Page 12: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Benzene

Part II. Science and Technical Considerations - Continued

11.0 Rationale

The guideline for benzene is established based on cancer end-points and is considered protective of both cancer and non-cancer end-points. In establishing this guideline, the Federal- Provincial-Territorial Committee on Drinking Water also took into consideration the need for the guideline to be measurable and achievable, as well as levels of exposure from drinking water in Canada. Both animal and human epidemiological studies report similar toxic effects following exposure to benzene, regardless of exposure pathway (inhalation or ingestion). The most sensitive end-points resulting from exposure to benzene in both animals and humans are those related to the blood-forming organs.

Benzene can be found in both surface water and groundwater; in surface water, benzene tends to volatilize into the atmosphere, although ice cover may interfere with this process in winter. Benzene may enter water through petroleum seepage and weathering of exposed coalcontaining strata and may enter air from volcanoes, forest fires, releases from plants, and anthropogenic sources. In Canada, levels of benzene in raw water sources have been reported as ranging between 0.02 and 0.42 µg/L, and levels in treated drinking water are generally less than 1 µg/L unless near a contamination source.

Several municipal-scale treatment processes can remove benzene from drinking water to levels below 0.005 mg/L. At the residential scale, drinking water treatment devices are available that have been certified as reducing concentrations of VOCs such as benzene to 0.001 mg/L, well below the MAC of 0.005 mg/L.

Based on the incidence of malignant lymphoma and bone marrow effects in animals following exposure to benzene by ingestion, the estimated lifetime risk associated with ingestion of water containing benzene at the MAC of 0.005 mg/L is 1.02 × 10-5 - 2.08 × 10-5 (derived by multiplying the unit risk by the MAC). The estimated lifetime cancer risk associated with the ingestion of drinking water containing benzene at 5 µg/L is partly within the range considered generally to be "essentially negligible," although the upper level of the risk range at the MAC extends beyond that range. The guideline for a carcinogen is normally established at a level at which the increased cancer risk is "essentially negligible" when a person is exposed at that level in drinking water over a lifetime. In the context of drinking water guidelines, Health Canada has defined this term as a range from one new cancer above background levels per 100 000 people to one new cancer above background levels per 1 million people (i.e., 10-5-10-6) Because exposure from drinking water represents only a small fraction (1-2%) of the total exposure to benzene, this slight exceedance in risk at the MAC is deemed acceptable. The estimated unit risks from the human epidemiological data overlap those estimated from the animal data, providing additional support for a MAC of 0.005 mg/L (5 µg/L) for benzene in drinking water.

In summary, the MAC of 0.005 mg/L (5 µg/L) for benzene was established on the basis that:

  • it is considered to present an "essentially negligible" risk;
  • it is measurable, with an estimated PQL of 0.4 μg/L;
  • treatment (both municipal and residential) is achievable at a reasonable cost. As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area and recommend any change(s) to the guideline that it deems necessary.
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