Arsenic has been classified by the U.S. Environmental Protection Agency as a human carcinogen. Long term exposure to arsenic has been linked to cardiovascular disease, immune disorders, diabetes, cancer and other medical issues. Arsenic occurs naturally in two forms, arsenic –three and arsenic-five. Many types of filtration can be effective at removing arsenic-five, including carbon, but arsenic-three can be more difficult. It is also more dangerous to humans.