from the respiratory epithelium, and were diagnosed as polypoid
adenomas. The male rat from the 100 ppm group with the adenoma had concurrent moderate
chronic inflammation of the nearby respiratory epithelium. Two male rats exposed to 400 ppm
MMA had squamous metaplasia of the respiratory epithelium in the anterior region of the nasal
cavity.
Lakeview Golden hamsters (56 per sex per group) were exposed to 0, 24.77, 100.06, or
398.68 ppm (0, 101, 410, or 1,632 mg/m3) MMA vapor for 6 h/day, 5 days/week, for 78 weeks
(duration adjusted to 0, 18, 73, or 291 mg/m3) (Hazelton Laboratories, 1979b). Clinical signs,
body weight, hematology, and gross and histopathology were monitored in the animals.
Microscopic examination was done only on tissues from control and high-exposure animals. At
week 78, mortality in the male hamsters exposed to 400 ppm was about twice that of controls.
Mortality in females in the high-level group was more than twice that of controls from weeks
0-52, but was about the same as controls at week 78. No treatment-related differences in body
weight, clinical signs, hematological parameters, or gross pathology were observed. There was an
increased incidence of blood in the nasal turbinates of high-exposure males which the authors
attributed to the necropsy procedure. No other significant differences were found on microscopic
examination of the hamsters.
4.2.3. Acute Oral Studies
Central nervous system effects were observed in Wistar rats given 500 mg/kg body
weight/day MMA in olive oil by gavage for 21 days (Husain et al., 1985; Husain et al., 1989).
Treated rats were observed to be lethargic and had gait defects and hind limb weakness for about
10 min after each treatment. Locomotor activity and learning ability were significantly decreased
and aggressive behavior was significantly increased in exposed rats compared with controls.
There were several significant alterations in biogenic amines in exposed animals. Significant
increases in cholesterol and triglycerides and a significant decrease in total phospholipids were
observed in the sciatic nerve of treated rats compared with controls.
Ghanayem et al. (1986) administered MMA in corn oil (0, 100, and 200 mg/kg/day, 5
days/7 days/week) for 2 weeks to male F344 rats. No significant increase of mucosal cell
proliferation or hyperkeratosis was observed following histopathologic examination of the
forestomach (only organ examined).
Microscopic examination of the livers of Swiss strain white mice (Swiss strain)
administered 1% to 20% MMA in olive oil by direct esophageal instillation showed a dose-
dependent increase in the frequency and severity of mild to moderate liver injury at doses of 6%
and greater (Mallory et al., 1973). Abnormalities consisted of swollen cells with nuclei altered in
size and shape and congested sinusoids at concentrations of 6% to 10%. At concentrations of
11%, there was evidence of central and mid-zonal fatty changes with central lobular alteration,
and at concentrations above 11% there was massive fatty infiltration with alteration and disruption
of the liver nuclei.
The LD50s for MMA have been estimated to be 7.9-9.4 g/kg, 5.9 g/kg, and 4.7 g/kg in
rats, guinea pigs, and dogs, respectively (Deichmann, 1941; Spealman et al., 1945). The lowest
26
-------
lethal dose in rabbits administered MMA by gavage was 6.5 g/kg (Deichmann, 1941). Toxic
symptoms included increased respiratory rate and motor weakness. These were followed by
decreased respiration at 15 to 40 min post-administration, shallow and irregular respiration,
increased urination and defecation, hemoglobinuria, loss of reflex activity, coma, and death.
4.2.4. Subchronic and Chronic Oral Studies
This section is a review of subchronic and chronic oral laboratory animal studies relevant
to the derivation of health benchmarks for MMA. An overall synthesis of this information and its
relation to the potential for MMA to cause noncancer and cancer effects is presented in Sections
4.5 and 4.6, respectively. These sections also contain summary tables of key subchronic and
chronic laboratory animal studies (Table 8) and carcinogen bioassays (Table 10).
Borzelleca et al. (1964) found no significant toxic effects in male and female dogs (2
males and 2 females per treatment group) receiving MMA via gelatin capsule in the diet at 10,
100, or 1,473 ppm daily for 1 year. The high exposure concentration represented a time-weighted
average based on the 1,000 ppm value increasing to 1,200 ppm at 5 weeks, to 1,400 ppm at 7
weeks, and to 1,500 ppm at 9 weeks.
Borzelleca et al. (1964) also exposed groups of 25 male and 25 female Wistar rats to
MMA in drinking water for 104 weeks. The initial exposure concentrations were 6, 60, and
2,000 ppm MMA. The low and medium exposures were increased to 7 and 70 ppm, respectively,
at the start of the fifth month, resulting in TWA exposure concentrations of 6.85 and 68.46 ppm
MMA. Survival of exposed rats was not significantly different from controls. An initial reduction
in body weight gain was observed in both males and females exposed to 2,000 ppm MMA, which
reverted to control levels by week 3 (females) and week 6 (males). This is likely the result of
reported reduced food intake during the first month, which was not observed in the second month
and beyond. No other effects on body weight gain were reported, but drinking water
consumption was significantly lower than controls in males and particularly females of the high-
exposure groups. Hematological parameters were normal throughout the study in all groups, and
no compound-related effects were observed on urinary protein or reducing substances. Tissues
examined included heart, lung, liver, kidney, urinary bladder, spleen, gastroenteric, skeletal,
muscle, skin, brain, thyroid, adrenal, pancreas, pituitary, and gonads. The only effect observed
was an increased kidney/body-weight ratio in female rats exposed to 2,000 ppm MMA. No
abnormalities or lesions related to MMA were identified from histopathological examination of
the tissues of exposed rats. In addition to testing with MMA, these authors also exposed Wistar
rats to ethyl acrylate (EA) under the same exposure regimen. The EA-exposed rats closely
paralleled the MMA-exposed rats with respect to all results except female body weight. EA
caused a significant reduction in the body weights of the females in the high-dose group.
Motoc et al. (1971) orally administered methyl methacrylate to albino rats for 3 (20
exposures), 5 (41 exposures), or 8 (63 exposures) months. Total doses were reported as 2,750,
5,500 and 8,125 mg/kg, respectively, for these exposure periods. The authors reported duration-
related increases in histopathologic alterations of the liver, ulcerations of the stomach, and
biochemical alterations (elevated serum enzyme activity), but no further details were described.
27
-------
4.3. Reproductive and Developmental Studies
This section is a review of reproductive and developmental studies relevant to the
derivation of health benchmarks for MMA. An overall synthesis and summary table (Table 9) of
this information and its relation to the potential for MMA to cause reproductive or developmental
effects is presented in Section 4.5. Certain studies that were considered to be inadequately
documented for the purposes of this assessment or used irrelevant dosing regimens may not have
been discussed in this section. A more complete listing/discussion of all types of repeat or single-
exposure studies can be found in other, more detailed reviews (ECETOC, 1995; U.S. EPA,
1991).
Pregnant Sprague-Dawley rats were exposed to «1 10 mg/L (1 10,000 mg/m3) MMA for
either 17.2 or 54.2 min/day (approximately 1/4 and 3/4 of the LT50 time required to kill 50% of
the exposed group, for this concentration) on days 6 through 15 of gestation (Nicholas et al.,
1979). Clinical signs, body weight, and food consumption were monitored in the dams. On the
20th day of gestation, the dams were necropsied and reproductive parameters were examined
(number of corpora lutea, number and position of living fetuses, and number and position of
resorptions and early fetal deaths). The crown-rump length, sex, and weight of each fetus was
determined and each was examined for gross and skeletal abnormalities. Maternal body weight in
the long-exposure group was significantly different from sham-treated and untreated controls on
days 11, 15, and 20. Maternal body weight in the short-exposure group was significantly
decreased compared to untreated controls on day 15 and from both control groups on day 20.
Initial deviation from control values for both groups was obvious from day 7, with untreated
controls having the highest weights, followed by sham-treated, short-exposure, and long-
exposure, in order of decreasing body weights. Normalized food consumption values were lower
throughout the exposure days (6-15) for exposed groups than for controls. Early fetal deaths
were significantly higher for the exposed animals. Fetal body weight and crown-rump length were
significantly lower (p < 0.05) for both the short- and long-exposure groups and appeared to
decrease with increasing exposure time. A dose-related increase in hematomas occurred, with the
difference from controls significant (p < 0.05) for the long-exposure group. An increase in short
crooked tails also occurred in the long-exposure group, but the difference was not significant.
There were increases in the incidence of fetal skeletal anomalies (delayed ossification of the
vertebrae and other vertebral anomalies, delayed ossification of the sternebrae, rudimentary 14th
ribs, and fused and stunted ribs) in the exposed group, but only the increases in the delayed
ossification of vertebrae in the long-exposure group and the delayed ossification of sternebrae in
the short- and long-exposure groups were significantly different (p < 0.05) from controls. The
LOAEL (and LOAELj^c) for developmental effects in these animals is 1 10,000 mg/m3.
Eighteen pregnant ICR white mice were exposed to the vapors of the liquid monomer of
the acrylic cement Simplex P (97.4% MMA, 2.6% N,N-dimethyl-p-toluidine, and 75 ppm
hydroquinone) (McLaughlin et al., 1978). Animals were exposed to 1,330 ppm (5,446 mg/m3)
for 2 h/day, twice daily from day 6 through day 15 of gestation. The two exposure periods were
separated by a 1-h period of no exposure. Fourteen pregnant mice of the same strain served as
unexposed controls. The majority of the fetuses from both groups of mice were normal (94.9% in
the control group and 96.2% in the treated group). There was no significant difference in the
28
-------
number of fetal resorptions, abnormal or dead fetuses, or litter size between the treated and
control animals. However, the average fetal weight of the exposed animals was significantly
greater (p < 0.001) than that of controls (0.84 g and 0.90 g for the control and treated groups,
respectively). The significance of this is unknown.
CD-BR rats (27 rats per group) were exposed to concentrations of 0, 99, 304, 1,178, or
2,028 ppm (0, 405, 1,245, 4,823, or 8,304 mg/m3) MMA for 6 h/day on days 6 to 15 of gestation
(Solomon et al., 1991, 1993). Clinical signs, body weight, food consumption, and morbidity and
mortality were monitored in the dams. Dams were sacrificed and necropsied on day 20 of
gestation, and reproductive developmental parameters were examined (uterus weight, number of
corpora lutea, implantation sites, resorptions, number of fetuses per litter and their location, fetus
weight and sex, and external, visceral, and skeletal alterations). Treatment-related decreases in
maternal body weight gain during the experimental period were observed in all exposure groups,
but were transient at the two lowest exposure concentrations. Food consumption was decreased
in all exposed groups during the exposure period. No gross pathological changes were observed
in the dams at necropsy. Unlike the study described above by Nicholas et al. (1979), in which
dams were exposed during gestation to much higher concentrations but shorter daily exposure
durations, no treatment-related effects on reproductive or developmental parameters were
observed. A NOAEL of 2,028 for developmental effects in rats is identified from the results of
this study.
Pregnant CD1 mice were exposed to 0 (38 mice), 116 (32 mice), or 400 (18 mice) ppm
(0, 475, or 1,638 mg/m3) for two 3-h periods per day, interrupted by a 1-h nonexposure period,
on days 4 to 13 of gestation (Tansy, 1979b). Condition and body weight of the mothers was
monitored. Mice were sacrificed on day 18 of gestation and offspring were examined for viability,
gross abnormalities, or skeletal and visceral abnormalities. No adverse effects of exposure were
observed in the dams. The mean weight of living fetuses was significantly lower (p < 0.05) in
both exposed groups than in the sham-treated controls, but there was no concentration-related
trend. No other adverse effects appeared to be related to treatment. The high concentration is
considered a NOAEL for reproductive and developmental effects (NOAEL of 400 ppm).
Pregnant rats were exposed by inhalation to 0, 0.52, or 4.48 mg/L (0, 520, or 4,480
mg/m3) MMA vapors for 2 h/day, every 3 days from days 6 to 18 of gestation (Luo et al., 1986).
There were no obvious toxic effects on the dams. There was a statistically significant (p < 0.01)
increase in resorptions in the high-level group compared to both the low-level and control groups.
Delayed ossification was observed in both groups, but the incidences and statistical significance
were not reported. The high concentration is considered a LOAEL for embryotoxicity. This
reference was an abstract and had no data tables or figures.
In a dominant lethal assay performed by ICI (1976a), CD-I male mice were exposed to
100, 1,000 or 9,000 ppm MMA, 6 h/day, for 5 days. Each male was mated with 2 different
unexposed female mice weekly over a period of 8 weeks. There were no significant differences in
the fertility of the exposed males or in the survival rate, total implants, and early or late post-
implantation loss in the offspring of exposed males compared with controls.
29
-------
ICI (1977) also exposed groups of 30 female Alderley Park SPF rats to 0, 100 and 1,000
ppm MMA, 5 h/day, from days 6 to 15 of gestation. The experiment was performed a second
time using the same exposure levels plus 25 ppm. In the first experiment, the 1,000 ppm exposure
level, which was slightly maternally toxic, produced an increase in the numbers of early
resorptions and possibly affected the total numbers of late resorptions. Delayed ossification was
also noted at this exposure level.
Smirnova and Blagodatin (1977, as reported in U.S. EPA, 1985) reported that rats
inhaling 54 mg/m3 MMA continuously for 1 to 4 months showed increased estrogen secretion by
the ovaries, and this apparently increased the follicle-stimulating activity of the pituitary. The
relevance of these data to a human hazard assessment is questionable.
Groups of 5 female Sprague-Dawley rats were administered i.p. doses of 0, 0.1328,
0.2656, and 0.4427 mL MMA/kg/bw (1/10, 1/5, and 1/3 of the acute LD50 value) on days 5, 10,
and 15 of gestation (Singh et al., 1972). No skeletal malformations were seen, but a dose-
dependent increase of gross abnormalities (haemangiomas) was found in fetuses. Maternal
toxicity to the dams was not examined. In addition, the extent to which the fetus is exposed
following maternal i.p. injection and the relevance of this route of administration to other
exposure routes remains unclear.
4.4. Other Studies Related to Noncancer or Cancer Effects From Chronic Exposure to Methyl
Methacrylate
This section is a review of other studies relevant to the derivation of health benchmarks
for MMA. An overall synthesis of this information and its relation to the potential for MMA to
cause noncancer and cancer effects is presented in Sections 4.5 and 4.6, respectively. Certain
studies that were considered to be inadequately documented for the purposes of this assessment
or used irrelevant dosing regimens may not have been discussed in this section. A more complete
listing/discussion of all types of studies can be found in other, more detailed reviews (ECETOC,
1995; U.S. EPA, 1991b).
4.4.1. Genotoxicity
Table 7 gives a summary of the genotoxicity data for MMA. MMA has demonstrated
positive and negative results in tests for genotoxicity. An increased incidence of chromosome
aberrations and sister-chromatid exchange (SCE) was noted in lymphocytes of workers
occupationally exposed to 114 to 400 ppm MMA (Marez et al., 1991). The study did not exclude
for several factors (e.g., vaccinations, virus infections, white blood counts, alcohol consumption,
30
-------
Table 7. Genotoxicity of methyl methacrylate
Test
Indicator
organism
Metabolic
activation"
Doseb
Response
Reference
In vitro bacterial gene
mutation assays
Salmonella typhimurium strains TA
100, TA 1535, TA 1537, TA 1538,
TA98
Salmonella typhimurium
strain TM677 (forward mutation to
8-azaguanine resistance)
Salmonella typhimurium
strain TM677 (forward mutation to
8-azaguanine resistance)
+ and - 100 to 10,000 ug/plate
0, 10,50, 100mMx2h
0, 10,50, 100 mM x 2h
Waegemaekers and Bensink, 1984; National Toxicology Program,
1986; Anderson et al., 1979; also Dupont, 1975; ICI, 1980;
Zeiger, 1987; Hachiya et al., 1981; and Jensen et al., 1991; as
referenced in ECETOC, 1995
Posset al., 1979
Posset al., 1979
In vitro mammalian cell L5178Y/TK+'~ mouse
gene mutation assays lymphoma cells
LSI 78 Y/TK+A mouse
lymphoma cells
+ and- 500 to 3,100 ng/mL
+ and - 0.125 to 1.5 uL/mL
Moore et al., 1988; and Dearfield et al., 1991, as referenced in
ECETOC, 1995
National Toxicology Program, 1986; also Cifone, 1981; and Myhr
et al., 1990, as referenced in ECETOC, 1995
In vitro chromosome
damage assays
Chinese hamster ovary cells
Chinese hamster ovary cells
160, 500, 1,600, 5,000
Hg/mL
750, 1,000, 1,600, 3,000
Hg/mL
Increase at National Toxicology Program, 1986
5,000 ug/mL
only
Slight dose-
related
National Toxicology Program, 1986
In vitro sister chromatid Chinese hamster ovary cells
exchange assays
Human lymphocyte cultures
+ and - Up to 3,000 ug/mL
NG Up to cytotoxic levels
Dose-related National Toxicology Program, 1986
increase
Cannasetal., 1987
31
-------
Table 7. Genotoxicity of methyl methacrylate (continued)
Test
Indicator
organism
Metabolic
activation"
Doseb
Response
Reference
Dominant lethal assay CD-1 male mice
NA
100 to 9,000 ppm 6 h/day, 5 days
ICI, 1976a
In vivo chromosome Rat bone marrow
damage laboratory animal
assays
Rat bone marrow cells
Rat bone marrow cells
NA
NA
NA
100 to 9,000 ppm 6 h/day, 5 days
NG
Non-dose-
related
chromosome
damage
ICI, 1979
Anderson et al. (1979)
0, 100, 1,000, 9,000 ppm, or 100, 400, Weak increase in Smith, 1980
700, 1,000 ppm, single 2-h exposures chromosone
or multiple exposure of 5 h/day for damage
5 days
to
In vivo chromosome
damage human
occupational studies
Cells from peripheral NA 1 to 72 ppm in air
lymphocytes of human
volunteers occupationally
exposed to MMA
Cells from peripheral NA 114 to 400 ppm
lymphocytes of human
volunteers occupationally
exposed to MMA
Marezetal., 1991
Seiji et al., 1994
Marezetal., 1991
"NA = not applicable.
bNG = not given.
32
-------
and smoking) that can affect SCE. In addition, the SCE increase was slight and in a small
subgroup of workers. The significance of SCE assay results for this and other studies has been
questioned (Tucker et al., 1993; ECETOC, 1995).
Positive results were obtained in a gene mutation assay with mouse lymphoma cells with
and without activation (National Toxicology Program, 1986), in a sister-chromatid exchange
assay using Chinese hamster cells (CHO cells) with and without activation; in a chromosome
aberration test using CHO cells with and without activation (no dose response, however, with
activation); in a chromosome damage assay using rat bone marrow cells (no dose response); and
in the induction of forward mutation to 8-azaguanine resistance in S. typhimurium strain TM677,
with activation (only at doses resulting in 80% cell mortality, according to National Toxicology
Program, 1986). The results of the mouse lymphoma cell assay, in which the response with
activation was proportional to concentration and was observed at lower concentrations than in
tests without activation, and positive results seen in the S. typhimurium strain TM677 test suggest
that a metabolite of MMA may be contributing to the observed genotoxicity.
Negative results were seen in the Ames assay using the standard tester strains and in a
bone marrow micronucleus test (Hachiya et al., 1982), and workers exposed occupationally to no
more than 72 ppm did not show signs of sister chromatid exchange or chromosome aberrations
(Marez et al., 1991; Seiji et al., 1994). In addition, several short-term tests on the carcinogenic
effects of MMA conducted at Imperial Chemical Industries' (ICI) Central Toxicology
Laboratories (CTL) were negative (Smith, 1980). In the BHK21 mammalian cell transformation
assay (so-called Styles test) MMA was found to be a nontransforming agent. Four other
tests—the sebaceous gland suppression test, the subcutaneous gland implant test, the tetrazolium
reduction test, and Rabin's degranulation of RER test—were also negative.
4.4.2. Allergies/Sensitization
Sensitization has not been tested in experimental animals following either oral or inhalation
exposure to MMA. There is currently no recognized and validated animal model available for the
prediction of respiratory sensitization hazard (ECETOC, 1995). However, MMA has been
extensively tested in more than 40 skin sensitization assays in guinea pigs or mice. A detailed
review of the numerous studies in this area is beyond the scope of this document. A complete
review was performed by the ECETOC Joint Assessment of Commodity Chemicals (ECETOC,
1995). In summary, positive reactions were obtained in skin tests of laboratory animals,
particularly when high concentrations were used and evaporation of the substance from the skin
was avoided by using a highly viscous solvent or occluded application. Cross reactions with other
methacrylic acid esters and stabilizers such as hydroquinone may contribute to the observed
sensitization reactions of MMA.
Repeated exposure of human volunteers to undiluted MMA has also led to skin
sensitization (Cavelier et al., 1981; Spealman et al., 1945). However, the data with respect to
MMA's potential to be a respiratory sensitizer are less clear (ECETOC, 1995). A case report
(Pickering et al., 1986) reported a delayed asthmatic response following challenge with MMA.
However, there is no evidence of a respiratory sensitization effect in several recent occupational
studies of workers exposed to MMA (ECETOC, 1995).
33
-------
4.4.3. Dermal and Ocular Effects
MMA has been determined in laboratory animal and human studies to be mildly irritating
to the skin and eyes, and to the mucosa of the respiratory tract (ECETOC, 1995). Single doses of
10 mL/kg MMA applied to the clipped abdomen of rabbits produced temporary local irritation,
but the animals recovered within an hour (Deichmann, 1941). Castellino and Colicchio (1969)
observed slight reaction in rabbit skin following the topical application of MMA for 15 days to the
shaved skin. Acute local necrotic toxicity was noted by Linder (1976) when MMA was injected
subperichondrially on the outer surface of the ears of rabbits.
Castellino and Colicchio (1969), Cavelier et al. (1981), and others (see ECETOC, 1995)
have observed reddening of the conjunctiva in rabbits after repeated instillations of MMA in the
eyes. Holyk and Eifrig (1979) studied the effects of MMA on rabbit eyes by injecting 500, 5,000,
and 50,000 ppm into the anterior chamber. At these high exposure concentrations, MMA was
highly toxic to the tissues of the anterior segment of the rabbit eye, causing limbal hyperemia,
corneal edema, corneal neovascularization, iris engorgement, anterior chamber inflammation, iris
atrophy, and cataract.
4.5. Synthesis and Evaluation of Major Noncancer Effects and Mode of Action
The absorption and hydrolysis of MMA to methacrylic acid and subsequent metabolism
via physiological pathways results in a low systemic toxicity by any route of exposure. However,
10% to 20% of inhaled MMA is deposited in the upper respiratory tract of rats, and the hydrolysis
of MMA by local nasal tissue esterases to methacrylic acid in this region has been cited as the
primary reason for MMA's selective olfactory toxicity (Lomax, 1992; Lomax et al., 1997).
Table 8 summarizes key subchronic and chronic laboratory animal studies of MMA.
Subchronic and chronic exposure of rats and mice to MMA by oral and inhalation (and dermal)
routes results in effects consistent with its irritant properties. In inhalation studies, dose-related
lesions have been observed in the upper respiratory tract, including rhinitis, inflammation
associated with necrosis, degeneration/loss of olfactory epithelium in the nasal turbinates, and
lung congestion. Exposures to very high levels of MMA ( > 1,000 ppm) can result in
neurochemical and behavioral changes, reduced body weight gain, and degenerative and necrotic
changes in the liver, kidney, brain, spleen, and bone marrow. Relatively low concentrations can
cause changes in liver enzyme activities. The data concerning MMA's ability to cause
cardiovascular effects are inconsistent. Several publications in the literature suggest that MMA
may have cardiovascular and/or neurotoxic effects in occupationally exposed human beings.
These effects may not represent neurotoxicity because they are generally nonspecific, and workers
were exposed to several other toxic compounds. In general, MMA has not resulted in
34
-------
Table 8. Key subchronic and chronic laboratory animal studies of methyl methacrylate
Animal Route
Number of
animals
Dose/concentration
Critical effects/NOAEL/LOAEL
Reference
Rat
Rat
Hamster
Not given
70 M,70 F
Not given
116 ppm for 3 or 6 mo (542 h or Damage to tracheal mucosa: small areas of focal
1,105 h)
0, 25, 100, 400 ppm x 6 h/d,
5 d/wk x 2 yr
0, 25, 100, 400 ppm x 6 h/day,
5 day/week x 18 mo
Tansy et al., 1980b
hemorrhage in 6-mo group. Epithelium denuded of cilia,
reduction in cellular covering of microvilli in 3-mo group.
LOAELs at 116 ppm (both durations) based on
extrathoracic respiratory effects.
Histopathological changes to the olfactory portion of the Hazelton Laboratories America, 1979b; Smith,
dorsal meatus in the anterior portions of the nasal cavity at 1980; Lomax, 1992; Lomax et al., 1997
100 or 400 ppm. NOAEL at 25 ppm; LOAEL at 100 ppm.
Cumulative mortality of males in 400-ppm group higher
than controls at week 78. NOAEL at 100 ppm; LOAEL at
400 ppm.
Hazelton Laboratories America, 1979a; Smith,
1980
Rat
Rat
Mouse
Mouse
50 M, 50 F Male - 0, 500, 1,000 ppm
Female - 0, 250, 500 ppm x 6
h/day, 5 day/week x 102 wk
10 M, 10 F 0, 500, 1,000, 2,000, 3,000,
5,000 ppm 6 h/day, 5 day/week
over 14 week
50 M, 50 F 0, 500, 1,000 ppm x
6 h/day, 5 d/week x 102 week
10 M, 10 F 0, 500, 1,000, 2,000 3,000,
5,000 ppm x
6 h/day, 5 d/week over
14 week
Inflammation of nasal cavity and degeneration of olfactory
sensory epithelium. LOAELs at 500 and 250 ppm (males
and females, respectively) based on extrathoracic
(olfactory) effects.
All died at 5,000 ppm dose, 1 male and 9 females died at
3,000 ppm dose. Necrosis and loss of olfactory epithelium
in nasal turbinates at 3,000 and 5,000 ppm in males and at
>2,000 ppm in females. Malacia and gliosis in 5/9 females
at 2,000 ppm. LOAEL at 500 ppm based on nasal effects.
Inflammation and epithelial hyperplasia of the nasal cavity,
degeneration of olfactory sensory epithelium. LOAEL of
500 ppm (males and females) based on olfactory effect.
8 males and 8 females died at 5,000, 3,000, 5,000 ppm 6
h/day, ppm. Inflammation of nasal turbinates, nasal
epithelium metaplasia. Renal cortical necrosis, renal
tubular degeneration, liver necrosis in males. LOAEL of
500 ppm based on olfactory effects.
National Toxicology Program, 1986
National Toxicology Program, 1986
National Toxicology Program, 1986
National Toxicology Program, 1986
Rat
Dog
orl
orl
25 M,25 F
2M, 2F
up to 2,000 ppm x 2 yr
up to 1,500 ppm x 2 yr
NOAEL of 2,000 ppm based on lack of exposure-related Borzelleca et al., 1964
effects.
No significant toxic effects. NOAEL of 1,500 ppm.
Borzelleca et al., 1964
35
-------
Table 9. Developmental and reproductive effects of methyl methacrylate
Route
Species
Dose
Fetal effects
Reference
Inhalation
Inhalation
Inhalation
Inhalation
Inhalation
Inhalation
Inhalation
Injection
Intraperitoneal
injection
Male CD-I mice
Female Aderley
Park SPF rats
Rats (strain not
stated)
Female ICR mice
Female Sprague-
Dawley rats
Female
Crl:CDRBRrats
Female CD-I
mice
Chickens (White
Leghorn), three-
day-old embryos
Sprague-Dawley
rats
100, 1,000 or 9,000 ppm, 6 h/day, for 5 days
0, 25, 100, and 1,000 ppm, 5 h/day from days 6 to 15 of
gestation
2 h/day, every 3 days from days 6 to 18 of gestation
1,330 ppm for 2 hrS, 2x/day during day 6 through day 15
of pregnancy
110 mg/L (26,646 ppm) for 17.2 or
54.2 min (approx. 1/4 and 3/4 or the LT50) daily on days 6
through 15 of gestation
0, 99, 304, 1,178, and 2,028 ppm, 6 h/day from days 6 to
15 of gestation
0, 116, and 400 ppm, 6 h/day from days 4 to 13 of
gestation
2.3,4.5,9, 18,36mol/peregg
0.4427, 0.2656, 0.1328 mL/kg on days 5, 10, and 15 of
gestation
No effect on fertility of males, nor on survival rate, total implants, and early or late
post-implantation death in offspring of exposed males.
The 1,000 ppm exposure level was slightly maternally toxic, and produced an
increase in numbers of early resorptions and possibly impacted total numbers of late
resorptions. Delayed ossification was also noted at this exposure level.
Statistically significant (p < 0.01) increase in resorptions in high-level group
compared with both the low-level and control groups. Delayed ossification both
groups, but the incidences and statistical significance were not reported.
Slight increase in average weight of fetuses.
Longer exposure group had significant deaths, decrease in fetal weight and crown-
rump length, increase of fetuses with hematomas, increase in occurrence of delayed
ossification, skeletal anomalies such as missing vertebral center and stunted ribs.
Neither group had significant alteration of implantations, resorptions, number of
living fetuses per litter. Both groups had delayed skeletal ossification.
Treatment-related decreases in maternal body weight gain in all exposure groups
(transient at the two lowest exposure concentrations). Decreased food consumption
in all exposed groups. No gross pathological changes in the dams at necropsy. No
treatment-related effects on reproductive or developmental parameters.
Decreased mean weight of living fetuses (psO.05) in both exposed groups, but no
concentration-related trend. No other adverse effects appeared to be related to
treatment.
67% of all embryos were affected in the 36 umol/egg group and in the 4.5, 9, and 18
umol/egg groups, respectively (i.e, a low-dose range). 71% of corneal and lid defects
occurred ED50 umol/egg.
5.9% resorptions, 16.7% gross abnormalities (hemangiomas) in 0.4427 mL/kg group.
No skeletal malformations.
ICI, 1976
ICI, 1977
Luoetal., 1986
McLaughlmetal., 1978
Nicholas etal., 1979
Solomon et al., 1991, 1993
Tansy, 1979b
Korhonen et al., 1983
Singh etal., 1972
36
-------
serious adverse effects to humans. In certain individuals it has been shown to induce allergic
dermatitis from skin contact. Mild eye irritation and respiratory tract irritation have been
reported, but the evidence available does not allow for a determination regarding respiratory
sensitization.
Table 9 summarizes developmental and reproductive studies of MMA. No oral studies
have investigated the developmental or reproductive toxicity of MMA. Evidence for
developmental effects from inhalation exposure is mixed and generally occurred at maternally
toxic exposure levels. Solomon et al. (1993) found no developmental effects in rats exposed 6
h/day during days 6-15 of gestation to atmospheric concentrations of up to 2,028 ppm (8,304
mg/m3). Tansy (1979b) and McLaughlin et al. (1978) found no developmental effects in mice
exposed 6 h/day to up to 400 ppm and 2 h/day to 1,330 ppm, respectively, during days 6-15 of
gestation. However, Nicholas et al. (1979) found evidence of developmental effects (early fetal
deaths, delayed ossification, decreased fetal body weight and crown-rump length, hematomas) in
Sprague-Dawley rats exposed for approximately 1 h/day during days 6-15 of gestation to levels
more than an order of magnitude higher (110,000 mg/m3). But nearly 20% of the exposed
pregnant rats died at this exposure level. In addition, ICI (1977) and Luo et al. (1986) describe
both delayed ossification and increased resorptions in rats exposed during days 6-15 of gestation
to 1,000 ppm MMA (5 h/day and 2 h/3 days, respectively). No adequate one- or two-generation
reproductive studies were available by any route of exposure. MMA did not reveal an effect on
male fertility in mice inhaling up to 9,000 ppm MMA for 6 h/day over a period of 5 days (ICI,
1976a). These data suggest that at high, maternally toxic doses, MMA can cause developmental
effects. However, there is no reason to believe that developmental toxicity should represent a
critical or co-critical effect in the RfC or RfD derivation. The lack of adequate reproductive
studies is not a major concern given the limited evidence for systemic or genotoxic effects from
MMA exposure, but will be considered in the determination of uncertainty factors (Sections 5 and
6).
4.6. Weight-of-Evidence Evaluation and Cancer Characterization
4.6.1. Genotoxicity and Animal Evidence
Table 7 gives a summary of the genotoxicity assay results. When tested at cytotoxic
concentrations, MMA does not appear to be mutagenic to bacteria. MMA has been shown to be
an in vitro clastogen in mammalian cell gene mutation and chromosomal aberration assays.
However, MMA has not been shown to result in clastogenic effects or dominant lethal mutations
following laboratory animal in vivo, inhalation (ICI, 1976a), or oral exposures (Hachiya et al.,
1981), and human data (Marez et al., 1991; Seji et al., 1994) are equivocal.
Table 10 gives a summary of carcinogen!city tests in experimental animals. One interest in
the testing for carcinogenicity is to determine whether prostheses and other medical applications
of MMA are carcinogenic in humans. Carcinogenic tests have been performed that suggest that
tumors can form when laboratory animals are subjected to subcutaneous implants of poly-MMA
(Laskin et al., 1954; Ferguson, 1977). While some researchers (Homsy et al., 1972; Bright et al.,
1972) have shown some leaching of monomeric MMA from poly-MMA surgical
37
-------
Table 10. Key carcinogen!city findings in laboratory animal studies of methyl methacrylate
Route of
administration
Species/strain
Dose
Duration
of exposure
Findings
Reference
Subcutaneous implants in
flank
A/BiF/F50+ mice
Implanted subcutaneously in
lateral abdominal wall
Subcutaneous injection in
back, painting on back, or
subcutaneous implantation in
abdominal wall
50 Harlan strain albino Swiss
mice, 6 weeks old
Sprague-Dawley and Donryu
rats (8 groups of 20 animals)
oo
Dermal (on back of neck)
10 rats
2.4 x 13 mm discs of poly-
MMA. Some with cellulose
filters bonded by Millipore
MF cement
83 weeks
1 x 1 on MMA film
(combined monomer and
polymer compressed into film
and polymerized)
Implants in form of perforated
bowl, unperforated square film
or resin tooth (combined
monomer and polymer
compressed and polymerized.
Injection or painting with 0.1
to 0.2 mL of monomer, 2x
week for 3 mo
Painting 3 x per week
(concentration not given")
First tumor observed at 257
days. Others at 405, 438,
454, and 469 days
3 mo, observed until natural
death or appearance of tumors
4 mo
Discs alone produced
sarcomas in 12% of mice at 64
weeks. Discs covered by 0.45
urn filters produced no
sarcomas up to 83 week
(p<0.001). Discs covered by
0.025 to 0.1 urn filters
produced sarcomas in 60% of
mice in 64 weeks (p<0.001)
25% incidence of
fibrosarcomas (5 of 20
surviving mice)
3 8.8% tumors (fibrosarcomas)
in rats with square film
implants, 26.3% tumors in rats
with square film implants +
painting, 30.8% tumors in rats
with resin tooth implant. No
tumors in rats with perforated
bowl implants, or in those
injected or painted.
No tumors induced
Ferguson, 1977
Laskinetal., 1954
Okada, 1966
Oppenehimer et al., 1955
38
-------
Table 10. Key carcinogenicity findings in laboratory animal studies of methyl methacrylate (continued)
Route of
administration
Species/strain
Dose
Duration
of exposure
Findings
Reference
Drinking water
Inhalation
Wistar rats
(25/sex/group)
F334/N rats
(50/sex/group)
0, 6, 60, 2,000 ppm in drinking 2 years
water. At 5th mo 6, 60 ppm levels
were raised to 7, 70 ppm and
continued for 2 years
Males: 0, 500, 1,000 ppm 102 weeks
Females: 0, 250, 500 ppm 6 h/day,
5 days/week
Hispathology performed on heart, lung,
liver, kidney, bladder, spleen,
gastroenteric, skeletal muscle, bone
marrow, skin, brain, thyroid, adrenal,
pancreas, pituitary, gonads. No
abnormalities or lesions.
Increased incidence of mononuclear cell
leukemia in females in 500-ppm group
compared with controls (control, 11/50;
250 ppm, 13/50; 500 ppm, 20/50); not
significantly by life table tests.
Significant dose-related decrease in
incidence of pituitary gland and preputial
gland tumors in males. Body weights of
male and female rats within 10% of
controls.
Borzelleca et al., 1964
National Toxicology
Program, 1986
Inhalation
VO
B6C3F; mice
(50/sex/group)
0, 500, 1,000 ppm 6 h/day, 5
days/week
102 weeks No neoplastic lesions were found.
Significant dose-related decrease in
incidence of alveolar/bronchiolar
adenomas or carcinomas (combined) in
males, hepatocellular adenomas in males
and females, pituitary gland adenomas or
adenocarcinomas (combined) and uterine
adeno-carcinomas in females.
National Toxicology
Program, 1986
Inhalation
Inhalation
Golden hamsters
(56/sex/group)
F-344 rats
(70/sex/group)
0, 25, 100, 400 ppm 6 h/day, 5
days/week
0, 25, 100, 400 ppm 6 h/day, 5
days/week
78 weeks Histophathological examination showed Hazelton, 1979a
no treatment-related tissue alterations.
Cumulative mortality of high-level males
higher than controls at week 78.
104 weeks Histophathological examination showed Hazelton, 1979b
no increased incidence of neoplasma in
treated animals compared with controls
39
-------
implants, Ferguson (1977) suggests that sarcomas that arise following subcutaneous implants of
poly-MMA can be attributed to mechanical processes involving topographic interaction of the
solid surface with normal cells, especially macrophages. Consistent with this explanation are the
experiments of Oppenheimer et al. (1955), in which no tumors were induced when monomeric
MMA was applied dermally to the back of the neck of rats. However, the exposure period in the
Oppenheimer study was just 4 mo and only 10 animals were tested.
In the studies by Hazelton Laboratories (1979a,b), Fischer 344 rats and Charles River
Lakeview Golden Hamsters were exposed to MMA vapors at 0, 25, 100, and 400 ppm for 6
h/day for 5 days/week for 2 years and 18 mo, respectively. No increase was seen in the number
or type of tumors in either rats or hamsters, indicating that MMA was not carcinogenic in these
two species under those conditions. Appearance of a polypoid adenoma in the nasal cavity of two
MMA-exposed male rats (one each from the 100 and 400 ppm groups) (Lomax, 1992; Lomax et
al., 1997) is not likely to be associated with MMA-exposure as these benign neoplasms have been
reported in control rats as well (Miller et al., 1985). Similarly, a 2-year NTP inhalation bioassay of
rats and mice exposed to up to 1,000 ppm gave negative results for carcinogen!city, although the
animals may not have been tested at the maximum tolerated dose (NTP, 1986; Chan et al., 1988).
Borzelleca et al. (1964) reported the absence of carcinogenic effects in groups of 25 male
and 25 female Wistar rats given drinking water containing 0, 6, 60, or 2,000 ppm MMA for 2
years. Taken together, the genotoxicity, chronic inhalation, and chronic oral studies available
suggest that MMA is not carcinogenic in laboratory animals.
4.6.2. Human Evidence
Limited epidemiological data are available to determine whether the incidence of various
malignancies is higher in groups occupationally exposed to MMA versus those not exposed, and
no studies have been reported on whether smoking is a related factor in the occurrence of
malignancies in MMA-exposed workers. One retrospective epidemiological study that relates to
malignancies was conducted at the Bristol Plant, PA, which manufactures plastics, leather
chemicals, etc. (Monroe, 1984; Walker et al., 1991). In this study of Bristol Plant employees
hired prior to 1946 (Early Bristol cohort), an excess of cancer of the large intestine and rectum
was noted. However, an increase in these types of cancers was not observed in similar
populations at separate sites, or in subsequent evaluations of the same site (Walker et al., 1991;
ECETOC, 1995; Collins et al, 1989). Collins et al. (1989) have noted that during the 1970s, the
county in which the plant was located had a high colorectal cancer rate, at the 75th percentile for
the United States.
Some evidence of an increased death rate from cancer and noncancer respiratory disease is
provided by the American Cyanamid (Collins et al., 1989) and Knoxville (Walker et al., 1991)
cohorts. However, in both of these cohorts, exposure to MMA was considerably lower than in
the Early Bristol cohort, which showed no such excess. Others have suggested that these
increases were lifestyle related (ECETOC, 1995).
40
-------
4.6.3. Structure-Activity Rel ati onship s
Acrylic acid, four monofunctional acrylates, eight polyfunctional (di- or tri-) acrylates, a
dimethacrylate, and a trimethacrylate have been tested in skin painting cancer bioassays. Acrylic
acid, 2-ethylhexyl acrylate, and three diacrylates caused skin tumors. Methyl acrylate (MA), ethyl
acrylate (EA), n-butyl acrylate (BA), and methyl methacrylate have been tested in chronic
inhalation bioassays and found to be negative with respect to carcinogenicity (Woo et al., 1988).
While the Borzelleca et al. (1964) drinking water studies did not report carcinogenicity for either
EA or MMA exposure, EA was found to cause forestomach tumors following gavage exposure
(NTP, 1983). However, the fact the EA has been found to cause forestomach tumors at high
gavage doses (NTP, 1983) does not necessarily implicate MMA. This is suggested by structure-
activity relationship studies that demonstrate that the addition of a methyl group to the acrylate
moiety tends to abolish carcinogenic activity (Woo et al., 1988) and gavage dosing of analogues
of EA demonstrating that the forestomach toxicity required the intact molecule (an ester moiety,
the double bond, and no substitution at carbon number 2) (Ghanayem et al., 1985). In another
paper, Ghanayem et al. (1986) reported that cell proliferation of the rat forestomach (believed to
be a precursor effect to tumors caused by this compound) was apparent in all rats (12/12)
following 2-week gavage administration of EA at both 100 and 200 mg/kg, but was not apparent
in any rats exposed to 100 mg/kg MMA (0/8) and in just 1/8 rats exposed to 200 mg/kg MMA.
This latter increase was not statistically significant and the effect was much less severe than the
effects caused by EA at either dose. Thus, structure-activity relationship analysis does not
suggest that MMA would be carcinogenic by any route.
4.6.4. Summary
Cases of sarcomas reported following implants of polyMMA are attributed to mechanical
processes, not MMA. Carcinogenic activity was not detected in four well-conducted chronic
inhalation bioassays in three different species: rats, mice, and hamsters (NTP, 1986; Hazelton,
1979a,b). Results of a 2-year drinking water study of Wistar rats (25/sex) (Borzelleca et al.,
1964), though not as well documented as the inhalation studies, also showed no carcinogenicity.
Mutagenicity data are mixed and human epidemiologic evidence is inadequate for basing a
carcinogenicity determination. Under the Proposed Guidelines for Carcinogenic Risk Assessment
(U.S. EPA, 1996a), MMA is considered not likely to be carcinogenic to humans because it has
been evaluated in two well-conducted studies in two appropriate animal species without
demonstrating carcinogenic effects.
4.7. Other Hazard Identification Issues
4.7.1. Possible Childhood Susceptibility
A number of factors may differentially affect children's responses to toxicants. The only
toxicity information on MMA of possible relevance to this issue is that several studies showed
that developmental effects are observed only at exposure levels that are maternally toxic, even
lethal. There is too little information to make any further statements about how children may be
differentially affected by methyl methacrylate, as there are no data regarding methyl methacrylate
41
-------
exposure prior to mating, from conception through implantation, or during late gestation,
parturition, or lactation.
4.7.2. Possible Gender Differences
No gender-related differences were observed in the current data on methyl methacrylate.
Male and female laboratory animals appear to respond similarly in all respects. It should be noted,
however, that all human epidemiology studies to date have involved solely male cohorts. No
epidemiologic data exist for females at this time.
5. Dose-Response Assessments
5.1. Oral Reference Dose (RfD)
5.1.1. Choice of Principal Study and Critical Effects
Relevant, quantitative human subchronic or chronic studies of MMA are not available. No
oral developmental or reproductive studies are available. There are three repeat exposure studies
that were of long enough exposure duration to be considered for use in the derivation of an oral
RfD, the Motoc et al. (1971) rat study, the Borzelleca et al. (1964) rat study, and the Borzelleca
et al. (1964) dog study. Of the three, only the Borzelleca et al. (1964) drinking water study in
rats was of chronic duration (2 years). Motoc et al. (1971) was a subchronic gavage study, and
the assessment of dogs by Borzelleca et al. (1964) involved the administration of MMA in gelatin
capsules. The Motoc et al. (1971) gavage study showed that large bolus doses can overwhelm
detoxification mechanisms and cause stomach ulcerations in rats. Thus, the less than chronic
gavage studies of Motoc et al. (1971) and Borzelleca et al. (1964) are considered less desirable
for use in the derivation of an RfD than the chronic drinking water study in rats of Borzelleca et
al. (1964). Borzelleca et al. (1964) reported an increase in kidney-to-body ratios reported for
female rats, but it was only marginally significant and was not associated with any histopathologic
findings. The fact that MMA was not reported to cause gastric toxicity in this study is not in and
of itself a reason to doubt the results of the study. Substitution on the number 2 carbon of acrylic
acid has been shown in gavage studies to abolish gastric toxicity (Ghanayem et al., 1985) and cell
proliferation (Ghanayem et al., 1986). Thus, the Borzelleca study is deemed adequate for RfD
derivation and the highest exposure level, 136 mg/kg/day (2,000 mg/L x 0.0313 L/rat/day divided
by the default body weight for Wistar rats of 0.462 kg), is considered a NOAEL for this study.
5.1.2. Method of Analy si s
The NOAEL discussed above will serve as the basis for the RfD. A benchmark dose
analysis could not be attempted from this study as no adverse effect was identified.
5.1.3. Chronic RfD Derivation
The following uncertainty factors are applied to this effect level: 10 for consideration of
intraspecies variation (UFH; human variability), an uncertainty factor of 3 for extrapolation for
interspecies differences (UFA; animal to human), and an uncertainty factor of 3 to account for a
deficient database (UFD), including the lack of a chronic study in a second species and the lack of
42
-------
other supporting studies (e.g., neurologic, developmental, and reproductive). The UFD is not
higher because the lack of oral studies is partially compensated for by a more complete inhalation
exposure database, though reproductive studies are lacking from the inhalation database as well.
The total UF = 10 x 101/2 x 101/2 « 100. No MF is applied.
RfD = 136 mg/kg/day - 100 = 1.4 mg/kg/day
5.2. Inhalation Reference Concentration
5.2.1. Choice of Principal Study and Critical Effect
No epidemiologic or occupational studies of MMA are available that adequately describe
inhalation exposure concentrations and effects. Most of the available human studies did not
account for one or more confounding exposures. Of the chronic laboratory studies available, only
Hazelton Laboratories America (1979a,b) tested exposure levels (to rats and hamsters) below 250
ppm. The original study found only mild rhinitis as the principal effect in all exposure groups,
with no mention of the olfactory effects so evident in all exposure groups (250 to 5,000 ppm) of
rats and mice studied by NTP (1986). As a result of EPA discussions with the Methacrylate
Producers Association (MPA), MPA commissioned a review (Lomax, 1992; Lomax et al., 1997)
of the histopathology of the nasal tissues in the Hazelton (1979a) study. Because of MMA's
propensity to cause effects in the olfactory epithelium as demonstrated in other studies (NTP,
1986), this reanalysis included a more thorough examination of the nasal cavity tissue blocks than
was done in the original study. The Hazelton (1979a) study and the Lomax (1992) reanalysis
were selected for use in the RfC derivation over the NTP (1986) because the combined analysis
was well conducted, involved an adequate number of test animals, and identified a NOAEL at an
exposure concentration 10-fold lower than the lowest exposure concentration in the NTP (1986)
study.
Tables 5 and 6 show the effects of MMA on both olfactory and respiratory epithelium and
the various exposure levels. The hydrolysis of MMA by carboxylesterase enzymes and
subsequent release of methacrylic acid in the olfactory tissue (Morris and Frederick, 1995) is
likely the cause of the greater effect in this region. Localization and severity of the lesion in the
olfactory epithelium is consistent with the greater esterase activity reported in the olfactory
epithelium as compared to respiratory epithelium in rodents (Dahl et al., 1987; Bogdanffy et al.,
1987; Bogdanffy, 1990; Frederick et al., 1994). Similar toxicity from acids produced via the same
metabolic route has been seen with ethyl acrylate (Miller et al., 1985), methyl and butyl acrylate
(Klimisch, 1984), dibasic esters (Keenan et al., 1990) and glycol ether acetates (Miller et al.,
1984). However, direct exposures to acrylic and acetic acids have also caused similar olfactory
specific lesions (Miller et al., 1981; Stott and McKenna, 1985), suggesting that greater esterase
activity in olfactory tissue is not the only factor leading to this specificity. Differing sensitivities
among nasal tissues to the acid metabolite or further metabolism of the acid may contribute as
well.
43
-------
5.2.2. Method of Analysis
A polynomial mean response regression model (THRESH, I.C.F. Kaiser, 1990a) and a
Weibull power mean response regression model (THRESHW, I.C.F. Kaiser, 1990b) were used to
fit data from Lomax (1992) and Lomax et al. (1997) by the maximum likelihood method. These
models were developed for use with dichotomous (incidence) data and provide the option of
assuming a zero or nonzero background response. The only olfactory effect noted in control
animals was minimal basal cell hyperplasia (5/39 control animals) (Table 6). For the purpose of
calculating a BMC, it appears reasonable to assume a zero background for slight, moderate, and
severe olfactory lesions. Minimal lesions were excluded from the BMC analysis and a zero
background was assumed. Using these criteria, BMC15, BMC10, BMC5, and EMCl analyses were
performed for all four olfactory lesions (male and female) listed in Table 6. Table 11 provides a
summary of these model runs.
From these data sets, data for degeneration/atrophy in males (0/39, 0/47, 35/48, and
38/38) were chosen for use in the derivation of the RfC because the concentration-response
curves generated by both THRESH and THRESHW models were similar and of reasonable
goodness of fit. In addition, the resultant BMC values were lower than the BMCs for
replacement by ciliated epithelium, the only other endpoint for which a good model fit could be
reached An EPA review of benchmark analysis performed for several upper respiratory toxicants
indicates that the BMC values for both the 5% and the 10% benchmark response (BMR) levels
for a given endpoint generally fall between the NOAEL and the LO AEL for that endpoint (Gift,
1996). The BMR chosen for use in the MMA RfC derivation is a 10% increase in the incidence
of a slight, moderate, or severe lesion. The 10% response level was chosen because of its closer
proximity to the actual experimental data and because of the overall mild severity of the effect.
The RfC is based on the BMC10, which is the lower 95% confidence bound on the maximum
likelihood estimate (MLE) of the concentration that causes a 10% increased incidence of this
lesion. The two model predictions for the BMC10 from degeneration/atrophy of male rat olfactory
epithelium were virtually identical, 39 (Weibull) and 35 (polynomial) ppm. The 35 ppm value was
chosen for use in the RfC calculation because it results in a slightly more environmentally
protective RfC. This value is slightly above the 25 ppm NOAEL and well below the 100 ppm
LOAEL for degeneration/atrophy and inflammation. More details of the BMC10 derivation for
this data set (model used, input assumptions, etc.) are provided in Appendix A. Appendix A also
discusses the limitations of this data set and the limitations of the analysis. The following
summarizes the results and describes how the BMC10 was used to derive the BMC10(HEC), which
serves as the basis for the RfC. Assuming 25 °C and 760 mmHg and a molecular weight of
100.11,
BMC10 (mg/m3) = 35 ppm x 100.11/24.45 = 143 mg/m3.
When the BMC10(mg/m3) is derived from a study in which laboratory animals are exposed
intermittently (e.g., 6 h per day, 5 days per week), an adjustment is usually applied to account for
the fact that the RfC is to protect against the worst-case scenario, continuous exposures.
However, the EPA guidelines (EPA, 1994c) recognize that, depending on the mechanism of
action, such duration adjustment may not always be appropriate. In the case of
44
-------
Table 11. Summary of benchmark analysis model runs on olfactory effects
Effect
Males
Degeneration/atrophy
Degeneration/atrophy
Basal cell hyperplasia
Basal cell hyperplasia
Replaced by ciliated
Replaced by ciliated
Inflammation
Inflammation
Females
Degeneration/atrophy
Degeneration/atrophy
Basal cell hyperplasia
Basal cell hyperplasia
Replaced by ciliated
Replaced by ciliated
Inflammation
Inflammation
Model
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
Weibull
Polynomial
BMC
15
(ppm)
46.2
42.3
21.2
22.7
156.3
167.4
36.8
25.8
20.7
28.9
33.8
118.1
73.3
89.2
89.6
BMC1
0
(ppm)
39.4
35.11
13.1
14.7
112.5
118.7
23.9
17.4
13.4
17.3
21.9
83.7
47.5
62.0
59.8
BMCO
5
(ppm)
30.2
25.55
5.8
7.2
63.4
62.8
11.6
9.0
6.5
7.3
10.7
47.3
23.1
33.5
29.4
BMCO
1
(ppm)
16.6
7.75
0.92
1.4
16.5
12.5
2.3
2.0
1.3
1.0
2.1
12.9
4.5
8.1
5.8
Chi-square
goodness of fit
0.087
0.968
12.7
11.9
0.167
0.256
5.3
5.4
8.9
10.1
8.07
7.5
1.9
2.13
0.77
1.16
Degrees of
freedom
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
P
value
0.768
0.616
0.0003
7
0.026
0.683
0.613
0.02
0.02
0.003
0.0015
0.045
0.023
0.16
0.144
0.38
0.28
acrylic acid, a compound that causes similar olfactory damage, there is information to suggest that
a limited C * T relationship of exposure to toxic effects is operative over the course of at least the
first 2 weeks of exposure at concentrations that cause minimal to moderate, reversible (if
exposure is discontinued) olfactory effects (Lomax et al., 1994). The lack of lesions in rats after
28 days of exposure to 100 ppm MMA (Greene, 1996), combined with the presence of lesions in
rats following chronic exposure to 100 ppm MMA (Lomax et al., 1997), suggests that these
effects can progress with increased exposure duration. Thus, it is reasonable to suggest that
continuous exposure to MMA could result in effects at concentrations below the NOAEL of an
intermittent exposure study, and that the application of an adjustment factor to account for this is
appropriate. Thus, the BMC10 of 143 mg/m3 is adjusted as follows:
BMC10(adj) = 143 x 6 h/24 h/day x 5 days/7 days/week = 25.6 mg/m3.
45
-------
The human equivalent BMC 10, BMC10(HEC), is calculated using default procedures
appropriate when peer-reviewed PBPK models are not available. For a category 1 gas,
gas:respiratory effect in the extrathoracic region is as follows:
Minute volume for the laboratory animal (Mva) = 0.25 L/min1
Minute volume for humans (Mvh) =13.8 L/min
Extrathoracic surface area for the laboratory animal [Sa(ET)J = 11.6 cm2
Extrathoracic surface area for humans [Sh(ET)] = 177 cm2
Regional gas deposition ratio (RGDR) = (MVa/Sa)/(MVh/Sh) = 0.28
BMC10(HEC) = 25.6 mg/m3 x RGDR = 7.2 mg/m3.
5.2.3. Chronic RfC Derivation
The BMC10(HEC) for degeneration/atrophy of olfactory epithelium described by Lomax
(1992) and Lomax et al. (1997) is estimated at 7.2 mg/m3. A partial threefold uncertainty factor
(UF) is applied to this effect level in consideration of possible intraspecies variation (UFH, to
protect sensitive human subpopulations), and a partial threefold interspecies uncertainty factor
(UFA) is applied because of possible toxicodynamic differences between rats and humans. The
total UF = 101/2 x 101/2 « 10. No modifying factor (MF) is applied.
RfC = 7.2 mg/m3 + 10 «0.7 mg/m3 = 7 E-l mg/m3.
5.3. Cancer Assessment
As discussed in Section 4.6, MMA is considered not likely to be carcinogenic to humans
because it has been evaluated in two well-conducted studies in two appropriate animal species
without demonstrating carcinogenic effects. No data exist to support a quantitative cancer
assessment for this compound.
6. Major Conclusions in Characterization of Hazard and Dose-Response
6.1. Hazard Identification
MMA is a colorless flammable liquid with a strong acrid odor. It is primarily used to
make a variety of resins and plastics, and is most often polymerized to polymethyl methacrylate,
which is used to make acrylic sheets, acrylic moldings, and extrusion powders. Because MMA is
relatively volatile (vapor pressure of 40 mmHg at 25°C) and widely used, significant occupational
exposure to the chemical can be expected to occur. Potential for significant exposure exists for
employees of manufacturers of MMA and its polymers, as well as doctors, nurses, dentists, and
dental technicians.
Calculated using Equation 4-4 of EPA (1994c) for male rats with a body weight of 380 g; confirmed by
actual measurements taken by Mauderly (1986) and Phalen (1984).
46
-------
MMA is rapidly absorbed and distributed in laboratory animals following oral and
inhalation exposure. It is similarly metabolized in animals and humans. From a toxicologic
standpoint, the key metabolic step is MMA hydrolysis to methacrylic acid, considered to be the
toxic moiety responsible for the irritant properties of MMA. The rate of this metabolic step has
been shown to proceed slower in human blood and nasal tissues than in the corresponding rat
tissues, but at a faster rate in human liver tissue. Subchronic and chronic exposures to various
laboratory animal species by dermal, oral, and inhalation routes produced effects consistent with
the irritant properties of MMA and methacrylic acid. In inhalation studies, exposure-related
lesions were observed in the respiratory tract at exposure levels at and above 100 ppm. The most
frequent, sensitive, and severe lesions generally occurred in the olfactory tissue and consisted of
olfactory epithelial loss and degeneration. Developmental, CNS, and other systemic effects were
sporadically reported, but generally at concentrations exceeding 1,000 ppm. No carcinogenic
potential was shown for MMA in four chronic inhalation bioassays (rats, mice, and guinea pigs)
and one chronic oral bioassay (rats).
Limited human epidemiologic information exists for MMA. Several occupational studies
are available, but none reported MMA-specific exposure information useful for the derivation of a
health benchmark. Many of the human studies were poorly reported and lacked details regarding
confounding exposures and the health status of exposed individuals. Nevertheless, the extensive
occupational literature does suggest that MMA would not be expected to cause death or serious
adverse health effects as a result of acute exposures. The distinct odor and low odor threshold of
MMA tend to assist workers in avoiding significant exposures. However, laboratory animal
studies and one human study (Schwartz et al., 1989) suggest that MMA may erode olfactory
function. This ability is considered to be MMA's most sensitive effect and perhaps its most
serious potential human hazard following inhalation exposure. Although other effects have been
noted (e.g., cardiovascular and neurologic effects), they are generally nonspecific, occur at higher
exposures, and are often not clearly attributable to MMA exposure. Sensitization from MMA
inhalation exposure has been reported in a single case study (Pickering et al., 1986). While the
potential for sensitization following either oral or inhalation MMA exposure can not be ruled out,
it has not been observed in several occupational studies and has only been clearly demonstrated
following dermal exposure to certain individuals. Epidemiology studies show no clear excess of
respiratory disease or cancer. Though a report suggesting increased colon cancer among ethyl
acrylate/MMA-exposed workers exists, a high background for this effect has been documented
for the location and time of this study, the effects were not reproduced in other similar and more
recent studies, a clear relationship between exposure and effect was not demonstrated, and the
extent that ethyl acrylate concurrent exposure confounded results could not be determined. Given
these considerations, the low potential for cancer from MMA exposure indicated in genotoxicity,
laboratory animal, and epidemiology studies suggests that MMA does not represent a
carcinogenic hazard to humans. Structure-activity relationship analysis relative to other acrylates
also does not suggest that MMA would be carcinogenic by any route. Under the Proposed
Guidelines for Carcinogenic Risk Assessment (U.S. EPA, 1996a), MMA is considered not likely
to be carcinogenic to humans because it has been evaluated in two well-conducted studies in two
appropriate animal species without demonstrating carcinogenic effects. Under EPA's (1987a)
Guidelines for Carcinogen Risk Assessment, MMA would be classified as evidence of non-
car cinogenicity for humans, or a Group E chemical.
47
-------
6.2. Dose-Response Assessment
The quantitative estimates of human risk as a result of low-level chronic exposure to
methyl methacrylate are based on laboratory animal experiments because adequate human data are
not available.
The human dose that is likely to be without an appreciable risk of deleterious noncancer
effects during a lifetime (the RfD) is 1.4 mg/kg/day. Because of the application of uncertainty
factors, this amount is approximately 1/100 of the dose that resulted in no effects in a chronic rat
drinking water study (Borzelleca et al., 1964).
The overall confidence in the RfD assessment is low to medium. The confidence in the
principal study is low to medium. The Borzelleca (1964) study is well documented, but does not
appear to be conducted in accordance with what would now be considered Good Laboratory
Practice and did not identify a LOAEL. Confidence in the database is judged to be low to
medium. Although repeat exposure inhalation studies, including developmental, reproductive,
and chronic studies, bolster the weak and dated oral database somewhat, no developmental or
reproductive studies by the oral route are available, and no multigenerational studies are available
by any route of exposure. Gastrointestinal irritation has been identified in a rat subchronic gavage
study (Motoc et al., 1971), but acute exposures to humans via the oral route are rare. Irritation is
still considered the most likely effect of concern from oral exposure to humans, however,
primarily because of extensive evidence from occupational studies and case reports that MMA is a
respiratory irritant in humans.
A full uncertainty factor for intraspecies differences (UFH) was used to account for
potentially sensitive human subpopulations. This UF was not reduced because of the lack of
human oral exposure information. A partial threefold uncertainty factor to account for laboratory
animal to human interspecies differences (UFA) was used. The slower blood metabolism of MMA
in humans (Bereznowski, 1995), combined with the fact that humans do not have a forestomach
(target organ in the Borzelleca et al., 1964 study) lowers the potential for a more pronounced
portal-of-entry effect in humans. However, complete elimination of this UF is not justified, given
the lack of human oral exposure information and remaining uncertainty regarding MMA's
potential to cause other effects in humans following chronic oral exposure.
The major areas of uncertainty in this assessment are the lack of an identified critical effect
to humans, the lack of a chronic study in a second species, and the lack of a neurologic study and
the lack of a developmental or reproductive toxicity study via the oral route (given that
developmental effects have been seen in laboratory animals following other routes of exposure).
A partial threefold database uncertainty factor (UFD) was employed, however, because a number
of repeat exposure inhalation studies, including developmental, reproductive, and chronic studies,
lend support to the oral database.
The daily exposure to the human population that is likely to be without an appreciable risk
of deleterious effects during a lifetime (the RfC) is 7 E-l mg/m3. This concentration is 1/10 of the
estimated BMC10(HEC) for degeneration/atrophy of olfactory epithelium described by Lomax
(1992) and Lomax et al. (1997).
48
-------
The overall confidence in this RfC assessment is medium to high. The RfC is based on a
long-term rat inhalation study (Hazelton Laboratories, Inc., 1979a) performed with relatively
large group sizes, in which, with additional investigations performed by Lomax et al. (1997),
thorough histopathologic analyses were performed on all relevant tissues. What is considered to
be the primary target organ, the nasal passage, was particularly well described, and the study was
able to identify both a NOAEL and a LOAEL. The scientific quality of the combined Hazelton
Laboratories (1979a) and Lomax et al. (1997) investigations is high.
The confidence in the inhalation database available for MMA is rated as medium to high.
Acceptable developmental studies were carried out in two species, rats and mice, with effects
observed only in offspring at levels more than 10-fold higher than the LOAEL for the chosen
critical (olfactory) effect. Two studies noted increased resorptions in rats at 1,000 ppm exposures
(Luo et al., 1986; ICI, 1977) and one did not (Solomon et al., 1993). However, the latter study
was peer reviewed whereas Luo et al. (1986) was an abstract and ICI (1977) was an unpublished
industry report. Multigenerational reproductive studies are not available for MMA; however,
MMA is so reactive at the portal of entry that the potential for systemic effects is deemed remote.
The observation of a portal-of-entry effect is consistent across both the oral and inhalation routes
of exposure. Given these considerations, the inhalation database and the RfC are given medium
to high confidence, and no uncertainty factor is applied to the RfC for database deficiencies.
A partial threefold intraspecies uncertainty factor is applied to the RfC for protection of
sensitive subpopulations. This UF is reduced by extensive human occupational studies and case
reports that consistently identify the irritant properties of MMA as the principal effect of concern
from MMA inhalation exposures. Little intraspecies variance is observed with respect to the
identified critical effect, olfactory degeneration in laboratory animals (ECETOC, 1995; Lomax et
al., 1997), and there is no reason to expect a high degree of intrahuman variability from this type
of effect. Although Pickering et al. (1986) reported delayed asthmatic response following
challenge with MMA, which would suggest that MMA is a possible respiratory sensitizer, no
occupational studies identified MMA as a respiratory sensitizer. A partial intraspecies uncertainty
factor of 3 is deemed sufficiently protective.
A partial threefold uncertainty factor is used for interspecies extrapolation to account for
potential toxicodynamic differences between rats and humans. This concern for potential
toxicodynamic differences is warranted given the fact that humans may be less capable of
recovering from olfactory damage than are rats. "Rapid potentially anatomically correct recovery
after massive destruction" is observed in rats when underlying basal cells are not damaged
(Youngentob, 1997) and small islands of intact olfactory epithelium are "sufficient to allow for
olfactory function" (Wong et al., 1997). In humans, however, it has been reported that patients
with relatively mild to moderate olfactory damage fail to recover olfaction and "...even when basal
cells remain intact, differentiating cells developing from them do not mature into receptor cells but
can develop into squamous cells...." (Yamagishi and Nakano, 1992). An attempt was made to
account for toxicokinetic differences between the rat and human in the derivation of
BMC10(HEC). The HEC calculation attempts to account for the morphological differences in the
species as reflected by the different ratio of normal minute volume to surface area in rats versus
humans.
49
-------
While there remain several differences between rats and humans that are not accounted
for, most of these differences suggest that rat nasal passages are likely to be affected at lower
MMA concentrations than those of humans. Most evidence suggests that the main metabolite of
MMA, methacrylic acid, is the toxic moiety of concern (Lomax et al., 1997; Bereznowski, 1995;
Morris and Frederick, 1995; ECETOC, 1995). Studies of carboxylesterase metabolic rates
suggest that humans metabolize MMA in blood (Bereznowski, 1995) and in olfactory tissue
(Mattes and Mattes, 1992; Greene, 1996) at a slower rate than rats, though at a slightly faster rate
in the liver (Greene, 1996). In addition, rats are obligate nose breathers, whereas humans can
breathe through the mouth during exertion and to avoid overpowering odors. EPA is aware of
PBPK models for MMA (developed for the Methacrylate Producers Association by Andersen et
al., 1996) and other acrylates (Morris and Frederick, 1995; Bogdanffy and Taylor, 1993) that
should eventually help to reduce uncertainly in the quantification of these differences. The use of
a PBPK model to update this assessment will be considered when EPA has completed its analysis
of these various model approaches. In the meantime, a majority of the dosimetric/toxicokinetic
evidence currently available suggests that humans would not be more sensitive than rats on this
basis and that further reduction of the BMC10(HEC) to account for interspecies
dosimetric/toxicokinetic uncertainty is not necessary.
50
-------
7. References
Andersen, ME; Barton, HA; Covington, TR. (1996) Applying a physiologically based deposition
model for methyl methacrylate in the olfactory regions of the rat and human nose to estimate
dosimetric adjustment factors. Report prepared by ICF Kaiser Engineers, Inc. for the
Methacrylate Producers Association, Inc.
Anderson, D; Longstaff, E; Ashby, J. (1979) An assessment of the carcinogenic and mutagenic
potential of methylmethacrylate. Toxicol Appl Pharmacol 48:A29.
Autian, J. (1975) Structure-toxicity relationships of acrylic monomers. EHP Environ Health
Perspect 11:141-152.
Bereznowski, Z. (1995) In vivo assessment of methyl methacrylate metabolism and toxicity. Int J
Biochem 27:1311-1316.
Bogdanffy, MS. (1990) Biotransformation enzymes in the rodent nasal mucosa: the value of a
histochemical approach. Environ Health Perspect 85:177-186.
Bogdanffy, MS; Kee, CR; Hinchman, CA; Trela BA. (1991) Metabolism of dibasic esters by rat
nasal mucosal carboxylesterase. Drug Metab Dispos 19:124-129.
Bogdanffy, MS; Randall, HW; Morgan, KT. (1987) Biochemical quantitation and histochemical
localization of carboxylesterase in the nasal passages of the Fischer-344 rat and B6C3F1 mouse.
Toxicol Appl Pharmacol 88:183-194.
Bogdanffy, MS; Taylor, ML. (1993) Kinetics of nasal carboxylesterase-mediated metabolism of
vinyl acetate. Drug Metab Dispos 21:1107-1111.
Borzelleca, JF; Larson, PS; Hennigar, GR, Jr; Huf, EG; Crawford, EM; Smith, RB, Jr. (1964)
Studies on the chronic oral toxicity of monomeric ethyl acrylate and methyl methacrylate. Toxicol
Appl Pharmacol 6:29-36.
Bratt, H; Hathway, DE. (1977) Fate of methyl methacrylate in rats. Br J Cancer 36:114-119.
Bright, DS; Clark, HG; McCollum, DE. (1972) Serum analysis and toxic effects of
methylmethacrylate. Surg Forum 23:455-457.
Cannas, M; Bigatti, P; Rossi, E; Rossi, P. (1987) In vitro research on the possibility of
chromosomal damage caused by polymethyl methacrylate in orthopaedics. A preliminary report.
Ital J Orthop Traumatol 13:387-391.
Castellino, N; Colicchio, G. (1969) Ricerche sperimentali sulla tossicita' acuta del metacrilato di
metile [Acute toxicity of methyl methacrylate]. Folia Med 52:337-347.
51
-------
Cavelier, C; Hervd-Bazin, B; Jelen, G; Foussereau, J. (1981) Irritation et allergie aux acrylates et
methacrylates: premiere partie, monoacrylates et monomethacrylates simples. Ann Dermatol
Venerol 108:549-556.
Chan, PC; Eustis, SL; Huff, JE; Haseman, JK; Ragan, H. (1988) Two-year inhalation
carcinogenesis studies of methyl methacrylate in rats and mice: inflammation and degeneration of
nasal epithelium. Toxicology 52:237-252.
Chmielewski, J; Renke, W. (1975) Clinical and experimental studies on the pathogenesis of toxic
effects of styrene. II. The effect of styrene on the respiratory system. Bull Inst Mar Trop Med
Gdynia 26:299-302.
Collins, JJ; Page, LC; Caporossi, JC; Utidjian, HM; Saipher, JN. (1989) Mortality patterns among
men exposed to methyl methacrylate. JOM, J Occup Med 31:41-46.
Corkill, JA; Lloyd, EJ; Hoyle, P; Crout, DHG; Ling, RSM; James, ML; Piper, RJ. (1976)
Toxicology of methyl methacrylate: the rate of disappearance of methyl methacrylate in human
blood in vitro. Clin Chim Acta 68:141-146.
Cromer, J; Kronoveter, K. (1976) A study of methyl methacrylate exposures and employee health.
U.S. Department of Health, Education and Welfare, National Institute for Occupational Safety
and Health; Cincinnati, OH; DHEW (NIOSH) publication no. 77-119. Available from NTIS,
Springfield, VA; PB-274789.
Crout, DHG; Lloyd, EJ; Singh, J. (1982) Metabolism of methyl methacrylate: evidence for
metabolism by the valine pathway of catabolism in rat and in man. Xenobiotica 12:821-829.
Dahl, AR; Miller, SC; Petridou-Fischer, J. (1987) Carboxylesterases in the respiratory tracts of
rabbits, rats and Syrian hamsters. Toxicol Lett 36:129-136.
Deichmann, W. (1941) Toxicity of methyl, ethyl and n-butyl methacrylate. J Ind Hyg Toxicol
23:343-351.
Delbressine, LPC; Seutter-Berlage, F; Seutter, E. (1981) Identification of urinary mercapturic
acids formed from acrylate, methacrylate and crotonate in the rat. Xenobiotica 11:241-247.
Donaghy, M; Rushworth, G; Jacobs, JM. (1991) Generalized peripheral neuropathy in a dental
technician exposed to methyl methacrylate monomer. Neurology 41:1112-1116.
Drees, JA; Tansy, MF; Smith, JM. (1979) Cardiovascular responses to chronic methyl
methacrylate inhalation in beagle dogs. Fed. Proc., Fed Am Soc Exp Biol 38:1135.
ECETOC. (1995) Joint Assessment of Commodity Chemicals No. 30: Methyl Methacrylate.
JACC Report No. 30 European Centre for Ecotoxicology and Toxicology of Chemicals
(ECETOC), Brussels, Belgium.
52
-------
Ferguson, DJ. (1977) Cellular attachment to implanted foreign bodies in relation to tumorigenesis.
Cancer Res 37:4367-4371.
Frederick, CR; Udinsky, JR; Finch, L. (1994) The regional hydrolysis of ethyl acrylate to acrylic
acid in the rat nasal cavity. Toxicol Lett 70:49-56.
Ghanayem, BI; Maronpot, RR; Matthews, HB. (1986) Association of chemically induced
forestomach cell proliferation and carcinogenesis. Cancer Lett 32:271-278.
Gift, JS. (1996) Deriving reference concentrations when adverse effects are reported in all
exposure groups. 1996 Society for Risk Analysis Meeting. New Orleans, LA, December 1996.
Greene, T. (1996) The metabolism of methyl methacrylate in the nasal tissues of rat and human.
Zeneca/Central Toxicology Laboratory Report No. CTL/R/1290, issued December 4, 1996.
Sponsor: CEFIC.
Guill, MA; Odom, RB. (1978) Hearing aid dermatitis. ArchDermatol 114:1050-1051.
Hachiya, N; Taketani, A; Takizawa, Y. (1982) [Mutagenicity of environmental substances].
Nippon Koshu Eisei Zasshi 29:236-239.
Harkonen, H. (1978) Styrene, its experimental and clinical toxicology. A review. Scand J Work
Environ Health (Suppl 2) 4:194-214.
Hawley, GG. (1981) Methyl methacrylate. In: The condensed chemical dictionary. 10th ed. New
York: Van Nostrand Reinhold Company, p. 684.
Hazleton Laboratories America, Inc. (1979a) 18-month vapor inhalation safety evaluation study in
hamsters: methyl methacrylate vapor, final report. Vienna, VA: Hazleton Laboratories America,
Inc.; project no. 417-354.
Hazleton Laboratories America, Inc. (1979b) A two-year vapor inhalation safety evaluation study
in rats: methyl methacrylate, final report. Vienna, VA: Hazleton Laboratories America, Inc.;
project no. 417-354.
Holyk, PR; Eifrig, DE. (1979) Effects of monomeric methylmethacrylate on ocular tissues. Am J
Ophthalmol 88:385-395.
Homsy, CA; Tullos, HS; Anderson, MS; Diferrante, NM; King, JW. (1972) Some physiological
aspects of prosthesis stabilization with acrylic polymer. Clin Orthop RelatRes 83:317-328.
Husain, R; Srivastava, SP; Seth, PK. (1985) Methyl methacrylate induced behavioural and
neurochemical changes in rats. Arch Toxicol 58:33-36.
Husain, R; Khan, S; Husain, I; Seth, PK; Pandya KP. (1989) Effect of methyl methacrylate on
selected lipids in rat brain and sciatic nerve. Ind Health 27(3): 121-124.
53
-------
ICF Kaiser, Inc., (1990a) THC: A computer program to compute a reference dose from
continuous animal toxicity data using the benchmark dose method. KS Crump Division, Ruston,
LA.
ICF Kaiser, Inc., (1990b) THWC: A computer program to compute a reference dose from
continuous animal toxicity data using the benchmark dose method. KS Crump Division, Ruston,
LA.
ICI. (1976a) Methylmethacrylate monomer: dominant lethal study in the mouse. Anderson, D;
Hodge, MCE, eds. Report CTL/P/295. ICI, Macclesfield, Cheshire.
ICI. (1977) Methylmethacrylate monomer: teratogenicity studies in the rat. Hodge, MCE; Palmer,
S, eds. Report CTL/P/316. ICI, Macclesfield, Cheshire.
ICI. (1979) Methylmethacrylate monomer: a second cytogenic study in the rat. Anderson, D;
Richardson, CR; Weight, TM, eds. Report CTL/P/449. ICI, Macclesfield, Cheshire.
Innes, DL; Tansy, MF. (1981) Central nervous system effects of methyl methacrylate vapor.
Neurotoxicology 2:515-522.
International Agency for Research on Cancer (IARC). (1994) Methyl methacrylate. In: IARC
monographs on the evaluation of the carcinogenic risk of chemicals to humans, vol. 60; some
monomers, plastics and synthetic elastomers, and acrolein. Lyon, France: World Health
Organization; pp. 445-474.
Jedrychowski, W. (1982) Styrene and methyl methacrylate in the industrial environment as a risk
factor of chronic obstructive lung disease. Int Arch Occup Environ Health 51:151-157.
Keenan, CM; Kelly, DP; Bogdanffy, MS. (1990) Degeneration and recovery of rat olfactory
epithelium following inhalation of dibasic esters. Fundam Appl Toxicol 15:381-393.
Klimisch, HJ. (1984) Carcinogenicity of acrylates: long-term inhalation studies on methyl acrylate
(MA) and n-butyl acrylate (BA) in rats. Toxicologist 4:53.
Korhonen, A; Hemminki, K; Vainio, H. (1983) Embryotoxic effects of acrolein, methacrylates,
guanidines and resorcinol on three day chicken embryos. Acta Pharmacol Toxicol 52:95-99.
Lang, Y; Tsai, T; Wang, W; Shie, Y; Yang, S. (1986) Observations on the effects of exposure to
methyl methacrylate on workers' health. J Chin Prevent Med 20(6):344-347.
Laskin, DM; Robinson, IB; Weinmann, JP. (1954) Experimental production of sarcomas by
methyl methacrylate implants. Proc Soc Exp Biol Med 87:329-332.
Lawrence WH; Autian, J. (1972) Possible toxic effects from inhalation of dental ingredients by
alteration of drug biologic half-life. J Dent Res 51(3):878.
54
-------
Linder, L. (1976) Tissue reaction to methyl methacrylate monomer: a comparative study in the
rabbit's ear on the toxicity of methyl methacrylate monomer of varying composition. Acta Orthop
Scand 47:3-10.
Lomax, LG. (1992) Histopathologic evaluation of the nasal cavities from Fisher 344 rats exposed
to methyl methacrylate vapor for two years. Rohm and Haas Company, Spring House, PA.
Lomax, LG; Brown, DW; Frederick, CB. (1994) Regional histopathology of the mouse nasal
cavity following two weeks of exposure to acrylic acid for either 6 or 22 hours per day. Rohm
and Haas Company, Spring House, PA.
Lomax, LG; Krivanek, ND; Frame, SR. (1997) Chronic inhalation toxicity and oncogenicity of
methyl methacrylate in rats and hamsters. Food Chem Toxicol 35:393-407.
Lorimer, WV; Lilis, R; Nicholson, WJ; Anderson, H; Fischbein A; Daum, S; Rom, W; Rice, C;
Selikoff, TJ. (1976) Clinical studies of styrene workers: intial findings. Environ Health Perspect
17:171-181.
Lozewicz, S; Davison, AG; Hopkirk, A; Burge, PS; Boldy, DAR; Riordan, JF; McGivern, DV;
Platts, BW; Davies, D; Newman Taylor, AJ. (1985) Occupational asthma due to methyl
methacrylate and cyanoacrylates. Thorax 40(ll):836-839.
Luo, SQ; Gang, SQ; Sun, SS. (1986) Study on embryotoxicity and fetotoxicity in rats by maternal
inhalation of low level methyl methacrylate. Toxicol Lett 31:80.
Mallory, TH; Stone, WA; St. Pierre, RL. (1973) Potential hepatotoxic effects of
methylmethacrylate monomer. Clin Orthop Relat Res 93:366-368.
Marez, T; Shirali, P; Hildebrand, HF; Haguenoer, JM. (1991) Increased frequency of sister
chromatid exchange in workers exposed to high doses of methylmethacrylate. Mutagenesis
6:127-129.
Marez, T; Shiral, P; Haguenoer, JM. (1992) Continuous ambulatory electrocardiography among
workers exposed to methyl methacrylate. Int Arch Occup Environ Health 64:373-375.
Marez, T; Edme, JL; Boulenguez, C; Shirali, P; Haguenoer, JM. (1993) Bronchial symptoms and
respiratory function in workers exposed to methyl methacrylate. Brit J Ind Med 50:894-897.
Mattes, PM; Mattes, WB. (1992) Alpha-naphthyl butyrate carboxylesterase in human and rat
nasal tissue. Toxicol Appl Pharmacol 114:71-76.
Mattia, MA. (1983) Hazards in the hospital environment: anesthesia gases and
methylmethacrylate. Am JNurs 83:73-77.
Mauderly, JL. (1986) Respiration of F344 rats in nose-only inhalation exposure tubes. J Appl
Toxicol 6(1):25-30.
55
-------
McLaughlin, RE; Reger, SI; Barkalow, JA; Allen, MS; Difazio, CA. (1978) Methylmethacrylate:
a study of teratogenicity and fetal toxicity of the vapor in the mouse. J Bone Jt Surg Am 60-A:
355-358.
McLaughlin, RE; Barkalow, JA; Allen, MS. (1979) Pulmonary toxicity of methyl methacrylate
vapors: an environmental study. Arch Environ Health 34:336-338.
MEDLARS II, Medical Literature Analysis and Retrieval System [database]. (1986) [Printout of
the CHEMLINE record on methyl methacrylate as of February]. National Library of Medicine,
MEDLARS Management Section, Bethesda, MD. Disc; Available for inspection at: U.S.
Environmental Criteria and Assessment Office, Research Triangle Park, NC.
Miller, RR; Ayres, JA; Jersey, GC; McKenna, MJ. (1981) Inhalation toxicity of acrylic acid.
Fundam Appl Toxicol 1:271-277.
Miller, RR; Hermann, EA; Young, JT; Calhoun, LL; Kastl, PE. (1984) Propylene glycol
monomethyl ether actetate (PGMEA) metabolism, disposition, and short-term vapor inhalation
toxicity studies. Toxicol Appl Pharmacol 75:521-530.
Miller, RR; Young, JT; Kociba, RJ; Keyes, DG; Bodner, KM; Calhoun, LL; Ayres, JA. (1985)
Chronic toxicity and oncogenicity bioassay of inhaled ethyl acrylate in Fischer 344 rats and
B6C3F1 mice. Drug Chem Toxicol 8:1-42.
Mizunuma, K; Kawai, T; Yasugi, T; Horiguchi, S; Takeda, S; Miyashita, K; Taniuchi, T; Moon,
C-S; Ikeda, M. (1993) Biological monitoring and possible health effects in workers occupationally
exposed to methyl methacrylate. Int Arch Occup Environ Health 65:227-232.
Money, C; Moss, S; Fortin, CM. (1987) An assessment of the health status of dental technicians
exposed to methyl methacrylate. Br Health Safe Soc Newsletter 15:11-15.
Monroe, CB. (1984) Interim communication on the results of a mortality study of Bristol plant
employees hired prior to 1946. Rohm and Haas Co., Philadelphia, PA, FYI-OTS-0384-0300.
Monroe, CB; Macherione, D; Defonso, L; Weiss, W. (1981) Respiratory health of workers in a
chemical manufacturing plant. 77th Annual Meeting of the American Lung Association and the
76th Annual Meeting of the American Thoracic Society, Detroit, MI, May 9-13, 1981. Am Rev
RespirDis 123(4, part 2): 145.
Morris, JB; Frederick, CB. (1995) Upper respiratory tract uptake of acrylate ester and acid
vapors. Inhal Toxicol 7:557-574.
Motoc, F; Constantinescu, S; Filipescu, G; Dobre, M; Bichir, E; Pambuccian, G. (1971) Noxious
effects of certain substances used in the plastics industry (acetone cyanohydrin, methyl
methacrylate, azobis-isobutylronitrile and anthracene oil). Relation between the aggressor agent
and its effects. Arch Mai Prof Med Trav Secur Soc 32:653-658.
56
-------
Muttray, A; Schmitt, B; Klimek, L. (1997) Effects of methyl methacrylate on the sense of smell,
Cent Euro J Occup Environ Med 3(l):58-66.
National Toxicology Program. (1983) Carcinogensis biosassay of ethyl acrylate in F344 rats and
B6C3FJ mice. Research Triangle Park, NC: U.S. Department of Health and Human Services,
National Institutes of Health; report nos. NTP-TR-259.
National Toxicology Program. (1986) Toxicology and carcinogenesis studies of methyl
methacrylate (CAS no. 80-62-6) in F344/N rats and B6C3FJ mice (inhalation studies). Research
Triangle Park, NC: U. S. Department of Health and Human Services, National Institutes of
Health; report nos. NTP-TR-314 and Nffl/PUB-87-2570. Available from: NTIS, Springfield, VA;
PB87-146742/XAB.
Nemec, JW; Kirch, LS. (1978) Methacrylic acid and derivatives. In: Kirk-Othmer encyclopedia of
chemical technology: v.15, matches to n-nitrosamines. 3rd ed. New York: John Wiley & Sons,
pp. 346-376.
Nicholas, CA; Lawrence, WH; Autian, J. (1979) Embryotoxicity and fetotoxicity from maternal
inhalation of methyl methacrylate monomer in rats. Toxicol Appl Pharmacol 50:451-458.
Oberly R; Tansy, MF. (1985) LC50 values for rats acutely exposed to vapors of acrylic and
methacrylic acid esters. J Toxicol Environ Health 16(6):811-822.
Okada, S. (1966) [A study of dental acrylic resin (methylmethacrylate) as carcinogenic agent].
Shikalgaku29:l-15.
Oppenheimer, BS; Oppenheimer, ET; Danishefsky, I; Stout, AP; Eirich, FR. (1955) Further
studies of polymers as carcinogenic agents in animals. Cancer Res 15:333-340.
Phalen, RF. (1984) Inhalation studies: foundations and techniques. Boca Raton, FL: CRC Press,
Inc., ISBN 0-8493-5469-2; p. 224, Table 7.
Pickering, CAC; Bainbridge, D; Birtwistle, Hi; Griffiths, DL. (1986) Occupational asthma due to
methyl methacrylate in an orthopedic theater sister. Br Med J 192:1362-1363.
Pickering, CAC; Niven, R; Simpson, J. (1993) A study of occupational asthma at the IOI acrylics
site at Darwen, Lancashire. ICI Acrylics, Darwen, Lancashire.
Pinto, PJ. (1997) Methyl methacrylate: 28-da subchronic inhalation study in rats. Zeneca/Central
Toxicology Laboratory Report No. CTL/P/5159, issued June 4, 1997. Sponsor: CEFIC.
Poss, R; Thilly, WG; Kaden, DA. (1979) Methylmethacrylate is a mutagen for Salmonella
tvphimurium. JBone Jt Surg Am Vol. 61-A: 1203-1207.
57
-------
Raje, RR; Ahmad, S; Weisbroth, SH. (1985) Methylmethacrylate: tissue distribution and
pulmonary damage in rats following acute inhalation. Res Commun Chem Pathol Pharmacol
50:151-154.
Sandmeyer, EE; Kirwin, CJ, Jr. (1981) Esters. In: Clayton, GD; Clayton, FE, eds. Patty's
industrial hygiene and toxicology: vol. 2A, toxicology. 3rd rev. ed. New York: John Wiley &
Sons, pp. 2259-2412.
Savonius, B; Keskinen, H; Tuppurainen, M; Kanerva, L. (1993) Occupational respiratory disease
caused by acrylates. Clin Exp Allergy 23:416-424.
Schwartz BS; Doty, RL; Monroe, C; Frye, R; Barker, S. (1989) Olfactory function in chemical
workers exposed to acrylate and methacrylate vapors. Am J Publ Health 79(5):613-618.
Scolnick, B; Collins, J. (1986) Systemic reaction to methylmethacrylate in an operating room
nurse. J Occup Med 28(3): 196-198.
Seiji, K; Inoue, O; Kawai, T; Mizunuma, K; Yasugi, T; Moon, C-S; Takeda, S; Ikeda, M. (1994)
Absence of mutagenicity in peripheral lymphocytes of workers occupationally exposed to methyl
methacrylate. Ind Health 32:97-105.
Singh, AR; Lawrence, WH; Autian, J. (1972) Embryonic-fetal toxicity and teratogenic effects of a
group of methacrylate esters in rats. J Dent Res 51:1632-1638.
Smirnova, ES; Blagodatin, VM. (1977) Effect of small concentrations of methyl methacrylate on
the reproductive organs of white rats. Gig Tr Prof Zabol 2:49-51. (As reported in U.S. EPA,
1985).
Smith, JM. (1980) Letter on review of toxicology of methyl methacrylate. Rohm and Haas Co.,
Philadelphia, PA, FYI-AX-0380-0063.
Solomon, HM; Hagan, JV; Swenson, RE; Wanner, FJ. (1991) Methyl methacrylate: inhalation
developmental toxicity study in rats. Rohm and Haas Company, Spring House, PA, Report No.
90R-056A.
Solomon, HM; McLaughlin, JE; Swenson, RE; Hagan, JV; Wanner, FJ; O'Hara, GP; Krivanek,
ND. (1993) Methyl methacrylate: inhalation developmental toxicity study in rats. Teratology
48:115-125.
Spealman, CR; Main, RJ; Haag, HB; Larson, PS. (1945) Monomeric methyl methacrylate: studies
on toxicity. Ind Med 14:292-298.
Stott, WT; McKenna, MJ. (1985) Hydrolysis of several glycol ether acetates and acrylate esters
by nasal mucosal carboxylesterase in vitro. Fundam Appl Toxicol 5:399-404.
58
-------
Tanii, H; Hashimoto, K. (1982) Structure-toxicity relationship of acrylates and methacrylates.
Toxicol Lett 11:125-129.
Tansy, MF. (1979a) Toxic mechanisms of inhaled methyl methacrylate vapor. NIOSH/00112356.
Tansy, MF. (1979b) Final report of teratology studies of mice exposed to methyl methacrylate
vapor. Rohm and Haas Company, Spring House, PA, Report No. 78RC-1021.
Tansy, MF; Hohenleitner, FJ; Landin, WE; Kendall, FM. (1980a) Chronic biological effects of
methyl methacrylate vapor: II. Body and tissue weights, blood chemistries, and gross metabolic
performance in the rat. Environ Res 21:108-116.
Tansy, MF; Hohenleitner, FJ; White, DK; Oberly, R; Landin, WE; Kendall, FM. (1980b) Chronic
biological effects of methyl methacrylate vapor: III. Histopathology, blood chemistries, and
hepatic and ciliary function in the rat. Environ Res 21:117-125.
Tansy, MF; Kendall, FM; Benhayem, S; Hohenleitner, FJ; Landin, WE; Gold, M. (1976) Chronic
biological effects of methyl methacrylate vapor: I. Body and tissue weights, blood chemistries,
and intestinal transit in the rat. Environ Res 11:66-77'.
Tucker, JD; Auletta, A; Cimino, MC; Dearfield, KL; Jacobson-Kram, D; Tice, RS; Carrano, AV.
(1993) Sister-chromatid exchange: second report of the Gene-Tox program. MutatRes 297:101-
180.
U.S. EPA. (1985) Health and environmental effects profile for methyl methacrylate. Office of
Health and Environmental Assessment, Environmental Criteria and Assessment Office; Cincinnati,
OH, report no. EPA/600/X-85/364. Available from: NTIS, Springfield, VA; PB88-178785/XAB.
U.S. EPA. (1987a) Risk assessment guidelines of 1986 (EPA/600/8-87/045, dated August 1987).
U.S. EPA. (1988a) Recommendations for and documentation of biological values for use in risk
assessment. EPA 600/6-87/008, NTIS PB88-179874/AS, February 1988.
U.S. EPA. (1988b) Health and environmental effects profile for methyl methacrylate.
NTIS/PB88-178785.
U.S. EPA. (1991b) Guidelines for developmental toxicity risk assessment, dated December 5,
1991. Federal Register 56 (234):63798-63826.
U.S. EPA. (1991c) Summary review of health effects associated with methyl methacrylate: health
issue assessment. Environmental Criteria and Assessment Office, Research Triangle Park, NC,
ECAO-R-092A.
U.S. EPA. (1994a) Peer review and peer involvement at the U.S. Environmental Protection
Agency, signed by U.S. EPA Administrator, Carol M. Browner, dated June 7, 1994.
59
-------
U.S. EPA. (1994b) Interim policy for particle size and limit concentration issues in inhalation
toxicity: notice of availability, dated October 26, 1994. Fed. Reg. 59, No. (206): 53799.
U.S. EPA. (1994c) Methods for derivation of inhalation reference concentrations and application
of inhalation dosimetry, EPA/600/8-90/066F, dated October 1994.
U.S. EPA. (1995a) Guidance on risk characterization, memorandum of the Administrator, Carol
Browner, dated March 21, 1995.
U.S. EPA. (1995b) (proposed) Guidelines for neurotoxicity risk assessment, dated October 4,
1995. Federal Register 60(192):52032-52056.
U.S. EPA. (1995c) Use of the benchmark dose approach in health risk assessment, EPA/630/R-
94/007, dated February 1995.
U.S. Environmental Protection Agency. (1996a, April 23) Proposed guidelines for carcinogen risk
assessment. Federal Register 61 (79):17960-18011.
U.S. EPA. (1996b) Guidelines for reproductive toxicity risk assessment, dated October 31, 1996.
Federal Register 61(212):56274-56322.
U.S. EPA. (1997) Integrated risk information system (IRIS) Online. NCEA, Cincinnati, OH.
University of Pennsylvania. Olfactory function in chemical workers exposed to acrylate and
methacrylate vapors with attachments, cover sheets, and letters dated 031488 and 081089
(sanitized). TSCATS/404117; EPA/OTS; Doc no. 86-890001519S.
Verkkala, E; Rajaniemi, R; Savolainen, H. (1983) Local neurotoxicity of methylmethacrylate
monomer. Toxicol Lett 18:111-114.
Waegemaekers, THJM; Bensink, MPM. (1984) Non-mutagenicity of 27 aliphatic acrylate esters
in the Salmonella-microsome test. MutatRes 137:95-102.
Walker, AM; Cohen, AJ; Loughlin, JE; Rothman, KJ; DeFonso, LR. (1991) Mortality from
cancer of the colon or rectum among workers exposed to ethylacrylate and methyl methacrylate.
Scand J Work Environ Health 17:7-19.
Weast, RC, ed. (1988) In: CRC handbook of chemistry and physics. 68th ed. Boca Raton, FL:
CRC Press, Inc.
Weiss, G, ed. (1980) Methyl methacrylate. In: Hazardous chemicals data book. Park Ridge, NJ:
Noyes Data Corporation; p. 619.
Wenzel, H; Garbe, A; Nowak, H. (1973) Untersuchungen zur pharmakokinetik von
monomethylmethacrylat. 1st Int. Kongr. Prothesentecknok funkt. Rehabil Wien (Cited in
Borchard, 1982).
60
-------
Windholz, M; Budavari, S; Blumetti, RF; Otterbein, ES, eds. (1983) Methyl ester, methyl
methacrylate. In: The Merck index: an encyclopedia of chemicals, drugs, and biologicals. 10th ed.
Rahway, NJ: Merck & Co., Inc.; p. 850.
Wines, RD. (1973) Possible hazard of polymethyl methacrylate. Br Med J 3:409.
Wong, BA; Dorman, DC; Asgharian, B. (1997) Developing specialized inhalation exposure
systems to address toxicological problems. CUT Activ 17(3): 1-8.
Woo, Y; Lai, DY; Arcos, JC; Argus, MF. (1988) In: Chemical induction of cancer: structure
bases and biological mechanisms. Vol. IIIC; Natural, metal, fiber, and macromolecular
carcinogens. Academic Press, Inc.: Boca Raton, FL, Appendix I, p. 617-621.
Yamagishi, M; Nakano, Y. (1992) A re-evaluation of the classification of olfactory epithelia in
patients with olfactory disorders. Eur Arch Otorchinolaryngol 249:393-399.
Youngentob, SL. (1997) Olfactory function following induced lesions of the olfactory epithelium
in the rat: a model for study of dysosmia in humans.
61
-------
8. APPENDICES
APPENDIX A: RFC BENCHMARK CONCENTRATION ANALYSES OF DATA FROM
LOMAX (1995)
Degeneration/atrophy of olfactory epithelium in male rats
(1) Computational Models-Discontinuous (Quantal) Data
The polynomial mean response regression model (THRESH, I.C.F. Kaiser, 1990a) and the
Weibull power mean response regression model (THRESHW, I.C.F. Kaiser, 1990b) were used to
fit data by the maximum likelihood method. The following are the forms of the two equations
used, excluding a background term (background = 0).
THRESH P(d) = l-exp[-qi (d-d0)r ... -qk (d-d0)k]
THRESHW P(d) = 1 - exp[-a(d-d0)p]
where:
d = dose
d0 = threshold
P(d) = probability of a response (health effect) at dose d
q^.^qk, a, p, k = estimated parameters
For data input to THRESH, the degree of the polynomial was set to the number of dose
groups minus one, the response type was extra [P(d) - P(0)] /I- P(0). For both models the
threshold, d(0), was set to zero. For THRESHW, the lower limit of p was set at 1.0.
(2) Data Set
Group Dose #Responses/#animals
1 0 0/39
2 25 0/47
3 100 35/48
4 400 38/38
62
-------
(3) Model Fit
Model fit was judged by thep values generated with the x2 goodness-of-fit generated by
THRESH or THRESHW.
(4) Results
Table A-l. THRESHW model results
Model
THRESHW
THRESH
BMC(IO)
(mg/m3)
39.4
35.1
Estimated
parameters
a=4.38E-10
p=4.7362
Qi=0; q2 =0
q3 =1.27E-06
p Value
0.768039
0.616323
x2 goodness-of-fit
8.698995E-02
0.967969
Degrees of
freedom
1
2
(5) Discussion
It is important to note that thep values and "goodness-of-fit" data given in the table above
should not be interpreted as an indication of confidence in the results of this model run. They
simply indicate that data were amenable to curve fitting, because there is only one data point in
the set that is not at the limits of the scale. The incidence in control and low-dose groups was 0,
and the incidence in the high-dose group was 100%. The incidence in the intermediate group was
also high at 70%. Thus, confidence in the overall curve fitting exercise is actually quite low, given
that there is no unique solution to fitting these points (a steeper or flatter curve could also be fit to
these points). However, it is clear that the 10% response level must lie between 25 and 100 ppm.
Given the nature of this effect and the gradual, not abrupt, increase in the severity of related
effects such as basal cell hyperplasia and olfactory cell inflammation, it is reasonable to assume
that a gradual increase in response begins at 25 ppm. This is also a conservative assumption
because 25 ppm is a clear NOAEL and is not necessarily the point at which a response is initiated.
The models reflect this gradual increase above 25 ppm, and the lower of the two model estimates
for the BMC10, 35 ppm, appears to be a reasonable and conservative estimate of the concentration
required to elicit a 10% response. This value is consistent with the NOAEL from the study and
other BMD10 values, and is chosen for further quantitation of the RfC.
63
-------
APPENDIX B: SUMMARY OF AND RESPONSE TO EXTERNAL PEER REVIEW
COMMENTS
The Toxicological Review for methyl methacrylate and all individual methyl methacrylate
assessments have undergone both internal peer review performed by scientists within EPA or
other Federal agencies and a more formal external peer review performed by scientists chosen by
EPA in accordance with U.S. EPA (1994a). Comments made by the internal reviewers were
addressed prior to submitting the documents for external peer review and are not part of this
appendix. Public comments also were read and carefully considered. The external peer reviewers
were tasked with providing written answers to general questions on the overall assessment and on
chemical-specific questions in areas of scientific controversy or uncertainty. A summary of
comments made by the external reviewers and EPA's response to these comments follows.
(1) General Comments
A. Comments: The reviewers suggested that EPA consider the following additional studies
in the review:
1. ECETOC. 1995. Joint assessment of commodity chemicals no. 30: methyl
methacrylate. JACC Report No. 30. European Centre for Ecotoxicology and
Toxicology of Chemicals (ECETOC), Brussels, Belgium.
2. Lomax, LG; Krivanek, ND; Frame, SR. 1997. Chronic inhalation toxicity and
oncogenicity of methyl methacrylate in rats and hamsters. Food Chem Toxicol 35:393-
407.
3. Muttray, A; Schmitt, B; Klimek, L. 1997. Effects of methyl methacrylate on the sense
of smell. Cent Euro J Occup Environ Med, 3(l):58-66
4. Ghanayem, BI; Maronpot,RR; Mathews, HB. 1986. Association of chemically
induced forestomach cell proliferation and carcinogensis. Cancer Lett 32:271-278.
5. Greene, T. 1997 (draft). Methyl methacrylate: the effect of carboxylesterase enzyme
inhibition on the development of a nasal lesion in rats. Zeneca: Central Toxicology
Laboratory Report No. CTL/R/1313, June 6, 1997. Sponsor: CEFIC.
6. Greene, T. 1996. The metabolism of methyl methacrylate in the nasal tissues of rat and
human. Zeneca: Central Toxicology Laboratory Report No. CTL/R/1290, issued
December 4, 1996. Sponsor: CEFIC.
7. Pinto, PJ. 1997. Methyl methacrylate: 28-da subchronic inhalation study in rats.
Zeneca: Central Toxicology Laboratory Report No. CTL/P/5159, issued June 4,
1997. Sponsor: CEFIC.
8. Frederick, CB, Lomax, LG;. Black, KA; Finch, L;. Bush, ML; Ultman, JS; Kimbell,
JS; Morgan, KT; Subramanian, RP; Morris, JB; Stott, WT; Young JT; Scherer, PW.
1997 (draft). Application of computational fluid dynamics and a physiologically-based
inhalation model for interspecies extrapolation of the dosimetry of acidic vapors in the
upper respiratory tract.
9. Bush, ML; Frederick, CB; Kimbell, JS; Ultman, JS. 1997 (draft). A computational
fluid mechanics-physiologically based pharmacokinetic hybrid model for simulating gas
and vapor uptake in the rat nose.
64
-------
10. Andersen, ME;. Barton, HA; Covington. TR. 1996. Applying a physiologically based
deposition model for methyl methacrylate in the olfactory regions of the rat and human
nose to estimate dosimetric adjustment factors. Prepared for the Methacrylate
Producers Association, September 27, 1996.
11. DeSesso, JM. 1993. The relevance to humans of animal models for inhalation studies
of cancer in the nose and upper airways. Quality Assurance: Good Practice Regul
Law2(3):213-231.
12. Mauderly, JL. 1986. Respiration of F344 rats in nose-only inhalation exposure tubes. J
Appl Toxicol 6(1):25-30.
13. Phalen, RF. 1984. Inhalation studies: foundations and techniques (Table 7). Boca
Raton, FL: CRC Press, Inc., ISBN 0-8493-5469-2, p. 224.
14. Pickering, CAC; Niven, R; Simpson, J. 1993. A study of the prevalence of
occupational asthma at the ICI acrylics site at Darwen, Lancashire. ICI Acrylics,
Darwen, Lancashire U.K. (available through ICI or TSCA 8[d]).
15. Two reviewers submitted additional references on other routes of exposure,
sensitization (primarily dermal exposure studies), epidemiology, genetic toxicology,
and metabolism.
Response: Citations that are either finalized industry reports (#6, #7, #13, #14), or
published articles in the peer-reviewed literature (#2, #3, #4, #10, #11, #12) were
considered for inclusion in the review. Of the 10 studies recommended that fall into either
of these categories, all but two were summarized in appropriate sections, considered in the
derivation of MMA benchmarks and weight-of-evidence, and added to the reference
section of the MMA document. The two that were not incorporated into the MMA
document at this time were the proposed PBPK model approach by Andersen et al. (1996;
#10) and the paper by Desesso et al. (1993; #11). The proposed PBPK model approach
of Andersen et al. (1996) is scheduled for review by an EPA-sponsored "Category 1"
dosimetry workgroup in early 1998. Though the model will not be formally cited and
used before the conclusions of this work group are known, preliminary analysis indicates
that the model would result in a regional gas deposition ratio (RGDR), referred to as a
dosimetric adjustment factor (DAF) in the Andersen et al. (1996) report, that is
approximately twofold higher than that estimated in this MMA Toxicological Review
document using the Agency's default dosimetric adjustment methods (0.64-0.67 vs. 0.28).
It is believed that EPA's default approach provides a reasonable margin of safety in the
interim. The paper by Desesso et al. (1993) was cited in support of a reviewer's position
that the Agency should use a higher minute volume for the rat in doing the HEC
calculation. This paper was a review and was not as relevant as other studies submitted by
the reviewer, such as Mauderly (1986) and Phalen (1984). Draft industry reports (#5, #8,
#9) will be considered, but cannot be included in the review and reference list until
finalized. Secondary references are not necessarily cited, but because it is recent and
complete, the ECETOC document (#1) has been cited as an alternative source of
information. For the most part, the list of less directly related references submitted by two
reviewers (#15) did not contain additional studies deemed necessary for the purposes of
the MMA IRIS document. In order to clarify the scope of the MMA document, however,
a paragraph similar to the following was inserted at the beginning of Sections 4.2, 4.3 and
4.4.
65
-------
"This section is a review of laboratory animal studies relevant to the derivation of
health benchmarks for MMA. An overall synthesis of this information and its
relation to the potential for MMA to cause noncancer and cancer effects is
presented in Sections 4.5 and 4.6, respectively. Certain studies that were
considered to be inadequately documented for the purposes of this assessment or
used irrelevant dosing regimens may not have been discussed in this section. A
more complete listing/discussion of all types of repeat or single-exposure studies
can be found in other, more detailed reviews (ECETOC, 1995; U.S. EPA, 1991)."
(2) Study Descriptions
A. Comment: One reviewer suggested that the document needs a NOAEL/LOAEL
summary table.
Response: Summary tables already exist for key noncancer and reproductive/
developmental studies (Tables 4-5 and 4-6). Columns have been added for recording
NOAELs and LOAELs from these studies.
B. Comment: One reviewer argued that too much emphasis is given in the allergies
section of the document (Section 4.4.2) to Chung and Giles (1977) as an indicator of
MMA sensitization potential, and that MMA is not a "very strong" sensitizer as
suggested in this section.
Response: The reviewer correctly points out that there are other studies from which
to draw to postulate a conclusion regarding the sensitization potential of MMA.
However, none of the available sensitization studies tested the oral or inhalation routes
of exposure. The dermal studies that are available are only qualitatively useful for the
purposes of the IRIS document. A few other studies, but not all, have added to the
Section 4.4.2 discussion, and the descriptive words "very strong" have been removed.
The overall qualitative conclusion that MMA is a sensitizer has not changed.
C. Comment: One reviewer suggested that too much emphasis is placed on Linder
(1976) and Holyk and Eifrig (1979) in the Dermal and Ocular Effects section (Section
4.4.3).
Response: Again, the entire large database of dermal and ocular acrylate studies does
not need to be covered to serve the purpose of this section, as none of these short-
term dermal or ocular studies are going to be useful in the establishment of an RfC,
RfD, or cancer assessment. A few current studies have been added to this section to
characterize the irritant nature of MMA. The reader is also referred to other, more
detailed reviews in this area, such as the 1995 European Center for Ecotoxicology and
Toxicology of Chemicals report (ECETOC, 1995)
D. Comment: One reviewer requested that the Marez (1991) publication should be
qualified and given less credibility.
66
-------
Response: The document was edited to reflect the fact that the reduced average
MEFV50 observed in workers by Marez et al. (1991) may have been due to acute
airway irritation caused by peak MMA exposures.
E. Comment: One reviewer claimed that the Schwarz (1989) study is more equivocal
than the MMA Toxicological Review document suggests.
Response: The document was edited to reflect the fact that Schwarz (1989) did not
report actual exposure concentrations. Further, it is stated that, "The reason effects
were reported in this study and not the Muttray et al. (1997) study could be due to a
higher co-exposure to other acrylates, the use of a more sensitive diagnosis method, or
a combination of both explanations."
(3) RfD/RfC Calculation
A. Comment: One reviewer suggested the use of a different weight for male rats to
calculate the daily dose from the Borzelleca et al. (1964) study.
Response: The reviewer is correct in asserting that the 0.295 kg weight used is low.
This weight was used in the risk assessment performed in the 1990 Health and
Environmental Effects Document (FIEED) for MMA, and was reported on page 9-10
of that document as being the "estimated weight at midpoint of study; based on
author's data." According to EPA's 1988 "Recommendations for and Documentation
of Biological Values for Use in Risk Assessment," the weight used for a chronic study
should be the "time weighted average (TWA) body weights ... from weaning to 730
days post weaning (chronic)." The EPA default body weight for male Wistar rats is
given in EPA (1988) as 0.462 kg for a chronic study. Data from the Borzelleca study
confirm that this default value is close to the TWA body weight for male rats over the
course of the study (approximated to be 0.506 kg). As requested by the reviewer, and
to be consistent with prior assessments involving this strain, the default value of 0.462
kg is used in lieu of 0.295 kg.
B. Comment: One reviewer commented that the rat minute volume for use in the FIEC
derivation is incorrect and should be recalculated. The default body weight for F344
rats of 0.38 kg should be used instead of the terminal body weight of 0.41 kg.
Response: The reviewer is correct in his assertion that the 0.13 L/min value used for
male rat minute volume in the RfC derivation is low relative to the data reported in the
literature references he provided for the F344 rat (-0.24 L/min). The value of 0.13
L/min was calculated using Equation 4-4 and coefficients in Table 4-6 of the EPA RfC
methods document (EPA, 1994). The 0.13 L/min value is low because the common
logarithm was used, as suggested by the Equation 4-4 text "log," instead of the natural
logarithm (LN), as stipulated in the text that follows the equation. The reviewer is
also correct in suggesting that the use of the default body weight of 0.38 kg would be
more consistent with past EPA practice. When the LN and the default body weight of
67
-------
0.38 kg are used in the equation, the minute volume is calculated to be 0.25 L/min.
This minute volume is in good agreement with the literature cited by the reviewer.
C. Comment: Two reviewers suggested that EPA should not duration-adjust the
exposure concentration used to derive the RfC because C x T, the assumption that
effect is proportional to concentration times duration exposed (time), does not hold
true for the effect of concern.
Response: C x T was assumed and the duration adjustment was applied to derive the
RfC primarily on the basis of a study that showed C x T to be operative for acrylic
acid for short exposure durations (Lomax, 1994). In this study, three dose groups
having similar C x T products (5 ppm x 22 h; 25 ppm x 4.4 h; 25 ppm x 6 h) all had
very similar incidence and severity of lesions in the nasal cavity of rats following 14
days of exposure. One of the reviewers correctly points out that there are differences
between substituted acrylates such as MMA and unsubstituted acrylates in the way
they are metabolized. Changes have been made to the document to acknowledge this
point. However, the acrylic acid study by Lomax (1994) cannot be discarded on this
basis because, while it is apparently less potent than acyrlic acid, MMA does cause a
similar, if not identical, toxicologic response in rats.
The reviewer also contended that a new 28-day study (Pinto, 1997) suggests that C x
T does not apply for extrapolation from less than chronic to chronic duration
exposures. The reviewer may be confusing the duration adjustment, which is applied
to convert an intermittent exposure to an approximate continuous exposure equivalent,
with the subchronic to chronic uncertainty factor adjustment. The subchronic to
chronic uncertainty factor adjustment was not used in the MMA RfC derivation
because the MMA RfC is already based on lesions from chronic exposure.
Nevertheless, the reviewer's contention that 100 ppm exposure to MMA in the Pinto
(1997) 28-day study resulted in lesions in F344 rats of the same severity as 100 ppm
exposures in the chronic study (Lomax et al., 1997) is not accurate. Pinto did observe
degeneration/necrosis at the 110 ppm exposure level following 1 and 2 days of
exposure. However, "regenerative changes" were noted following 5 and 10 days of
exposure, and no lesions were noted after 28 days of exposure to 100 ppm MMA.
First of all, there is a great deal of uncertainty with respect to diagnosing the extent of
olfactory damage following apparent regeneration. Other studies have shown that
some functional loss occurs in rats despite apparent complete regeneration of olfactory
epithelium (Wong et al., 1997; Youngentob, 1997). There is also a question as to
whether humans have this ability to regenerate olfactory epithelium and recover even
partial olfaction (Yamagishi et al., 1992). Second, as noted by the author of the 28-
day study (Pinto, 1997), the lack of recognizable olfactory lesions following 28 days of
exposure to 110 ppm MMA and the report of lesions following chronic exposure to
100 ppm (Lomax, 1997) suggest that degenerative olfactory epithelial changes do
develop over an extended period of time. Thus, the extent to which rats and humans
tend to recover with continuous exposure to MMA is still under investigation, and
duration of exposure cannot be ruled out as a contributing factor toward the
development/progression of the adverse olfactory effects of MMA.
68
-------
D. Comment: One reviewer questioned whether the surface area adjustment should be
done based on the entire extrathoracic region or target tissue (olfactory) area.
Response: The use of target tissue, as opposed to the entire extrathoracic region, for
dosimetric extrapolations between rats and humans would be possible if the relative
contributions of airflow patterns were well understood. This is part of what the EPA-
sponsored "Category 1" dosimetry workgroup discussed previously (and below)
should address in February of 1998. Until then, the margin of safely provided by the
default method (RGDR = 0.28) is deemed adequate given preliminary model results
(RGDR = 0.64-0.67).
E. Comment: Two reviewers urged that the PBPK model submitted by Andersen et al.
(1996) be used in the derivation of the RfC.
Response: This model was submitted on September 20, 1996, and EPA has agreed to
review it, along with other model approaches for acrylate olfactory toxicants (e.g.,
vinyl acetate and ethyl acrylate). The review process is under way and an EPA-
sponsored work group meeting has been scheduled. Although the model will not be
formally cited and used before the conclusions of this work group are known,
preliminary analysis indicates that the model would result in a RGDR, referred to as a
dosimetric adjustment factor (DAF) in the Andersen et al. (1996) report, that is
approximately twofold higher than that which is estimated in the MMA document
using the Agency's default dosimetric adjustment methods (0.64-0.67 vs. 0.28). Thus,
use of the EPA default approach provides a reasonable margin of safely in the interim.
(4) Uncertainty Factors
A. Comment: Interspecies UF for RfD should be reduced.
Response: Two reviewers suggested that the interspecies UF be lowered from 10 for
the purposes of the RfD derivation. The primary basis given for this position is that
(1) large uncertainty with respect to animal-to-human extrapolation is not warranted
for a local irritant such as MMA, and (2) metabolic/pharmacokinetic data suggest that
rats convert MMA to the presumed irritant, methacrylic acid, at a faster rate. It is
agreed that the slower blood metabolism of MMA in humans (Bereznowski, 1995),
combined with the fact that humans do not have a forestomach (target organ in the
Borzelleca et al., 1964 study) lowers the potential for a more pronounced portal-of-
entry effect in humans. Since portal-of-entry irritation forms the basis of the MMA
RfD, the interspecies UF has been lowered from 10 to 3. Complete elimination of this
UF is not justified, however, given the lack of human oral exposure information and
remaining uncertainty regarding MMA's potential to cause other effects in humans
following chronic oral exposure.
69
-------
B. Comment: Database UF for RfD should be eliminated.
Response: A threefold database UF was applied for the purposes of deriving the
MMA RfD. The major areas of uncertainty in this assessment were identified as (1)
the lack of an identified critical effect to humans, (2) the lack of a chronic study in a
second species, (3) the lack of a neurologic study, and (4) the lack of a developmental
or reproductive toxicity study via the oral route (given that developmental effects have
been seen in laboratory animals following other routes of exposure). The number of
repeat exposure inhalation studies, including developmental, reproductive, and chronic
studies, were recognized as lending support to the oral database and obviating the
need for a 10-fold database UF. Two reviewers suggested the elimination of the
database UF, and one reviewer provided a copy of the original report (dated August 8,
1992), which included raw data, including individual animal records, used in the
Borzelleca et al. (1964) study. The availability of this information increases
confidence in the Borzelleca study and reduces the possibility of missed effects in
rodents significantly. Largely because of this information, EPA is no longer
considering the application of an additional modifying factor (see discussion below).
However, database uncertainty remains because of the lack of human oral exposure
information and the lack of any oral developmental or reproductive studies. Thus, the
threefold partial database UF is retained.
C. Comment: Intraspecies UF for RfC should be reduced.
Response: One reviewer suggested that the intraspecies UF for the RfC should be
reduced below 10, suggesting that a 10-fold UF is "enormously conservative when
dealing with a point-of-contact irritant at subthreshold concentrations." New studies
provided by the reviewer lend support to this position. The intraspecies UF was
lowered to threefold and the following text was added to Section 6.2:
"A partial threefold intraspecies uncertainly factor is applied to the RfC for
protection of sensitive subpopulations. This UF is reduced by extensive human
occupational studies and case reports that consistently identify the irritant
properties of MMA as the principal effect of concern from MMA exposures.
Little intraspecies variance is observed with respect to the identified critical
effect, olfactory degeneration in laboratory animals (ECETOC, 1995; Lomax
et al., 1997), and there is no reason to expect a high degree of intrahuman
variability from this type of effect. Although Pickering et al. (1986) reported
delayed asthmatic response following challenge with MMA, which would
suggest that MMA is a possible respiratory sensitizer, no occupational studies
identified MMA as a respiratory sensitizer. A partial intraspecies uncertainty
factor of 3 is deemed sufficiently protective."
D. Comment: The interspecies UF for the RfC should be eliminated.
Response: One reviewer suggested that the interspecies UF for the RfC should be
eliminated. The reasons given were (1) "...differences between animals and humans
70
-------
are already factored into the calculation of the RGDR"; (2) "...differences in the
response of rodent vs human tissues that would justify MORE protection/uncertainty,
are unlikely"; (3) "...metabolic differences in carboxylesterase activity in the olfactory
epithelium"; and (4) "...dosimetric/anatomical/airflow/tissue deposition considerations
that make rodents' tissue 4 times more susceptible for chemical contact with nasal
tissue than humans'." The reviewers' reasons 1, 3, and 4 were taken into account and
were the primary reasons that the interspecies UF was lowered from 10 to 3.
However, these reasons all deal with the potential for toxicokinetic differences. While
the reviewer may be correct in asserting (reason 2 above) that significant
toxicodynamic differences are not likely, some studies suggest that humans may be less
capable of recovery from olfactory damage when it occurs. "Rapid potentially
anatomically correct recovery after massive destruction" is observed in rats when
underlying basal cells are not damaged (Youngentob, 1997) and small islands of intact
olfactory epithelium are "sufficient to allow for olfactory function" (Wong et al.,
1997). In humans, it has been reported that patients with relatively mild to moderate
olfactory damage fail to recover olfaction and "...even when basal cells remain intact,
differentiating cells developing from them do not mature into receptor cells but can
develop into squamous cells..." (Yamagishi and Nakano, 1992). Thus, a partial 3-fold
interspecies UF is retained.
(5) Weight-of-Evidence/Confidence Levels
A. Comment: There is not a clear "weight-of-evidence" assessment for each section.
Response: "Weight-of-evidence" and data synthesis is described in Sections 4.5 and
4.6. To some extent the data are also synthesized in earlier sections. However, in
response to this comment, a paragraph similar to the following has been inserted at the
beginning of Sections 4.2, 4.3 and 4.4.
"This section is a review of laboratory animal studies relevant to the derivation of
health benchmarks for MMA. An overall synthesis of this information and its
relation to the potential for MMA to cause noncancer and cancer effects is
presented in Sections 4.5 and 4.6, respectively. Certain studies that were
considered to be inadequately documented for the purposes of this assessment or
used irrelevant dosing regimens may not have been discussed in this section. A
more complete listing/discussion of all types of repeat or single exposure studies
can be found in other, more detailed reviews (ECETOC, 1995; U.S. EPA, 1991)."
B. Comment: Confidence in the RfD should be increased. Assignment of low
confidence to RfD is "penalization" for MMA showing low oral irritation toxicity at
concentrations limited by solubility in water and palatability in drinking water.
Response: Two reviewers suggested that confidence in the RfD database, particularly
confidence in the Borzelleca et al. (1964) study, should be increased from "low." As
discussed earlier, a copy of the original report (dated August 8, 1992), which included
details and raw data from the Borzelleca et al. (1964) study, was submitted in support
71
-------
of this contention. The availability of this information increases confidence in the
Borzelleca study and in the overall database. However, database uncertainty remains
because of the lack of human oral exposure information and the lack of any oral
developmental or reproductive studies. Overall confidence in the RfD has therefore
been changed from "low" to "low to medium."
(6) Comments on Chemical-Specific Questions
A. Question: MM A has been classified as not likely to be carcinogenic to humans under
the Proposed Guidelines for Carcinogenic Risk Assessment (U.S. EPA, 1996). Is
there sufficient evidence to support this classification? If so, has the evidence to
support this classification been clearly presented?
Comments: All reviewers essentially responded "yes" to both questions. One
suggested EPA consider the potential for MMA to be a tumor promoter. Another
suggested greater emphasis be given to an ICI dominant lethal study (ICI, 1976a) in
the mutagenicity section.
Response: The genotoxicity section of the document (Section 4.4.1) has been
expanded to include discussions of additional relevant studies.
B. Question: As described in the Appendix to this report, there are limitiations to the
benchmark analysis used to derive the RfC. Does the estimated BMC(IO) serve as an
appropriate basis for the RfC, or should the NOAEL of 25 ppm have been used?
Comments: One reviewer suggested NOAEL; all others suggested BMC(IO)
analysis.
Response: The majority of the reviewers suggested the use of the BMC(IO)
approach. The NOAEL approach would result in a slightly more conservative RfC,
and the dose-response curve for use in the BMD analysis is difficult to model because
of the high response level in the effected exposure group. However, one of the
reviewers that suggested use of the BMC(IO), a highly qualified pathologist, felt that
the effect at the 100 ppm exposure level was slight and that the true NOAEL was
likely closer to 100 ppm than to 25 ppm. Thus, an attempt at extrapolating, however
difficult, was deemed justified and appropriate.
C. Question: The oral database for MMA is limited and a database uncertainty factor of
3 was employed in the derivation of the RfD. The study used in the derivation of the
RfD was also questionable and an additional "modifying factor" (MF) has been
suggested to account for this uncertainty surrounding the quality of the principal study
itself. Ethyl acrylate (EA), a related compound, was found to severely irritate and
cause cancer in the forestomach of gavaged rats (NTP, 1983), yet no effect was
reported by Borzelleca et al. (1964) for either EA or MMA- exposed rats. This could
be due to a lower dose and different dosing regimen, but could also mean that
72
-------
Borzelleca and colleagues did not closely examine all aspects of the gastrointestinal
tract. Although short-term strucure-activity relationship studies (discussed below)
demonstrate that MMA may not cause the forestomach effects observed for EA, it is
nevertheless curious that Borzelleca and colleagues did not report any such effects
from EA chronic drinking water exposure. Is a modifying factor necessary to account
for this uncertainty?
Comments: One reviewer suggested use of the Motoc (1971) study over Borzelleca
et al. (1964). All others argued for use of the Borzelleca study. No reviewer
suggested the need for a modifying factor. One reviewer argued strongly for increased
confidence in the Borzelleca study.
Response: As discussed above, confidence in the Borzelleca study is increased with
the submission by a reviewer of the original detailed report, raw data, and individual
animal records. The Motoc (1971) study was a 32-week gavage study and is
considered to be a less appropriate and potentially irrelevant exposure regimen when
compared to the Borzelleca chronic drinking water study. One reviewer provided
strong evidence in support of the EA toxic response being highly dependent on the
exposure regimen, and that "changing the mode of oral administration to continuous
small doses in drinking water allows efficient detoxification of EA and does not
overwhelm glutathione-binding [detoxification] mechanisms." Studies submitted by
this reviewer were used to improve the argument in Section 5.1.1 for use of the
Borzelleca et al. (1964) study as the principal study.
D. Question: Olfactory tissue is considered the primary target organ with respect to
inhalation exposure. Is there reason to suspect systemic toxicity from inhalation
exposure? If so, would the calculated RfC be protective against such systemic
toxicity?
Comments: All reviewers essentially said that the literature did not present a reason
to expect systemic toxicity from MMA exposure.
Response: No change necessary.
E. Question: Is the information in the Toxicological Review sufficient to consider
methyl methacrylate as having a low potential for causing reproductive effects?
Comments: For the most part the reviewers answered "yes" to this question. One
reviewer commented that greater emphasis should be given in the Reproductive
Toxicity section to the lack of effects on reproductive organs in lifetime exposure
studies in several species and an ICI dominant lethal study, and that less emphasis
should be given to the Singh intraperitoneal study.
Response: No significant changes were deemed necessary. Minor edits were made to
Section 4.3, Reproductive and Developmental Studies, to reflect reviewer comments.
73
------- |