Xylenes
CASRN 1330-20-7 (endorsed 2013)
Guideline
Based on aesthetic considerations (taste and odour), the concentration of xylenes in drinking water should not exceed 0.02 mg/L.
Based on health considerations the concentration of xylenes should not exceed 0.6 mg/L.
General description
The term ‘xylenes’ encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- (o-) [CASRN 95-47-6], meta- (m-) [CASRN 108-38-3], and para- (p-) [CASRN 106-42-3], which specify to which carbon atoms of the benzene ring the two methyl groups are attached. o-Xylene is also known as 1,2-dimethylbenzene, m-xylene is also known as 1,3-dimethylbenzene, and p-xylene is also known as 1,4-dimethylbenzene. The mixture is a slightly greasy, colourless liquid commonly encountered as a solvent.
Xylenes occurs naturally in petroleum and coal tar and represent about 0.5–1% of crude oil, depending on the source (hence xylenes are found in small amounts in petrol and aviation fuels). Xylenes can also be formed naturally during forest fires (ATSDR, 2007). It is mainly produced from reformate, but is also obtained from coal carbonisation derived from coke ovens.
Xylenes have a taste and odour threshold of 0.02 mg/L.
Typical values in Australian drinking water
Xylenes have only rarely been identified in Australian drinking waters. Natural concentrations in most water sources are usually very low. Xylenes can occur naturally in groundwater as a result of proximity to, or contact with, coal seams, petroleum and gas deposits, and shales. It may be mobilised by extraction activities (Lesage et al., 1997; Leusch and Bartkow, 2011; Volk et al, 2011). However, contamination can occur, usually via exposure to petrochemicals in surface waters or groundwater. Known sources of groundwater contamination include leakage from sub-surface fuel storage tanks (do Rego & Netto, 2007) and nearby hydrocarbon deposits (IPCS, 1997). Emissions of fuel components from boating use is a known source of contamination of multiple-use lakes and reservoirs (Schmidt et al., 2004). Xylenes were reported in 3% of samples from an extensive groundwater survey undertaken in Denmark with the highest concentration being 0.00003 mg/L (Juhler & Felding, 2003). Concentrations in groundwater in the USA were generally <0.001 mg/L, but were as high as 1 mg/L in contaminated areas (IPCS, 1997; ATSDR, 2007). Xylenes have been reported at up to 0.0005 mg/L in municipal drinking water in Croatia (Karaconji et al., 2006), and are occasionally detected in drinking waters in the USA (Williams et al., 2004), up to a maximum of 0.012 mg/L (IPCS, 1997).
Treatment of drinking water
Volatile organic chemicals such as xylenes are most commonly treated in drinking water by aeration stripping and/or adsorption to granular activated carbon (GAC). A conventional biologically active sand filter has been shown to be highly effective for the removal of xylenes from contaminated water, under suitable conditions (Arvin et al., 2004). Effective bioremediation of highly contaminated groundwaters has also been demonstrated (Sedran et al., 2004; Zein et al., 2006).
Measurement
A purge and trap gas chromatographic procedure can be used for the analysis of xylenes (APHA, AWWA & WEF, 2012). An inert gas is bubbled through the sample and xylenes are trapped on an adsorbent. The adsorbent is then heated and xylenes analysed using gas chromatography with mass spectrometric (GC-MS) detection (Method 6200 B) or photoionisation (PI) detection (Method 6200 C) (APHA, AWWA & WEF, 2012). The method detection limit is 38 ng/L for GC-MS and 24 ng/L for GC-PI (APHA, AWWA & WEF, 2012).
Health considerations
Xylenes are readily absorbed after inhalation and metabolised almost completely to methyl benzoic acid. They can cross the placenta. No data are available on human absorption after ingestion, or on health effects of oral exposure in humans.
A 2-year gavage study using rats and mice reported decreased growth at high doses (500 mg/kg body weight per day) but no xylene-related lesions (NTP 1986). There was no evidence of carcinogenicity in oral and skin administration studies using rats and mice, and xylenes were not mutagenic in tests using bacteria and mammalian cells.
The International Agency for Research on Cancer has concluded that xylenes are not classifiable as to their carcinogenicity in humans (Group 3, inadequate evidence in humans and in animals) (IARC 1989).
Derivation of guideline
The USEPA (2009) has set a drinking water guideline of 10 mg/L for total xylenes, while the WHO (2011) proposes a guideline of 0.5 mg/L.
The health-based guideline value for xylenes in drinking water was determined as follows:
where:
250 mg/kg body weight per day is the no effect level based on a 2-year gavage study using rats (NTP 1986).
70 kg is the average weight of an adult
0.1 is the proportion of total daily intake attributable to the consumption of water
2 L/day is the average amount of water consumed by an adult
1000 is the safety factor in using the results of an animal study as a basis for human exposure (10 for interspecies variations, 10 for intraspecies variations and 10 for the limited toxicological end point)
5/7 is used to convert data based on a 5 day per week feeding study to a 7-day week equivalent.
The WHO guideline value of 0.5 mg/L is based on an adult body weight of 60 kg. The difference in guideline values is not significant.
The health-based guideline value exceeds the taste and odour threshold of xylenes in water of 0.02 mg/L.
References
American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF) (2012). Standard Methods for the Examination of Water and Wastewater, 22nd Edition. Eds. Rice EW, Baird RB, Eaton AD and Clesceri LS.
Arvin, E., Engelsen, P. and Sebber, U. (2004) Biodegradation of gasoline compounds (BTEX) in a water works sand filter. Water Science & Technology: Water Supply, 4(5-6), 29-33.
ATSDR (2007). Toxicological profile for xylene. US Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, USA.
do Rego, E. C. P. and Netto, A. D. P. (2007) PAHs and BTEX in groundwater of gasoline stations from Rio de Janeiro City, Brazil. B. Environ. Contam. Tox., 79(6), 660-664.
IARC (International Agency for Research on Cancer) (1989) IARC Monographs on the Evaluation of Carcinogenic Risks to Humans: some organic solvents, resin monomers and related compounds, pigments and occupational exposures in paint manufacture and painting., Lyon.
IPCS (1997) Environmental Health Criteria 190: Xylenes. International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland.
Juhler, R. K. and Felding, G. (2003) Monitoring methyl tertiary butyl ether (MTBE) and other organic micropollutants in groundwater: Results from the Danish National Monitoring Program. Water Air Soil Poll., 149(1-4), 145-161.
Karaconji, B., Skender, L. and Karacic, V. (2006) Benzene, toluene, ethylbenzene, and isomeric xylenes in various water samples in Croatia. B. Environ. Contam. Tox., 76(3), 458-462.
Lesage, S., Hao, X., and Kent, S. (1997) Distinguishing Natural Hydrocarbons from Anthropogenic Contamination in Ground Water. Groundwater, 35(1), 149-160
Leusch, F. and Bartkow, M. (2010) A short primer on benzene, toluene, ethylbenzene and xylenes (BTEX) in the environment and in hydraulic fracturing fluids http://www.ehp.qld.gov.au/management/coal-seam-gas/pdf/btex-report.pdf (accessed 19 March 2013)
NTP (National Toxicology Program) United States Department of Health and Human Services (1986) Toxicology and carcinogenesis gavage studies of xylenes (mixed) in F344/N rats and B6C3F1 mice. NTP Report No. 327.
Schmidt, T. C., Haderlein, S. B., Pfister, R. and Forster, R. (2004) Occurrence and fate modeling of MTBE and BTEX compounds in a Swiss Lake used as drinking water supply. Water Res., 38(6), 1520-1529.
Sedran, M. A., Pruden, A., Wilson, G. J., Suidan, M. T. and Venosa, A. D. (2004) Biodegradation of methyl tert-butyl ether and BTEX at varying hydraulic retention times. Water Environ. Res., 76(1), 47-55.
USEPA (2009) National Primary Drinking Water Regulations. United States Environmental Protection Agency, Washington, DC, USA.
WHO (2011) Guidelines for Drinking Water Quality, 4th Edition. World Health Organisation, Geneva, Switzerland.
Williams, P. R. D., Benton, L. and Sheehan, P. J. (2004) The risk of MTBE relative to other VOCs in public drinking water in California. Risk Anal., 24(3), 621-634.
Zein, M. M., Suidan, M. T. and Venosa, A. D. (2006) Bioremediation of groundwater contaminated with gasoline hydrocarbons and oxygenates using a membrane-based reactor. Environ. Sci. Technol., 40(6), 1997-2003.
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