Zone 1, Zone 21
Primary grade of release gives rise to Zone 1 & Zone 21:
Zone 1: As a place in which an explosive atmosphere consisting of a mixture with air or flammable substances in the form of gas, vapours and mists is likely to occur occasionally during normal operation.
Zone 21: As a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation. For example, bagging points and inspection ports that are frequently opened.