A mixture of combustible substances in the form of gas, vapour, dust or fibres with air, under atmospheric conditions in which – after ignition – combustion spreads to the entire unburned mixture.

§ 5 of the Polish Regulation of the Minister of Economy dated 8 July 2010 and concerning the Minimum Occupational Health and Safety Requirements for the Potential of Explosive Atmosphere at Workplaces (Dz.U. 2010.137.931; ATEX 137) states in Section 4.4:

The employer shall perform a comprehensive assessment of the (explosion) risks related to the potential of an explosive atmosphere to form by considering:

• The probability and duration of presence of an explosive atmosphere;

• The probability of ignition sources to occur and become active;

• The installations in operation, the substances and mixtures used, and the processes carried out;

• The magnitude of the foreseeable consequences of an explosion.

§ 5 of the Polish Regulation of the Minister of the Interior and Administration of dated 7 June 2010 and concerning the Fire Protection of Buildings, Civil Structures, and Land (Dz.U. 2010.109.719) states in Section 37:

For the buildings and their on adjoining lands where processing is carried out with materials that may form explosive mixtures […], an explosion hazard assessment shall be performed.

The explosion hazard assessment shall include classification of Ex (explosion hazard) zones. Correct Ex zone classification is critical.