Arsenic is a unique carcinogen. It is the only known human carcinogen for
which there is adequate evidence of carcinogenic risk by both inhalation
and ingestion.While arsenic is released to the environment from natural
sources such as wind-blown dirt and volcanoes, releases from anthropogenic
sources far exceed those from natural sources. Oral exposure of
arsenic to human beings however, is usually not the result of anthropogenic
activity as it is with many carcinogens, but the result of natural
contamination of well-water supplies by arsenic-rich geologic strata.