Arsenic has many uses that are beneficial. The substance is used to treat wood and as a pesticide. But high levels of arsenic can lead cause severe health problems and is a known cause of cancer. Minor arsenic ingestion has little or no effects of most people. The U.S. Environmental Protection Agency (EPA) measures arsenic toxicity and has set limits on arsenic to make sure that dangerous levels don't seep through the soil, air or water. As for water, the EPA's legal level is 10 parts per billion for public water system. In some areas of the country, arsenic levels are naturally higher and the effects on the water supply have been known to increase the levels of skin cancer. Pesticides are major cause of soil contamination because until a few decades ago, many of them contained high levels of arsenic. The EPA has banned most arsenic compounds found in pesticides.