At 20 0C, para-xylene (p-xylene) is a liquid that floats on water and is moderately
volatile. Characterized by low solubility, it will volatilize rapidly once dissolved and moderately
adsorbs to organic matter. In soil, pure p-xylene can be found as either a solid or liquid (melting
point of 13 0C). During a spill, this compound will partially evaporate but the majority of the
product will enter into the soil where it may solidify or migrate into a waterway and create a film
on the water surface. Solid 1,4-xylene in the soil will dissolve and volatilize slowly. Once
dissolved, the majority of 1,4-xylene will volatilize, but may also migrate into the water table.
The adsorbed p-xylene in the vadose and saturated zones will take some time to disappear, liberating
contamination primarily in the gaseous, or in the dissolved state. The resulting plumes (gaseous or
dissolved) will be relatively limited in size.
Xylene isomers (ortho-, meta-, and para-) are aromatic hydrocarbons naturally present
in crude petroleum and coal tar. Mixed xylene (a mixture of ethylbenzene and the three isomers of
xylene, where m-xylene predominates) is produced industrially in large amounts from petroleum or
coal tar. Approximately 70 % of mixed xylene is used in the production of ethylbenzene and the o-,
m-, and, especially, p- (five times more than any other) isomers of xylene. The remaining mixed
xylene is primarily used as a solvent in the printing, rubber and leather industries, in products
such as paints and coatings, or blended into airplane fuels and gasoline.
Xylene isomers are used primarily for internal industrial operations as solvents and
intermediates in synthetic reactions. o-Xylene is a chemical intermediate in the synthesis of
phthalic anhydride (for plasticizers), phthalonitrile, 4,4-(trifluoro-1-(trifluoromethyl)
ethylidene) diphthalic anhydride (for polyimide polymers), o-toluic acid, vitamins and
pharmaceuticals. m-Xylene is a chemical intermediate in the production of isophthalic acid and
isophthalonitrile; isophthalic acid, in turn, is used in the manufacture of polyesters. p-Xylene is
a chemical intermediate in the synthesis of terephthalic acid (for polyesters resins and fibers),
dimethyl tetrachloroterephthalate, vitamins and pharmaceuticals. Both o-xylene and p-xylene are used
as components of insecticides.
Because individual xylene isomers are used in large amounts in industrial settings, people who
work or live near these locations may receive a higher exposure to one xylene isomer compared to the
other isomers. However, since xylenes are present as a mixture in gasoline and in the solvent
components of commonly used commercial products (paint, coatings, etc.), exposure of the general
population is expected to be primarily to xylenes as a mixture, and not to the separate xylene
isomers. Xylenes may be introduced into groundwater by fuel oil, gasoline or solvent spills, leaking
underground petroleum storage tanks, or leaching from disposed wastes.