Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Benzene

Part I. Overview and Application - Continued

3.0 Application of the guideline

Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.

Benzene is a human carcinogen, which means that exposure to any level in drinking water may increase the risk of cancer. Jurisdictions may establish more stringent limits than the MAC.

Generally, benzene is not a concern for the majority of Canadians who rely on surface water as their source of drinking water, because it volatilizes easily. However, ice cover in the winter may pose a concern, since it will impair benzene volatilization from surface waters. Benzene is not a widespread problem in Canada, affecting only some groundwater supplies, but accidental releases of benzene may occur at any stage of the production, storage, use, and transport of isolated benzene and crude oil and gasoline, including emissions resulting from fuel combustion.

The drinking water guideline is based on lifetime exposure (70 years) to benzene from drinking water. For drinking water supplies that occasionally experience short-term exceedances above the guideline value, it is suggested that a plan be developed and implemented to address these situations. For more significant, long-term exceedances that cannot be addressed through treatment, it is suggested that alternative sources of water for drinking, showering, and bathing be considered.

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). 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. 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 MAC is also achievable and measurable with current treatment technology.

The overall risk associated with exposure to benzene in drinking water is reported as a range (Table 1), since lifetime exposure to benzene has been linked to several types of cancers in animals.
Table 1: Estimated lifetime range of risk of excess cancers associated with various concentrations of benzene in drinking water.
Benzene levels in drinking water (µg/L) Estimated lifetime range of risk of excess cancersa (×10-6)Table 1 Footnote a

Table 1 footnotes

Table 1 footnote a

The estimated lifetime risk of excess cancer is calculated from the risk range associated with ingesting 1 µg/L of benzene in drinking water. This estimated unit risk range is 2.03 × 10-6 to 4.17 × 10-6, with the lower bound representing malignant lymphoma and the upper bound representing bone marrow tumours in mice (Health Canada, 2005a).

Return to Table 1 footnote a referrer

1 2.0-4.2
5 10.1-20.8
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