When an area is evaluated for hazardous area classification on the basis of the presence of dust particles, it’s called zone system. There are three different zones categorized on the basis of hazardous material risks. Zone 20 is where ignitable filers or combustible dust particles have been present for a long time.
Zone 21 is the area where dust particles are present only during operational conditions. Zone 22 is the area where there are no harmful dust particles present. Even if they are present, it’s for a very short period.