Arsenic in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
Provisional guideline value
Inorganic arsenic compounds are classified by IARC (1987) in Group 1 (carcinogenic to humans) on the basis of sufficient evidence for carcinogenicity in humans and limited evidence for carcinogenicity in animals.
Although there is a substantial database on the association between both internal and skin cancers and the consumption of arsenic in drinking-water, there remains considerable uncertainty over the actual risks at low concentrations. US NRC (2001), in its updated evaluation, concluded “that the available mode-of-action data on arsenic do not provide a biological basis for using either a linear or nonlinear extrapolation.” The maximum likelihood estimates, using a linear extrapolation, for bladder and lung cancer for US populations exposed to 10 µg of arsenic per litre in drinking-water are, respectively, 12 and 18 per 10 000 population for females and 23 and 14 per 10 000 population for males. The actual numbers, indicated by these estimated risks, would be very difficult to detect by current epidemiological methods. There is also uncertainty over the contribution of arsenic in food — a higher intake of inorganic arsenic from food would lead to a lower risk estimate for water — and the impact of factors such as variation in the metabolism of arsenic and nutritional status. It remains possible that the estimates of cancer risk associated with various arsenic intakes are overestimates.
The concentration of arsenic in drinking-water below which no effects can be observed remains to be determined, and there is an urgent need for identification of the mechanism by which arsenic causes cancer, which appears to be the most sensitive toxicity end-point.
The practical quantification limit for arsenic is in the region of 1–10 µg/litre, and removal of arsenic to concentrations below 10 µg/litre is difficult in many circumstances. In view of the significant uncertainties surrounding the risk assessment for arsenic carcinogenicity and the practical difficulties in removing arsenic from drinking-water, the guideline value of 10 µg/litre is retained. In view of the scientific uncertainties, the guideline value is designated as provisional. In many countries, this guideline value may not be attainable; where this is the case, every effort should be made to keep concentrations as low as possible.