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Xylenes

Inhalation Health Effect Reference Values for Xylene - All Isomers
(CASRNs: Mixed Isomers - 1330-20-7; m-Xylene - 95-47-6; 0-Xylene -
108-38-3; p-X ylene - 106-42-3)

The reader is strongly encouraged to read Section 1 of the following report for critical
background information regarding the health effect reference values discussed in this summary:
Graphical Arrays of Chemical-Specific Health Effect Reference Values for Inhalation Exposures
[FinalReport] (U.S. EPA. 2009). This report is available on-line at
http://cfpub.epa.gov/ncea/cfm/recordisplav.cfm?deid=211003.

In general, inhalation health effect reference values have been included which have been
developed and formally reviewed by an authoritative governing body (government agency or
professional association) for use in assessments of risk to support regulatory decision-making.
This is a review of existing reference values, including the basis for each of the reference values
as provided in the available technical support documents for those values, along with some basic
contextual references; this is not a comprehensive review of the health effects literature for
xylene.

Background

Xylene (CeHz^CHa^); MW = 106.17) can occur as one of three isomers: ortho - 1,2-
dimethylbenzene or o-xylene; meta - 1,3-dimethylbenzene or w-xylene; and para -
1,4-dimethylbenzene or /^-xylene. Synonyms for mixed xylenes include dimethylbenzene,
methyltoluene, and xylol. Commercial grade xylene is most often a mixture of isomers, which is
predominantly w-xylene (up to 60%) with approximately equal levels of o- and /^-xylene (-20%
each); technical grade xylene also contains ethylbenzene between 6% and 15%. Xylene is a
colorless, non-corrosive, flammable liquid with an aromatic odor similar to that of benzene
(Henderson. 2001). The odor threshold for xylene ranges from 0.09 to 0.4 ppm, and is
noticeably irritating for very short durations at 750 ppm.

Production and Uses

Xylene is one of the BTEX aromatics (benzene, toluene, ethylbenzene, and xylene).
Global production of xylene in 2010 was nearly 44 million metric tons (SRI. 2011). Mixed
xylene is used as a solvent for cleaning and in paints and coatings, and is blended into gasoline.
Specific isomers are used as a feedstock in chemical manufacturing with a diverse array of final
products associated with each isomer.

Exposure Potential

Xylene is typically detected in ambient air, and can also be found in groundwater and
food (ATSDR. 2007). Ambient air concentrations in the United States have been reported to

3	3

average 0.18 ppb (0.78 |ig/m ) in remote rural areas and up to 99 ppb (430 |ig/m ) near industrial
facilities and roadways (NLM. 1998). The Toxic Release Inventory (TRI) for the 2010 reporting
year (U.S. EPA. 2010) reported a total of 12,871,595 pounds of xylene were emitted to air from
all industrial sources in the United States, with 8,344,898 pounds emitted from point sources
(stacks, vents, ducts, or pipes) and 4,526,697 pounds coming from fugitive sources (equipment
leaks, evaporative losses from surface impoundments and spills, and releases from building
ventilation systems).

Overview

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Xylenes

Xylene can be absorbed into the human body via inhalation, ingestion, or dermal. There
are more similarities between the isomers than differences on key issues, including: toxic
potency; physical characteristics; toxicokinetics (how it behaves once absorbed); and
toxicodynamics (how the body handles it). Each of the reference values discussed in this
summary handles the individual isomers and any mixture of isomers as the same compound.

Potential Health Effects

Acute inhalation exposure to mixed xylenes in humans has been associated with
shortness of breath, and irritation of the nose and throat; gastrointestinal effects (e.g., nausea,
vomiting, and gastric discomfort); mild transient eye irritation; and neurological effects (e.g.,
impaired short-term memory, impaired reaction time, decreases in numerical ability, and
alterations in equilibrium and body balance) (ATSDR. 2007). Exposure to a mixture of toluene
and xylenes resulted in more than additive respiratory and neurological toxicity in humans and
animals. Acute animal studies have also reported cardiovascular, liver, and kidney effects from
inhalation exposure to mixed xylenes in addition to the effects previously noted.

Chronic inhalation exposures to xylenes in worker cohorts have primarily shown
neurological effects such as headache, dizziness, fatigue, tremors, incoordination, anxiety,
impaired short-term memory, and inability to concentrate (ATSDR. 2007). Other effects
reported from chronic exposure include labored breathing, impaired pulmonary function,
increased heart palpitation, severe chest pain, abnormal EKG, and possible effects on the
kidneys. Mixed xylenes have not been extensively tested for chronic effects, although animal
studies show effects on the liver and CNS from inhalation and oral exposures and effects on the
kidneys from oral exposure to mixed xylenes.

Cancer Potential

The U.S. EPA (2003) found that "data are inadequate for an assessment of the
carcinogenic potential of xylenes " Similarly, IARC (1999) assessed the cancer potential and
found that "Xylenes are not classifiable as to their carcinogenicity to humans (Group 3). "
Therefore, no reference values for cancer (e.g., cancer slope factors) are available for xylene.

Emergency Response Values

The emergency response reference values for xylene are limited to the Acute Exposure
Guideline Levels (AEGLs); no Emergency Response and Planning Guideline (ERPG) values
have been developed for xylene. Interim AEGL values for xylene were developed for all three
severity levels (1 = mild, transient effects; 2 = irreversible effects or impeding ability to escape;
and 3 = threshold for life threatening effects). The Protective Action Criteria developed by the
U.S. Department of Energy adopts the AEGL (or ERPG) values when available for the three
severity levels defined by the AEGLs, and also includes a Temporary Emergency Exposure
Level (TEEL) value for a zero (0) severity level, which is defined as "the threshold
concentration below which most people will experience no adverse health effects. "

Occupational Exposure Limits

The occupational values for xylene show a very high level of concordance. All of the
values based on a time-weighted average (TWA) for daily exposures (up to 10 hours per day,
and 5 days a week) for a working career (up to 40 years) were set at 100 ppm, which include the
OSHA PEL, NIOSH REL, and the ACGIH TLV®. Similarly, both of the short-term exposure
limits (STELs) for exposures less than 15 minutes developed by NIOSH and ACGIH were

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established at 150 ppm. All of the occupational values for xylene were based on irritation and a
concern for neurotoxicity in the form of narcosis. In addition to the TWA and STEL values,
NIOSH has also developed an Immediately Dangerous to Life and Health (IDLH) value of 900
ppm for a 30 minute exposure.

Special Use Occupational Values

In addition to the standard occupational values, a set of special use occupational values
are also available which have been developed by and/or reviewed by the National Research
Council (NRC): Spacecraft Maximum Acceptable Concentrations (SMACs) for durations of 1
and 24 hours, and 7, 30, 180, and 1000 days; and values derived for submarine crews - the
Emergency Exposure Guideline Levels (EEGLs) for 1 and 24 hours, and the Continuous
Exposure Guideline Levels (CEGL) for 90 days. The EEGL values (150 ppm for 1-hour and 100
ppm for 24-hours) are fairly consistent with the AEGL-1 and traditional occupational TWA
values. Largely due to the confined working environment with no opportunity to be removed
from exposure, however, the CEGL value of 50 ppm is lower than the 100 ppm occupational
TWA values. Similarly, all of the SMACs are lower than other occupational reference values for
similar durations, including the EEGLs. The SMACs for durations of 7-days or longer were
based on concerns for potential narcosis and neurotoxic effects, including hearing loss
(ototoxicity) for the 180 and 1000 day values.

General Public Values (Routine Non-emergency Exposures)

Several reference values have been developed for exposures of the general public to
xylene. The California Reference Exposure Levels (CA-RELs) were developed by the Office of
Environmental Health Hazard Assessment (OEHHA 2008. 2000) for both acute (1-hour, 22
mg/m3 or 5 ppm) and chronic (0.7 mg/m3 or 0.2 ppm) durations. Minimal Risk Levels (MRLs)
were developed by the Agency for Toxic Substances and Disease Registry (ATSDR 2007) for
acute (1-14 days,), intermediate (15-365 days), and chronic (>1 year) durations. A Reference
Concentration (RfC) value of 0.1 mg/m (0.02 ppm) was developed by the U.S. Environmental
Protection Agency (U.S. EPA 2003) for the Integrated Risk Information System (IRIS).

Both of the acute values (CA-REL and MRL) use the exposure level of 50 ppm as the
point-of-departure (POD). The 50 ppm POD for the acute 1-hour CA-REL was the result of an
adjustment to the 100 ppm NOAEL for a 30-minute exposure observed in the study by Hastings
et al. (1986) using a simple C x t relationship (e.g., Haber's rule), whereas the 50 ppm LOAEL
for a 2-hour exposure in humans from the study by Ernstgard et al. (2002) was not duration-
adjusted for the acute MRL. Regardless, these organizations arrive at very different final values
based on differences in the application of uncertainty factors (UF); a Total UF of 10 was applied
in deriving the CA-REL versus 30 applied in derivation of the acute MRL; details on the
application of UFs are presented in Table 1.

The intermediate MRL was based on a study on neurotoxicity in rats (Korsak et al..
1994). which was also used in the derivation of the RfC; however, there was no duration
adjustment in the derivation of the POD for the MRL (50 ppm), while in the derivation of the
RfC the POD was adjusted to match a continuous exposure scenario. Other differences in the
derivation of the RfC and intermediate MRL values from this study was in the application of
uncertainty factors (UFs): a total UF of 90 was applied in the derivation of the intermediate
MRL, whereas the RfC was calculated with a total UF of 300.

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Xylenes

Likewise, both the chronic CA-REL and chronic MRL were derived based on the same
study (Uchida et al.. 1993) with an observed lowest observed adverse effect level (LOAEL) of
14 ppm for a combination of irritation and neurological effects in workers. The POD for the CA-
REL was based on duration adjustments and a total UF of 10, whereas the chronic MRL made no
duration adjustments but applied a total UF of 300 which included a modifying factor of 3 for
database uncertainty. As noted in Table 1, these differences in derivation lead to final reference
values which diverged considerably from the use of the same observations.

In conclusion, the effects most noted in association with exposure to xylene across all
durations are neurotoxic in nature (narcosis/intoxication, incoordination, headache, ototoxicity,
etc.). Short-duration exposures to elevated concentrations (e.g., > 50 ppm) have been associated
with irritation of the eyes and upper respiratory system. At higher exposure levels, changes in
body weight, liver morphology, liver weight, and enzymatic functions have all been noted (U.S.
EPA. 2003). In addition, gestational exposure of animals to xylenes has resulted in
neurodevelopmental and other possible developmental effects, but only at levels above those at
which neurobehavioral effects in adult rats were reported. Finally, no reproductive effects have
been reported in a one-generation reproductive/developmental study of male and female rats
exposed to 500 ppm xylenes or in male rats exposed to 1000 ppm (U.S. EPA 2003).

Summary

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Xylene (mixed isomers): Comparison of Reference Values

June 2011

1.E+05

1.E+04 v O

CO
£
o>
E

6
c
o
o

»
X

ACGIH-STEL'

1 E+03 --NI0SH-STEL

1.E+02 -

1.E+01 -

1.E+00 -

1.E-01 -

1.E-02

'TEEL-0

o SMAC

X

CA-REL (Acute)

O CEGL*
O SMAC* O SMAC*<> SMAC*

+ SMAC*

ATSDR-MRL (1-14 d)

	I

~ SM/ C*

ATSDR-MRL (15-365d)

*-

	I

-X

CA-REL (Chronic)

X	X

ATSDR-MRL (>1yr)
™

tttH

EPA/IRIS RfC

H-

—AEGL-3
O AEGL-2
^^AEGL-1
O TEEL-0
O ACGIH-STEL*

•	NIOSH-STEL*

•	NIOSHIDLH*

- O OSHA-PEL (TWA)*
-O ACGIH-TLV (TWA)*
O NIOSH-REL(TWA)*

•	SMAC*

O EEGL*

O CEGL*

X CA-REL (Acute)
-X- ATSDR-MRL(1-14d)
-X- ATSDR-MRL(15-365d)
-X- ATSDR-MRL(> 1yr)
—X— CA-REL (Chronic)
—¦— EPA/IRIS RfC

0.1

10

-100^. ..1000 . 10000

Duration (hours)

100000 1000000

O		Cl


05
Q.
D
O
O

O

o
n

D
Q_

TO


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Markup from Peer Review Comments - October 2011

Table 1. Details on derivation of the available health effect reference values for inhalation exposure to xylenes



Reference



Reference Value



Point of



Principal

Uncertainty

Notes on

Review



Value Name

Duration

(mg/m3)

(ppm)

Health Effect

Departure

Qualifier

Study

Factors2

Derivation

Status



AEGL-3

10 minutes
30 minutes
1 hour
4 hours
8 hours

>LEL5o%
16,000
11,000
5,600
4,300

^ELsoo,3
3600
2500
1300
1000

Rats

exhibited
prostration
followed by full
recovery

2800 ppm
(4 hours,
used in
PBPK
model)

NOAEL
for

Lethality

Carpenter
etal. (1975)

Total UF = 3
UFH=3
UFa=1

A human
PBPK model
was run for
each time
period to
determine the
exposure
producing the
target internal

Final
(NRC, 2010)

4>

AEGL-2

10 minutes
30 minutes
1 hour

11,000
5,600
4,000

2500
1300
920

Rats

exhibited
poor coordination
2 h into a 4-h
exposure

1300 ppm
(2 hours,
used in
PBPK
model)

LOAEL

Carpenter
etal. (1975)

Total UF = 3
UFh=3
UFa=1

dose



=

©



4 hours

2,200

500















a

VI



8 hours

1,700

400















w
=
0)
0/j
•~

E

AEGL-1

10 minutes
30 minutes
1 hour
4 hours
8 hours

560
560
560
560
560

130
130
130
130
130

Eye irritation in
human volunteers
exposed to mixed
xylenes

400 ppm
(30 minutes)

LOAEL

Hastings et
al. (1986)

Total UF = 3
UFh=3
UFa=1

No duration
scaling due to
irritant effects





TEEL-0

1 hour

400

100

NR

NR

NR

NR

NR

(DOE, 2008)

Revision 26
(DOE. 2010)

2	UFh - inter-human variability; UFA - animal to human variability; UFL - LOAEL to NOAEL adjustment; UFS - subchronic to chronic adjustment;
UFdb - database uncertainty

3	LEL = Lower Explosive Limit. The AEGL-3 for 10 minutes = 7,200 ppm which is greater than 50% of the lower explosive limit for xylene.

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Xylenes

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Reference



Reference Value



Point of



Principal

Uncertainty

Notes on

Review



Value Name

Duration

(mg/m3)

(PPm)

Health Effect

Departure

Qualifier

Study

Factors2

Derivation

Status



OSHA PEL

8 hour TWA

435

100

Irritant, narcotic,

NR

NR

NR

NR

NR

Final



(TWA)







and chronic effects











(NIOSH. 2007)



NIOSH STEL

<10 minutes

655

150

Irritation and
narcotic effects,

Various

NA

NIOSH
(1975)

NA

WOE Approach

Final
(NIOSH, 2007)



NIOSHREL
(TWA)

10 hour
TWA

435

100

with potential for
chronic effects

Various

NA











NIOSH IDLH

30 minutes

3710

900

Acute inhalation
toxicity data in
animals

Various

Effects
levels

Various4

NA







ACGIH

<15 minutes

651

150

Potential for eye

200 ppm

TOAET

Carpenter et

NA

WOE Approach

Final



TLV-STEL







and upper
respiratory
irritation; also
protective for
narcosis



(Human)

al. (1975)





(ACGIH,
2007)



ACGIH

8 hour TWA

434

100

100 ppm

NOAET

Nelson et al.









TLV-TWA









(Human)

(1943)









EEGLs

1 hour

870

200

Eye irritation

200 ppm

Various

Various^

None

WOE Approach

Final
(NRC. 2008a)





24 hour

435

100

Eye irritation

100 ppm

NOAET

Hake et al.















(repeated)





(1981)







=

©

CEGL

90 days

217

50

Eye and throat

50 ppm

NOAET

Various"







a

3









irritation













SMACs

1 hour

217

50

Eye, nose, and
throat irritation and
headache

50 ppm
(2 hours)

TOAET

Emstgard et
al. (2002)

None

Not adjusted,
concentration
dependent

Final
(NRC, 2008b)

w
w

o



24 hour

74

17



Total UP = 3
UFl=3





7 day and

74

17

Neurotoxicity

50 ppm

NOAET

Korsak et al.

Total UP = 3

No time







30 day





(motor function)

(3 months)

(rat)

(1994)

UFa=3

adjustment







180 day

37

8.5

Ototoxicity

250 ppm

1000 ppm
(Rat, 18 h/d,
61 days)

TOAETadj

TOAET
(rat)

Nylen and

Hagman

(1994)

Total UF = 30
UFl= 10
UFa=3

Duration
adjusted using
C x t (Haber's)







1000 days

6.5

1.5



8.5 ppm

180 d
SMAC





180 d SMAC
adjusted to
1000 d



4	(DeCeaurriz et al.. 1981: Harper et al.. 1975: Cameronet al.. 1942)

5	(Hastings et al.. 1986: Hake et al.. 1981: Carpenter et al.. 1975)

6	(Ungvarv. 1990: Carpenter et al.. 1975: Jenkins et al.. 1970)

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Reference



Reference Value



Point of



Principal

Uncertainty

Notes on

Review



Value Name

Duration

(mg/m3)

(ppm)

Health Effect

Departure

Qualifier

Study

Factors2

Derivation

Status



Acute
CA-REL
(Mild Effects)

1 hour

22

5

Subjective reports
of eye, nose, and
throat irritation (50
healthy human
volunteers)

50 ppm
100 ppm

NOAELadj

NOAEL
(30-min)

Hastings et al.
(1986)

Total UF = 10

UFh= io

Duration
extrapolation
via Cn x t
(n = 1) from 30-
min to 1 hr

Final
COEHHA.
2008)



Acute ATSDR

1-14 days

8.7

2.0

Discomfort in the

50 ppm

LOAEL

Emstgard et

Total UF = 30



Final



MRL







eyes and nose,
detection of solvent
smell, and feeling of
intoxication

(human
volunteers,
2 hours)



al. (2002)

ufh= io

UFl = 3



(ATSDR.
2007)



Intermediate

15-365 days

2.6

0.6

Dose-related

50 ppm

LOAEL

Korsak et al.

Total UF = 90







ATSDR MRL







increase in the
failure rate of rats
on the rotarod
performance test





(1994)

UFa=3

ufh= io

UFL =3







Chronic

Chronic

0.7

0.2

Eye irritation, sore

5.1 ppm

LOAEL ADJ

Uchida et al.

Total UF = 30

Adjustments:

Final



CA-REL







throat, floating
sensation, and poor

14.2 ppm

LOAEL

(1993)

UFl=3
UFh = 10

occupational
breathing rate

(OEHHA,
2000)

3
s
a.









appetite (175
exposed and 241
control workers)

(Human, 7-yr
avg.)







(10 m3 vs.
20 m3) and
5 days per
week.



«

Chronic

Chronic

0.22

0.05

neurotoxicity

14 ppm

LOAEL

Uchida et al.

Total UF = 300

Not adjusted

Final

e

0

ATSDR MRL

(> 1 year)





(anxiety,
forgetfulness,
floating sensation),
and nasal, throat
and

eye irritation

(Human, 7-yr
avg.)



(1993)

UFh = 10

ufl = 10

MF7 = 3

due to rapid
clearance of
xylene from the
body

(ATSDR,
2007)



Chronic RfC

Chronic

0.1

0.02

Impaired motor

39 mg/m3

LOAEL ADJ

Korsak et al.

Total UF = 300

NOAEL

Final



(IRIS)







coordination

(decreased

rotarod

performance) in a
subchronic
inhalation
study in male rats

217 mg/m3
(50 ppm)

100 ppm

LOAEL
LOAEL

(1994)

UFa=3
UFh = 10
UFl=3
UFdb = 3

adjusted from 6
h/d and 5 d/wk
to continuous
exposure
(24 h/d, 7 d/wk)

(U.S. EPA,
2003)

7 A Modifying Factor was applied to account for the lack of supporting studies evaluating the chronic neurotoxicity of xylene.

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Further Reading

U.S. EPA (U.S. Environmental Protection Agency). Xylenes, Air Toxics Web Site.
http://www.epa.gov/ttn/atw/hlthef/xylenes.html

REFERENCES

ACGIH (American Conference of Governmental Industrial Hygienists). (2007). 2007 TLVs and
BEIs: Based on the documentation of the threshold limit values for chemical substances
and physical agents and biological exposure indices. Cincinnati, OH.

AT SDR (Agency for Toxic Substances and Disease Registry). (2007). Toxicological profile for
xylene [ATSDR Tox Profile], Atlanta, GA: U.S. Department of Health and Human
Services, Public Health Service.

http://www.atsdr.cdc. gov/ToxProfiles/tp.asp?id=296&tid=53
Cameron. GR; Courtice. FC: Foss. GL. (1942). Effect of exposing different animals to a low

concentration of phosgene 1:1,000,000 4 mg/m3 for 5 hours (third report). In First report
on phosgene poisoning (pp. Chapter IX). (Porton Report No. 2349). Washington, DC:
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Carpenter. CP: Kinkead. ER; Geary. PL. Jr; Sullivan. LJ; King. JM. (1975). Petroleum

hydrocarbon toxicity studies V Animal and human response to vapors of mixed xylenes.
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DeCeaurriz. JC: Micillino. JC: Bonnet. P; Guenier. JP. (1981). Sensory irritation caused by
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DOE (U.S. Department of Energy). (2008). Temporary emergency exposure limits for
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Action Criteria (PAC) Rev 26 based on applicable 60-minute AEGLs, ERPGs, or TEELs
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Hake. CL; Stewart. RD; Wu. A: Graff. SA; Forster. HV; Keeler. WH; Lebrun. AJ; Newton. PE;
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Institute for Occupational Safety and Health.

Harper. C: Drew. RT; Fouts. JR. (1975). Benzene and p-xylene: A comparison of inhalation

toxicities and in vitro hydroxylations. In J Jollow (Ed.), Biological reactive intermediates,
formulation, toxicity, and inactivation (pp. 302-311). New York, NY: Plenum Press.
Hastings. L; Cooper. GP; Burg. W. (1986). Human sensory response to selected petroleum

hydrocarbons. In HN MacFarland; CE Holdsworth; JA MacGregor; RW Call; ML Lane
(Eds.), Advances in Modern Environmental Toxicology Volume VI Applied Toxicology
of Petroleum Hydrocarbons (pp. 255-270). Princeton, NJ: Princeton Scientific Publishers.
Henderson. RF. (2001). Aromatic hydrocarbons: Benzene and other alkylbenzenes. In E

Bingham; B Cohrssen; CH Powell (Eds.), Patty's toxicology (5 ed., pp. 231-301). New
York, NY: John Wiley and Sons.

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I ARC (International Agency for Research on Cancer). (1999). Re-evaluation of some organic
chemicals, hydrazine, and hydrogen peroxide. Lyon, France.
http://monographs.iarc.fr/ENG/Monographs/vol71/volume71.pdf
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toluene, o-xylene, and cumene on experimental animals. Toxicol Appl Pharmacol 16:
818-823.

Korsak. Z; Wisniewska-Knypl. J: Swiercz. R. (1994). Toxic effects of subchronic combined
exposure to n-butyl alcohol and m-xylene in rats. Int J Occup Med Environ Health 7:
155-166.

Nelson. KW: Ege. JF. Jr; Ross. M; Woodman. LE; Silverman. L. (1943). Sensory response to

certain industrial solvent vapors. J Ind Hyg Toxicol 25: 282-285.

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Office of Research and Development
National Centerfor Environmental Assessment

Research Triangle Park, NC	Xylenes

Markup from Peer Review Comments - October 2011

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