Gas

Zone 0
relates to areas in which a potentially explosive atmosphere consisting of a mixture of air and gases, vapours or mist exists continuously, for long periods or frequently.

Zone 1
relates to areas in which it can be considered that a potentially explosive atmosphere of gases, vapours or mist occurs occasionally.

Zone 2
relates to areas in which it is unlikely that a potentially explosive atmosphere of gases, vapours or mist might occur, but if it does occur then in all probability only seldom and for a short period.

Dust

Zone 20
relates to areas in which a potentially explosive atmosphere consisting of a mixture of dust and air exists continuously, for long periods or frequently.

Zone 21
relates to areas in which it can be considered that a potentially explosive atmosphere of a mixture of dust and air occurs occasionally.

Zone 22
(note the difference between conductive and non-conductive dust!) relates to areas in which it is unlikely that a potentially explosive atmosphere of suspended dust might occur, but if it does occur then in all probability only seldom and for a short period.