Arsenic is one of the chemicals of greatest health concern in some natural waters. Arsenic is found widely in Earth’s crust in oxidation states of –3, 0, +3 and +5, often as sulfides or metal arsenide’s or arsenates. In water, it is mostly present as arsenate (+5), but in anaerobic conditions, it is likely to be present as arsenite (+3). It is usually present in natural waters at concentrations of less than 1–2 μg/l.

In view of the practical difficulties in removing arsenic from drinking-water, as well as the practical quantification limit in the region of 1–10 μg/l, the guideline value of 10 μg/l is retained and designated as provisional.

Arsenic has not been demonstrated to be essential in humans. The acute toxicity of arsenic compounds in humans is predominantly a function of their rate of removal from the body. Arsine is considered to be the most toxic form, followed by the arsenide’s, the arsenates and organic arsenic compounds.

Numerous epidemiological studies have examined the risk of cancers associated with arsenic ingestion through drinking-water. Many are ecological-type studies, and many suffer from methodological flaws, particularly in the measurement of exposure.