Agency for Toxic Substances and Disease Registry Fact Sheet contains the following information on arsenic.
Summary: Exposure to higher than average levels of arsenic happens mostly in the workplace near hazardous waste sites, or in areas with high natural levels. Arsenic is a powerful poison. At high levels, it can cause death or illness. This chemical has been found in at least 781 of 1,300 National Priorities List sites identified by the Environmental Protection Agency.