Class I Division 1 hazardous areas are locations that are defined by the National Electrical Code (NEC) Handbook with articles 500, 501, 502, 503, 505, and 506.  Class I locations are those in which flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures (NEC 501).  Division 1 is a subset of Class I and is classified as an area where the explosive or flammable gases, vapors or liquids mentioned above can exist under normal, everyday operating conditions.