March  31,  1987
               820K87109
                                      XYLENES
                                  Health Advisory
                              Office of Drinking Water
                        U.S.  Environmental  Protection Agency
I. INTRODUCTION
        The Health Advisory  (HA)  Program, sponsored by the Office of Drinking
   Water (ODW),  provides information on the  health effects,  analytical method-
   ology and treatment  technology that  would be useful in dealing with the
   contamination of drinking water.   Health  Advisories describe  nonregulatory
   concentrations  of  drinking water  contaminants at which adverse health effects
   would not be  anticipated  to  occur over specific exposure  durations.  Health
   Advisories contain a margin  of safety to protect sensitive members of the
   population.

        Health Advisories  serve as informal  technical guidance to assist Federal,
   State and local officials responsible for protecting public health when
   emergency spills or  contamination situations occur.  They are not to be
   construed as  legally enforceable  Federal  standards.  The  HAs  are subject to
   change as new information becomes available.

        Health Advisories  are developed for  One-day, Ten-day, Longer-term
   (approximately  7 years, or 10% of an individual's lifetime) and Lifetime
   exposures based on data describing noncarcinogenic end points of toxicity.
   Health Advisories  do not  quantitatively incorporate any potential carcinogenic
   risk from such exposure.   For  those  substances  that are known or probable
   human carcinogens, according to the  Agency classification scheme (Group A or.
   B),  Lifetime  HAs are not  recommended. The chemical concentration values for
   Group A or B  carcinogens  are correlated with carcinogenic risk estimates by
   employing a cancer potency (unit  risk) value together with assumptions for
   lifetime exposure and the consumption of drinking water.  The cancer unit
   risk is usually derived from the  linear multistage model with 95% upper
   confidence limits.  This  provides a  low-dose estimate of cancer risk to
   humans that is  considered unlikely to pose a carcinogenic risk in excess
   of the stated values.  Excess  cancer risk estimates may also be calculated
   using the One-hit, Weibull,  Logit or Probit models.  There is no current
   understanding of the biological mechanisms involved in cancer to suggest that
   any one of these models is able to predict risk more accurately than another.
   Because each  model is based  on differing assumptions, the estimates that are
   derived can differ by several  orders of magnitude.

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         This Health Advisory  is based on information presented in the Office of
    Drinking Water's Health Effects Criteria Document (CD) for Xylenes (U.S. EPA,
    1985a).  The HA and CD formats  are similar for easy reference.  Individuals
    desiring further information  on the toxicological data base or rationale for
    risk  characterization should  consult the CD.   The CD is available for review
    at each EPA Regional Office of  Drinking Water counterpart (e.g., Water Supply
    Branch or Drinking Water Branch),  or for a fee from th National Technical
    Information Service, U.S.  Department of Commerce, 5285 Port Royal Rd.,
    Springfield, VA 22161,  PB  # 86-117942/AS. The toll-free number is (800)
    336-4700; in the Washington,  D.C.  area:  (703) 487-4650.


II.  GENERAL INFORMATION AND PROPERTIES

                                        Xylene     Ortho-

    CAS No.                           1330-20-7

    Structural Formula                            CH3         CH3         CH3


                                                                          CH3

    Synonyms

         0  Xylols; dimethylbenzene

    Uses

         0  As solvents for paints, inks and adhesives, and as components of
            detergents and other  industrial and household products.

    Properties  (Verschueren,  1983)

                                        Xylene     Ortho-     Meta-      Para-

    Chemical Formula                               C8H10      C8H1      C8H10
    Molecular Weight                               106.16     106.16     106.16
    Boiling Point                                  144.4C    139.0C    138.4C
    Melting Point                                  -25C      -48C      -13C
    Density
    Vapor Pressure, mm Hg, 20 C                      5          6          6.5
    Water Solubility, mg/1, 20 C                  175        160
                            25 c                                    198
    Log Octanol/Water Partition                    3.12       3.20       3.15
      Coefficienta
    Taste Thresholdb              0.3-1.0 mg/L
    6dolr~Threshold
    Conversion Factor             1 ppm =4.3 mg/m3

    a Leo et al.  (1971)
    b National  Inst. for Water Supply (1977)

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     Occurrence

          0  Xylene occurs naturally as a component of petroleum oil.

          0  Xylenes are produced in large amounts, 5 billion Ibs in 1982 (U.S. ITC,
             1984).  Xylenes are also produced indirectly in large volumes during
             gasoline refining and other operations.  Xylene content of gasoline
             can be as high as several percent.

          0  Releases of xylenes to the environment are largely to air due to their
             volatile nature,  with smaller amounts to water and soil.  Releases of
             xylenes to water are due to spills and leaks of gasoline and other
             petroleum products and,  to a lesser extent,  from the disposal of waste
             from paints, inks and other industrial products.  Because of the wide-
             spread use of petroleum products, releases of xylene occur nationwide.

          0  Xylenes degrade rapidly in air with a half life of a few days.  Xylenes
             released to surface water rapidly volatilize to the air.  Xylenes
             released to the ground bind somewhat to soil and slowly migrate with
             ground water.  Xylenes are biodegraded readily in soils and surface
             waters.  In the absence of biodegradation, xylenes are expected to be
             stable in ground water.

          0  Xylenes occur at low levels in drinking water, food, and air.  Xylene
             occurs in both ground and surface public water supplies, with higher
             levels occurring in surface water supplies.   The EPA's Community
             Water Supply Survey, found 3% of all ground water derived public
             drinking water systems sampled had levels greater than 0.5 ug/L.  The
             highest level reported in ground water was 2.5 ug/L.  The survey
             reported that 6% of all surface water derived drinking water system
             are contaminated at levels higher than 0.5 ug/L.  Hone of the systems
             were reported to contain levels higher than 5.2 ug/L.  No information
             on the occurrence of xylene in foods has been identified.  Xylenes
             are found in the air of urban and suburban areas at levels of approxi-
             mately 2 ug/L.  Because of the widely dispersed low levels of xylenes
             reported in water, air is likely to be the major source of exposure.


III. PHARMACOKINETICS

     Absorption

          0  Xylenes are absorbed readily after inhalation.  Data on absorption
             after ingestion are not available.  Sedivec and Flek (1976) exposed
             human volunteers to 0.2 mg/L (200 mg/m3)  o-,  m- and p-xylene vapors
             and also to their mixture at a ratio of 1:1:1 for an 8-hour period.
             The amount absorbed or retained was 63.6% * 4.2% for all isomers.

     Distribution

          0  Using whole-body autoradiography to detect radiolabeled xylene  and
             metabolites, Bergman (1978) reported distribution of the compound in
             many tissues and organs  of exposed mice.   14C-m-Xylene was administered

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            to mice  by inhalation for 10 minutes.  Whole mice were frozen and
            sectioned before exposure to X-ray film.   In  mice sacrificed immediately
            after xylene  exposure,  radiolabel  was  detected  in the  lungs, liver
            and kidneys.   Rapid distribution to the brain and adipose tissue also
            was evident.   Two hours after  exposure, radiolabel  in  the lungs was
            restricted to the bronchi.  In addition to the  previously noted
            organs,  radiolabel was  detected in the intestine  after two hours.
            The last traces of xylene were detected in the adipose tissue of mice
            sacrificed four hours after  exposure.

         0  In rats  exposed to 14c-p-xylene for 1-8 hours by  inhalation at 208
            mg/m3 (48 ppm), radiolabel was detected (in decreasing concentration)
            in the kidneys, subcutaneous fat,  sciatic  nerve,  blood,  liver,  lungs,
            spleen,  muscles, cerebrum and  cerebellum  (Carlson,  1981).  Distribution
            to all tissues  was rapid,  allowing near maximal levels in tissues
            (except  in kidneys and subcutaneous fat) within one hour.

    Metabolism

         0  Metabolism of xylenes varies somewhat  according to  the isomer but,  in
            general, proceeds by oxidation of  methyl  groups and ring hydroxylation.
            The resulting metabolites  include  methyl hippuric acid (95%) and
            xylenols (1-2%) (Harper et al. 1975).
    Excretion
            Elimination of xylenes  is primarily through urinary excretion of
            metabolites, representing nearly 95% of the absorbed dose,  and the
            remaining 5% by pulmonary exhalation of unchanged solvent (Sedivec
            and Flek, 1976; Astrand et al.,  1978).
IV. HEALTH EFFECTS

    Humans
         0  Tho lowest oral lethal dose (LDLo)  for humans  has been reported as
            50 mgAg (NIOSH,  1978).

         0  Xylenes produce central nervous system disturbances as reflected in
            changes in numerative ability, short-term memory and electroencephalo-
            graphic patterns.

         8  Gamberale et al.  (1978) observed no adverse health effects in fifteen
            male subjects at rest following 70 minutes of  inhalational exposure
            to xylene at 435 and 1300 mg/m3.  However, in  another experiment,
            eight male subjects were exposed to xylene at  1300 mg/m3 with 30
            minutes of exercise on a bicycle ergometer which was continued during
            behavior tests.  The authors concluded that there was evidence of
            reduction in the performance level on three of the four tests.  The
            tests conducted were:  simple addition and choice reaction time,
            short-term memory and critical Hicker fusion frequency.

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        Savolainen et al. (1980) observed adverse effects on the psycho-
        physiological functions in eight male students following inhalational
        exposure to m-xylene at 391  mg/m3 for five consecutive days and one
        day after the weekend.  Body balance, reach on time and manual coordi-
        nation were impaired.  However, tolerance against the observed effects
        developed during one working week.
Animals
Short-term exposure
        In rats,  oral LD50 values range from 4,300 to 5,000 mg/kg(NIOSH, 1978),
        whereas inhalation IG^QB (four hours) are 20,600 to 29,000 mg/m3
        (Carpenter et al., 1975; Harper et al., 1975).
Long-term Exposure

     8  Carpenter et al. (1975) exposed rats to mixed xylenes at 770, 2000 or
        3500 mg/m3 for six hours/day, five days/week for 13 weeks duration.
        No effects on body weight gain, hematology, blood chemistry, kidney,
        or liver weights or tissue histology were reported at two lower dose
        levels.  At the highest dose level, one rat treated at 3500 mg/m3 for
        seven weeks showed slight renal tubular regeneration and, at 13 weeks,
        the response was noted in rats in a non-dose-related manner.

     0  Jenkins et al.  (1970) reported the results of repeated (30 exposures)
        or continuous inhalation exposure (90 days) to o-xylene by rats,
        guinea pigs, monkeys and dogs.  The exposure levels were 337 or
        3,358 mg/m3 in rats and monkeys.  One of the dogs exhibited tremors
        of varying severity throughout exposure.  No significant effects were
        observed with respect to body weight, hematology, and histopathological
        examination at the lower dose of 337 mg/m3 xylene.

     0  Ultrastructural hepatic effects have been reported in rats following
        subchronic oral exposure (200 mg/kg diet for up to 6 months) (Bowers
        et al., 1982).  Two types of intracellular vesicles in rats treated
        with o-xylene were observed.  One type appeared to be an extension of
        the endoplasmic reticulum and the second vesicle type was associated
        with the hepatocyte plasmalemma.

     0  Tatrai et al. (1981) reported hepatomegally and ultrastructurally
        evident proliferation of the smooth endoplasmic reticulum following
        chronic inhalation exposure of 4750 mg/m3, eight hours/day, seven
        days/week for one year in rats.

Reproductive Effects

     0  No information was found in the available literature on the repro-
        ductive effects of xylene.

Developmental Effects

     e  Twenty CFY rats, 240 to 280 g, were exposed to 1,000 mg/m3 of mixed
        xylenes 24 hours/day during days 9 to 14 of pregnancy, and although

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           there were increased incidences of fused sternebrae and extra ribs in
           the offspring, the authors interpreted these as signs of embryotoxicity
           rather than teratogenicity (Hudak and Ungvary,  1978).  No signs of
           maternal toxicity were noted.  In-another study, Charles River rats
           aged 12 weeks were exposed to 0,  100 or 400 ppm of xylenes (0, 434 or
           1,730 mg/m3) during days 6 to 15 of pregnancy (25 rats per group); the
           authors reported no signs of teratogenicity whether on the per-fetus
           or the per-litter basis  (Litton Bionetics, 1978).

   Mutagenicity

        0  Xylene was not mutagenic in the Ames test with  or without activation
           or in other short-term in vitro assays (Litton Bionetics, 1978).

   Carcinogenicity

        0  A long-term carcinogenicity bioassay in rats and mice has been
           conducted by the NTP; however, the final report is not yet released
           by the NTP (1986).
V. QUANTIFICATION OF TOXICOLOGICAL EFFECTS

        Health Advisories (HAs)  are generally determined for One-day,  Ten-day,
   Longer-term (approximately 7  years) and Lifetime exposures if adequate data
   are available that identify a sensitive noncarcinogenic end point of toxicity.
   The HAs for noncarcinogenic toxicants are derived using the following formula:

                 HA - 
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Step 1:  Determination of the Total Absorbed Dose (TAD)

      Total absorbed dose = <1300 mg/m^)^^^) (p. 64) = 1U9 mg/kg/day


where :

        1,300 mg/m3 = NOAEL based on absence of reduction in the performance
                      level on tests in humans.

               1 m3 = assumed amount of air inhaled during one hour by a human

               0.64 = 64% absorption factor in humans (Sedivek and Flek,  1976).

              70 kg = assumed body weight of an adult.

Step 2:  Determination of a One-day HA
  One-day HA = <11-9 ngAg/day) (10 kg) . n.9 mg/L = 12 mg/L (12,000 ug/L)
                    (10) (1  L/day)
where:

        11.9 mgAg/day = TAD.

                    10 = uncertainty factor, chosen in accordance with NAS/ODW
                         guidelines for use with a NOAEL from a human study.

                 10 kg = assumed body weight of a child.

               1 L/day  assumed daily water consumption of a child.


Ten-day Health Advisory

     Insufficient data using oral exposure to calculate a Ten-day Health
Advisory are available currently.  The Longer-term HA for the 10 kg child
(7800 ug/L) is recommended for a ten-day exposure.


Longer-term Health Advisory

     The study by Carpenter et al. (1975) is the most appropriate basis for
calculating a Longer-term HA.  A group of male rats were exposed by inhalation to
mixed xylenes at 770,  2000, or 3500 mg/m3 for six hours/day on five days/week
for up to thirteen weeks.  No effects on body weight, hematology, blood
chemistry, kidney or liver weight, or tissue histology were observed at 770
or 2000 mg/m3 exposure levels of xylene.  Based on the 2000 mg/m3 exposure
level as a NOAEL, a Longer-term HA may be derived for a 10 kg child as follows:

Step 1:  Determination of the Total Absorbed Dose (TAD)

  TAD - (2,000 mg/m3)  (6 hours/day) (1 m3/hour) (5/7) (0.64) = ?8
                               (70 kg)

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 where:

         2,000 mg/m3  = NOAEL based on  the absence of various toxicological
                      parameters in rats.

            6 hr/day   duration of exposure.

             i m3/hr  = assumed respiratory volume for a rat.

                 5/7  = Conversion of 5 day/week dosing regimen to  daily dosing
                      regimen.

                0.64  - absorption efficiency  in humans.

               70 kg  = assumed body weight of an adult.


 Step 2:   Determination  of  the Longer-term HAs

      For a 10  kg child:

        Longer-term HA = (78 mg/kg/day)  (10 kg) = 7>8   /L  (7 800  ug/D
                            (100)  (1 L/day)

 where:

         78 mg/kg/day   TAD.

                10 kg =  assumed body weight of a child.

                  100 -  uncertainty factor, chosen  in accordance with NAS/ODW
                        guidelines  for use with a NOAEL from an  animal study.

              1  L/day =  assumed daily  water consumption of  a child.

      For a 70 kg adult:

       Longer-term HA -  (78 mg/kg/day) (70 kg)  27.3 mg/L  (27,300 ug/L)
                           (100)  (2 L/day)

 where:

         78 mgAg/day *  TAD.

                70 kg   assumed body weight of an adult.

                  100   uncertainty  factor, chosen  in accordance with NAS/ODW
	guidelines for use with  a NOAEL from an  animal study.
              2 L/day = assumed daily water consumption of an adult.

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Lifetime Health Advisory

     The Lifetime HA represents that portion of an individual's total exposure
that is attributed to drinking water and is considered protective  of  noncar-
cinogenic adverse health effects over a lifetime exposure.  The Lifetime HA
is derived in a three step process.  Step  1 determines the Reference  Dose
(RfD), formerly called the Acceptable Daily Intake (ADI).  The RfD is an esti-
mate of a daily exposure to the human population that is  likely to be without
appreciable risk of deleterious effects over a lifetime, and is derived from
the NOAEL (or LOAEL), identified from a chronic  (or subchronic) study, divided
by an uncertainty factor(s).  From the RfD, a Drinking Water Equivalent Level
(DWEL) can be determined (Step 2).  A DWEL is a medium-specific (i.e., drinking
water) lifetime exposure level, assuming 100% exposure from that medium, at
which adverse, noncarcinogenic health effects would not be expected to occur.
The DWEL is derived from the multiplication of the RfD by the assumed body
weight of an adult and divided by the assumed daily water consumption of an
adult.  The Lifetime HA is determined in Step 3 by factoring in other sources
of exposure, the relative source contribution (RSC).  The RSC from drinking
water is based on actual exposure data or, if data are not available, a
value of 20% is assumed for synthetic organic chemicals and a value of 10%
is assumed for inorganic chemicals.  If the contaminant is classifed as a
Group A or B carcinogen, according to the  Agency's classification  scheme of
carcinogenic potential (U.S. EPA, 1986), then caution should be exercised in
assessing the risks associated with lifetime exposure to this chemical.

     Compound-specific, chronic ingestion data for xylenes do not exist at
this time.  In the absence of appropriate  ingestion studies, the Lifetime
Health Advisory for xylenes will be derived from the inhalation studies of
Jenkins et al. (1970) instead of the Bowers et al. (1982) study, even
though the route and duration of exposure used in the Jenkins study are not
ideal for development of a Lifetime HA.

     The study by Bowers et al. (1982) was designed primarily to investigate
the first hepatocyte changes following long-term exposure to low levels of
o-xylene or other methylbenzenes administered to aged male rats, weighing 800
to 900 g,  in the diet at 200 mgAg food for up to six months prior to electron
microscopic examination of their livers, but not other tissues.   Certain
major weaknesses of this study rule out its consideration in the development
of a Lifetime HA.  These weaknesses are:   (1) the use of aged animals weighing
800 to 900 g in the experiment; (2) the stability of o-xylene was  not monitored
(i.e., any loss due to evaporation not mentioned);  (3) the use of a single
exposure level;  (4) the lack of histological examination of tissues other
than liver of animals on test diet; and (5) ultrastructural changes in the
hepatocytes of rats ingesting o-xylene was not stated specifically for o-xylene.

     The inhalation study by Jenkins et al. (1970)  was selected  as the basis
for the Lifetime HA.   In sthis study,  o-xylene was  administered  by inhalation
to rats, guinea pigs, monkeys and dogs for 30 repeated exposures at 3,358
mg/m3, eight hours/day,  five days/week or 90 days continuous exposure at
337 mg/m3.  At 3,358 mg/m3,  two rats died on the third day of exposure and
another rat and one monkey died on day seven;  one of the dogs exhibited
tremors of varying severity throughout the exposure.  Besides the  above
mentioned observations,  no significant effects were observed with  respect to
body weight, hematology, and histopathological examination at,either dose.

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     Using 337 mg/m3 as a NOAEL, the Lifetime HA for a 70 kg adult is calcu-
lated as follows:

Step 1:  Determination of the Total Absorbed Dose (TAD):

            TAD = 037 mg/m3) (20 m^day) (0.64) = 6U62 mg/kg/day
                              70 kg

where:

        337 mg/m3  NOAEL based on the absence of toxicological effects in rats.

        20 m3/day = assumed respiratory volume per day of a rat.

             0.64 = assumed absorption factor for xylenes (64%).

            70 kg = assumed body weight of an adult.

Step 2:  Determination of the Reference Dose (RfD)

                  RfD = 61.62 mg/kg/day _, o.06162 mg/kg/day
                             (1,000)

where:

        61.62 mg/kg/day = TAD.

                  1,000 - uncertainty  factor, chosen in accordance with NAS/ODW
                          guidelines for use with a NOAEL from an animal study
                          of less-than-lifetime duration.^ _^

Step  3: Determination of the Drinking  Water Equivalent  Level  (DWEL)

         DWEL -  <0.06162 mgAg/day)  (70 kg) = 2.16 mg/L (2,200 ug/L)
                             (2  L/day)

where:

        0.06162 mgAg/day - RfD.

                    70 kg * assumed  body weight of an adult.

                  2 L/day   assumed daily water consumption  of an adult.

Step  4: Determination of Lifetime  HA:

              Lifetime HA -  2 mg/L x 0.20 - 0.4 mg/L  (400 ug/L)

where:

         2 mg/L  - DWEL.

           0.20   assumed relative  source contribution of water.

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          It should be noted that an estimated concentration for detection by
     taste and odor in surface water was 0.3 to 1.0 mg/L (National Inst. for Water
     Supply, 1977) and that the HA may exceed these thresholds for some individuals.


     Evaluation of Carcinogenic Potential

          0  The carcinogenic potential of xylene will be assessed when the report
             of the NTP animal bioassay for carcinogenicity (1986) is available
             for review.

          0  IARC has not evaluated the xylenes for their carcinogenic potential.

          0  Applying the criteria proposed in EPA's guidelines for assessment of
             carcinogenic risk (U.S. EPA,  1986),  the xylenes may be classified in
             Group D: Not classified.  This category is for agents with inadequate
             animal evidence of carcinogenicity.


 VI. OTHER CRITERIA,  GUIDANCE AND STANDARDS

          0  NAS (1980) calculated SNARLs  (Suggested-no-adverse-response-levels)
             for xylenes  in drinking water.  The  NAS-SNARL value was  21  mg/L
             xylene for a 1-day exposure and 11.2 mg/L for a 7-day exposure (for
             a 70 kg adult).

          0  U.S. EPA (1981) also provided draft  HAs for a 1-day,  10-day,  and
             longer-term  exposure to xylenes in drinking water.  These HAs for a
             10 kg child  were 12.0 mg/L,  1.2 mg/L,  and 0.62 mg/L of xylene,
             respectively.

          0  ACGIH (1981) has recommended  a TWA of 100 ppm.


VII. ANALYTICAL METHODS

          0  Analysis of  xylene(s) is by a purge-and-trap gas  chromatographic pro-
             cedure used  for the determination of volatile aromatic and unsaturated
             organic  compounds in water (U.S.  EPA,  1985b).   This method  calls for
             the bubbling of an inert gas  through the sample and trapping  benzene
             on an adsorbent material.   The adsorbant material is  heated to drive
             off xylene(s) onto a gas chromatographic column.   The gas chromatograph
             is temperature programmed  to  separate the method  analytes which are
             then detected by the photoionization detector.  This method is
             applicable to the measurement of  xylene(s)  over a concentration range
             of 0.02  to 1500 ug/L.  Confirmatory  analysis for  xylene(s)  is  by mass
             spectrometry (U.S. EPA,  1985c).  The detection limit for  confirmation
             by mass  spectrometry is 0.2 ug/L.

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VIII. TREATMENT TECHNOLOGIES

           0  Treatment technologies which will remove  xylene  from water  include
              granulated activated carbon (GAG) and aeration.   Limited data suggest
              that conventional treatment may be partially  effective  in xylene
              removal.

           0  Dobbs and Cohen (1980) developed adsorption isotherms for several
              organic compounds including p-xylene.  It was reported  that Filtrasorb*
              300 carbon exhibited adsorptive capacities of 130 mg, 85 mg,  54 mg
              and 35 mg p-xylene/g carbon when the initial  xylene concentrations
              were 10,  1.0,  0.1  and 0.01  mg/L, respectively.   These values  along
              with Freundlich constants  of K = 85 and 1/n  0.19  indicate that
              p-xylene and its closely related isomers,  o-xylene  and  m-xylene,  should
              be amenable to carbon adsorption.  Powdered activated carbon (PAC)
              added at the well field to xylene-contaminated water containing 0.03
              to 0.5 ug/L removed 60 to  >99% of the xylene  (U.S.  EPA,  1985d).  The
              higher the xylene load the less efficient the adsorption.  GAC was
              slightly less effective when used o" water containing 0.05  ug/L in
              xylene.  In 16 samples tested the average removal efficiency was  50%
              (McCarty et al., 1979a).  When the m-xylene  (0.046  ug/L) and  p-xylene
              (0.012 ug/L) were measured separately only 20% and  17%  removals were
              experienced using adsorption on GAC.  Each of these studies,  however,
              were conducted on wastewater containing a number of organic contaminants
              and therefore are not completely representative  of  what might be
              expected with potable water treatment.

           8  Xylene is amenable to aeration on the basis  of its  Henry's  Law Constant
              of 255 atoms at 20C (U.S. EPA, 1985d).  Although only  19%  of the
              xylene in wastewater could be removed by aeration,  the  process was
              much more successful in the treatment of  potable well water contaminated
              by a gasoline spill (McCarty et al., 1979b).   At air-to-water ratios
              of 17 to 1 or greater, 80  to 100% removal of  all three  xylene isomers
              was accomplished.  At low air to water ratios (8:1), poor removal per-
              formance was experienced.   Average influent concentrations  for the
              o, m and p isomers were 10, 2.9 and 6.9 ug/L, respectively.

           0  Air stripping is an effective, simple and relatively inexpensive
              process for removing xylene and other organics from water.   However,
              use of this process then transfers the contaminant  directly to the
              air stream.  When considering use of air  stripping  as a treatment
              process, it is suggested that careful consideration be  given to the
              overall environmental occurrence, fate, route of exposure,  and various
              hazards associated with the chemical.

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IX. REFERENCES

    ACGIH.   1981.   American Conference of Governmental Industrial Hygienists.
         TLVs - Threshold limit values for chemical substances in workroom air
         adopted by ACGIH for 1981.   Cincinnati,  OH.

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Xylenes                                                    March 31, 1987

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Xylenes                                                    March 31, 1987

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