The state divides them into three districts according to the time and quantity of combustible dust during normal operation. It is 20, 21 and 22.

Area 20: During the normal operation, flammable dust appears or often appears, and its number is sufficient to form a places and containers that may not be controlled and/or may not be controlled and/or may not be controlled.

Area 21: During the normal operation, the number of combustible dust may occur in place where combustible dust and air mixture may occur but have not been included in districts 20. This area includes: places where the dust layer and normal operation may produce flammable dust and air mixture where the dust layer and normal operation may produce a place where the dust layer and normal operation occur.

Area 22: Under abnormal conditions, combustible dust occasionally appears and only occasionally exists or flammable dust occasionally accumulate or may exist in the dust layer and produce flammable dusty air mixture places. If it is not guaranteed to eliminate the accumulation of flammable dust or dust layer, it should be divided into areas 21.