Page 12: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Toluene, Ethylbenzene and the Xylenes

11.0 Rationale

Toluene, ethylbenzene and xylenes are volatile, flammable and colourless liquids used primarily in the synthesis of specific chemical compounds or as industrial solvents. All three compounds occur naturally in small quantities in crude oil and are further added to gasoline and other fuels. Entry into drinking water sources can occur from leaching of fuel at various sites, including refineries and fuel filling stations, as well as from spills during the transportation and storage of fuels. Given the volatility of toluene, ethylbenzene and xylenes, the contribution from inhalation and dermal exposure during bathing and showering was also estimated using a multiroute exposure approach. As part of its ongoing guideline review process, Health Canada will continue to monitor new research and recommend any change to the guidelines that is deemed necessary.

11.1 Toluene

Toluene is considered to be not classifiable with regard to carcinogenicity due to insufficient animal and human carcinogenicity data. The health effects of toluene in occupationally exposed workers include an array of neurological effects, such as loss of colour vision as well as disturbances in memory, concentration and cognitive function. Oral exposure in animals has resulted in altered behaviour, changes in neurotransmitter levels and brain necrosis, which support adverse neurological effects as a critical endpoint of toluene toxicity.

The MAC for toluene in drinking water has been determined to be 0.06 mg/L (60 µg/L), based on several neurological endpoints reported in human occupational studies.

The lowest reported odour threshold for toluene is 0.024 mg/L (24 µg/L); this is close to the MAC, indicating that toluene may not be detected by smell at the MAC.

11.2 Ethylbenzene

Ethylbenzene is considered to be possibly carcinogenic to humans based on sufficient evidence of carcinogenicity in experimental animals but inadequate data in humans. The health effects of ethylbenzene in humans are relatively unknown, due to the lack of occupational settings with predominant exposure to ethylbenzene. Studies of rats and mice exposed to ethylbenzene via inhalation and ingestion have identified liver and kidney as primary targets for ethylbenzene. Inhalation and ingestion of ethylbenzene in rats and mice lead to enlarged liver and kidney, effects on the pituitary gland, as well as increased severity of nephropathy. Chronic exposure of animals by inhalation and ingestion also suggests tumour formation at various sites, including the liver, kidney and lung. As a result, both cancer and non-cancer endpoints were considered in deriving the MAC.

A MAC of 0.14 mg/L (140 µg/L) for ethylbenzene in drinking water has been determined based on kidney effects and body weight reduction in rats and on liver and pituitary gland effects in mice; this MAC is protective of both cancer and non-cancer endpoints.

The lowest reported odour threshold for ethylbenzene is 0.0016 mg/L (1.6 µg/L); this is much lower than the MAC, indicating that drinking water containing ethylbenzene will become unpalatable at a concentration much lower than that which may cause adverse health impacts.

11.3 Xylenes

The primary health effects associated with exposure to xylenes in animals are effects on the central nervous system by all routes of exposure, effects on the respiratory tract following inhalation exposure and hepatic, renal and body weight effects following higher oral exposures. Occupational exposure studies have also reported neurological effects in workers.

A MAC of 0.09 mg/L (90 µg/L) for xylenes in drinking water has been determined based on neuromuscular effects in rats.

The lowest reported odour threshold for xylenes is 0.02 mg/L (20 µg/L); this is close to the MAC, indicating that xylenes may not be detected by smell at the MAC.

11.4 Analytical and treatment considerations

The MACs and aesthetic objectives for toluene, ethylbenzene and xylenes can be measured by available analytical methods and are achievable by municipal and residential treatment technologies. A number of residential treatment devices are available to remove toluene, ethylbenzene and xylenes from drinking water.

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