Page 3: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Toluene, Ethylbenzene and the Xylenes
2.0 Executive summary
Toluene, ethylbenzene and xylenes are primarily synthetic chemicals. These compounds are mainly found in petroleum hydrocarbons, such as gasoline and diesel fuel, or used as industrial solvents or as intermediates in styrene or benzene production. They can enter drinking water through leaching from contaminated sites or from industrial discharges of chemical manufacturing plants, or as a result of a spill during transportation or storage.
Because these chemicals are generally found together, and based on their similar chemical properties and treatment strategies, they have been grouped in one guideline technical document although individual guideline values are derived for each substance. This guideline technical document reviews and assesses all identified health risks associated with toluene, ethylbenzene and xylenes in drinking water, incorporating all relevant routes of exposure from drinking water—namely, ingestion as well as inhalation and skin absorption from showering and bathing.
It assesses new studies and approaches and takes into consideration the availability of appropriate treatment technology in order to establish maximum acceptable concentrations that are protective of human health, measurable and achievable by both municipal and residential scale treatment technologies. Based on this review, the drinking water guidelines are maximum acceptable concentrations of 0.06 mg/L (60 µg/L) for toluene, 0.14 mg/L (140 µg/L) for ethylbenzene, and 0.09 mg/L (90 µg/L) for xylenes.
2.1 Health effects
There is currently insufficient information from both animal and human studies to determine whether toluene is carcinogenic to humans. The health effects of toluene have been studied in humans in various occupational settings. These studies have revealed an array of neurological effects including loss of colour vision, and disturbances in memory, concentration and cognitive function in general upon long-term inhalation of toluene. Studies of oral exposure in animals support adverse neurological effects as the endpoint of concern to derive a guideline for toluene in drinking water.
Ethylbenzene is classified as possibly carcinogenic to humans, based on sufficient evidence of carcinogenicity in experimental animals but inadequate data in humans. Chronic studies in rats and mice suggest that exposure to ethylbenzene may lead to tumour formation at various sites, including kidney (male and female rats), lung (male mice), liver (female mice) and testes (male rats).
As for non-cancer health effects, animal data has identified liver and kidney as primary targets of ethylbenzene. Inhalation and ingestion of ethylbenzene in rats and mice lead to enlarged liver and kidney, increased severity of renal disease and effects on the pituitary gland. Effects on the liver and pituitary gland have been identified as the endpoints of concern to derive a guideline for ethylbenzene. There are little data available on the effects of ethylbenzene in humans, due to the lack of occupational settings in which exposure to ethylbenzene is predominant.
Xylenes are not classifiable with respect to carcinogenicity in humans, based on insufficient information. Data from both animal and human studies show that the primary health effects following exposure to xylenes will depend on the route of exposure: xylenes can affect the central nervous system by all routes of exposure, the respiratory tract following inhalation exposure, and the liver, kidney and body weight following oral exposures. Adverse neurological effects have been identified as the endpoint of concern to derive a guideline for xylenes in drinking water.
2.2 Aesthetic considerations
The presence of toluene, ethylbenzene and xylenes can be detected by their odour when present in water at levels below the MACs. Although there are no adverse effects associated with these levels, they will affect the acceptability of the water by consumers. The aesthetic objectives for these compounds are established at the level of the lowest reported odour thresholds, specifically 0.024 mg/L (24 µg/L) for toluene, 0.0016 mg/L (1.6 µg/L) for ethylbenzene, and 0.02 mg/L (20 µg/L) for xylenes.
The general population is primarily exposed to toluene, ethylbenzene and xylenes from inhalation of ambient air and usage of various products including gasoline and diesel fuels and solvents. To a lesser extent, exposure can also occur from drinking water, contaminated soil, or from food. Toluene, ethylbenzene and xylenes are rarely detected in Canadian drinking water supplies. Canadians may also be exposed to toluene, ethylbenzene and xylenes from drinking water through inhalation and dermal absorption.
2.4 Analysis and treatment
The establishment of a drinking water guideline must take into consideration the ability to both measure the contaminant and remove it from drinking water supplies. Toluene, ethylbenzene and xylenes can be reliably measured in drinking water at the MACs.
At the municipal level, conventional treatment techniques are not effective at removing toluene, ethylbenzene and xylenes. The best available technologies for removing all three compounds from drinking water are packed tower aeration and granular activated carbon. Taking into consideration currently available technologies, municipal treatment plant are expected to be able to consistently achieve concentrations below the MACs and aesthetic objectives.
At the residential scale, there are certified point-of-use treatment devices available that can remove volatile organic chemicals (VOCs) such as toluene, ethylbenzene and xylenes from drinking water to meet the MACs and aesthetic objectives. They rely on adsorption (activated carbon) and reverse osmosis technologies. Filtration systems may be installed at the faucet (point-of-use) or at the location where water enters the home (point-of-entry). Point-of-entry systems are preferred for the reduction of VOCs, because they provide treated water for bathing and laundry as well as for cooking and drinking. This will reduce the potential for VOC exposure through inhalation.
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