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

11.0 Rationale

Humans are exposed to many forms of arsenic that have different toxicities. The acute toxicity of the various arsenic compounds in humans is predominantly a function of their rate of removal from the body. Metallic arsenic (0 valence) is not absorbed from the stomach and as such does not have any adverse effect. Inorganic arsenic has historically been considered to be the predominant form of arsenic responsible for toxic and carcinogenic effects in humans. Inorganic arsenic that is not immediately removed from the body may enter a methylation pathway, which was believed to be a detoxification process. Although some organic arsenic compounds have little or no toxicity or are rapidly eliminated from the body in the urine, forms such as MMAIII and DMAIII have recently been found to be more toxic than inorganic arsenite (As(III)); however, further research is required to confirm these findings. There is no evidence that children or other groups such as pregnant women are at a greater risk of developing health effects from exposure to arsenic compared with the general population.

Arsenic can be found in both surface water and groundwater sources, with levels generally higher in groundwater. Most provinces and territories across Canada report some areas where arsenic can be detected in drinking water supplies. Levels of arsenic tend to be higher in groundwater than in surface water. Levels of arsenic naturally found in waters generally range between 0.001 and 0.002 mg/L, but arsenic may occur in much higher concentrations. Data collected indicate that the levels of arsenic in Canadian drinking water are generally less than 0.005 mg/L.

Several advanced municipal-scale treatment processes can remove arsenic from drinking water to levels of 0.001-0.005 mg/L. However, given their complexity and cost, these processes may not be practical for smaller communities. Alternative processes, such as adsorption and membrane systems, are suitable for reduction of arsenic to low concentrations (<0.003 mg/L) in small to mid-sized communities. At residential scale, drinking water treatment devices available to date have been certified as reducing arsenic concentrations to 0.01 mg/L, although lower levels may be achieved with the use of these devices.

Since arsenic is classified in Group 1 (carcinogenic to humans), the MAC is derived based on the estimated lifetime cancer risk; consideration was also given to available practical treatment technology and the PQL.

A MAC of 0.01 mg/L (10 µg/L) for arsenic is established on the basis of the following considerations:

  • The concentration of arsenic in drinking water representing an "essentially negligible" risk is 0.3 µg/L. Levels of arsenic in drinking water should be as close as possible to this level.
  • The MAC must be measurable. The PQL, based on the ability of laboratories to measure arsenic within reasonable limits of precision and accuracy, is 0.003 mg/L.
  • The MAC must be achievable at reasonable cost. Both municipal-scale and residential-scale treatment options can remove arsenic from drinking water to below the guideline value.

The estimated lifetime cancer risk associated with the ingestion of drinking water containing arsenic at 0.01 mg/L (10 µg/L) is greater than the range that is considered generally to be "essentially negligible" (i.e., between 10-5 and 10-6). Based on the incidence of internal (lung, bladder, liver) cancers in individuals in southwestern Taiwan, the estimated lifetime risk associated with ingestion of water containing arsenic at 0.01 mg/L (10 µg/L) is 3.0 × 10-5 to 3.9 × 10-4 (derived by multiplying the unit risk by the MAC).

Although arsenic is a documented human carcinogen, limited data on the mode of action of arsenic do not strongly justify the use of either a linear or non-linear quantitative risk assessment model. The use of a non-linear extrapolation method to estimate the risks of internal organ cancers from exposure to low levels of arsenic as well as confounding factors (e.g., genetic differences, differences in health status, arsenic metabolism, and nutritional status of the southwestern Taiwanese study population) may lead to an overestimate of the risks of internal organ cancers. Although some recent studies in the United States have found no clear association between lung and bladder cancer risks and arsenic levels in drinking water between 0.01 and 0.05 mg/L, the weight of evidence still lies with the southwestern Taiwanese cohort data. Given the current uncertainties, the carcinogenic potential of arsenic, and the different practical difficulties associated with removing arsenic from drinking water at the small municipal and residential levels, every effort should be made to reduce arsenic levels in drinking water to as low as reasonably achievable.

In considering both the treatment costs associated with achieving arsenic concentrations in drinking water at or below the health-based guideline value and the health risks associated with concentrations of arsenic in drinking water above the guideline value, the Federal-Provincial-Territorial Committee on Drinking Water has concluded that a MAC of 0.01 mg/L (10 µg/L) should be adopted. This value is the result of a risk management decision, since it exceeds the health-based guideline value.

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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