Arsenic has not been demonstrated to be essential in humans. It is an important
drinking-water contaminant, as it is one of the few substances shown to cause cancer
in humans through consumption of drinking-water. There is overwhelming evidence
from epidemiological studies that consumption of elevated levels of arsenic through
drinking-water is causally related to the development of cancer at several sites, particularly
skin, bladder and lung. In several parts of the world, arsenic-induced disease,
including cancer, is a significant public health problem. Because trivalent inorganic
arsenic has greater reactivity and toxicity than pentavalent inorganic arsenic, it is generally
believed that the trivalent form is the carcinogen. However, there remain considerable
uncertainty and controversy over both the mechanism of carcinogenicity
and the shape of the dose–response curve at low intakes. Inorganic arsenic compounds
are classified by IARC in Group 1 (carcinogenic to humans) on the basis of sufficient
evidence for carcinogenicity in humans and limited evidence for carcinogenicity in
animals.