An EU directive that describes the work that may be safely carried out in an explosive atmosphere. The areas or zones in a process plant are classified according to the type of hazards, the location, and size, and the likelihood of an explosion. It is applied to mining operations, offshore processing, petrochemical plants, and flour mills, where potentially explosive atmospheres may exist. The name is derived from the French title for the EU directive Appareils et systèmes de protection pour les atmosphères explosibles.

http://www.hse.gov.uk/fireandexplosion/atex.htm Official website of the Health and Safety Executive, UK, outlining information on ATEX and explosive atmospheres.