Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Arsenic
Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.
Every effort should be made to maintain arsenic levels in drinking water as low as reasonably achievable. Arsenic is a human carcinogen, which means that exposure to any level in drinking water may increase the risk of cancer. Subpopulations (such as children and pregnant women) are not at a greater risk of developing health effects from exposure to arsenic than the general population.
The drinking water guideline is based on lifetime exposure to arsenic 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, then it is suggested that alternative sources of water for drinking and food preparation be considered.
The guideline for a carcinogen is normally established at a level where the increased cancer risk is "essentially negligible" when a person is exposed at that level in drinking water over a lifetime (70 years). In the context of drinking water guidelines, Health Canada has defined this term as a range from one new cancer above background per 100 000 people to one new cancer above background per 1 million people (i.e., 10-5 to 10-6 ). In the case of arsenic, the guideline is higher than the concentration that would present an "essentially negligible" risk of internal organ cancers, since it represents the lowest level of arsenic in drinking water that can be technically achieved at reasonable cost, especially for smaller public systems and private wells.
Table 1 lists the estimated lifetime (70 years) risk of excess internal cancers associated with the ingestion of arsenic in drinking water at various concentrations. The overall risk is is reported as a range, because lifetime exposure to arsenic could be linked to several types of cancer, including liver, bladder, and lung cancers.
|Level of Arsenic in Drinking Water (µg/L)
|Estimated Lifetime Range of Risk of Excess Internal Organ CancersTable 1 footnote a (× 10-5)
Table 1 Footnotes
|0.3 ("essentially negligible" risk)
Exposure to the MAC over a lifetime (70 years) may result in an increased risk, as estimated using a population in southwestern Taiwan exposed to very high levels (ranging from 350 to 1140 µg/L) of arsenic in their drinking water; the genetic make-up, health status, arsenic metabolism, and nutritional status of the study population may not be representative of the North American situation. However, the Taiwanese study population was chosen because it represents long-term exposure to arsenic and follow-up, extensive pathology data, homogeneity between lifestyles of the population, and a large population (approximately 40 000 people).
Most existing studies on the health effects of arsenic in drinking water have reported links between internal organ cancers and very high concentrations of arsenic. Recent studies conducted in the United States have not found a clear association between cancer risks and arsenic in drinking water at levels greater than 10 µg/L (and below 50 µg/L). However, these recent studies cannot be used to derive a guideline until their results are confirmed by further research.
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