Explosion Hazardous Areas are divided into zones according to risk probabilities. Within the European Union, these are defined by IEC/EN standards (IEC/EN 60079-10-1 or 2) as directed by ATEX.
There are three zones for gas: Zone 0, Zone 1 and Zone 2. And there are three corresponding zones for dust: Zone 20, Zone 21 and Zone 22. Put simply, the 0-zones (0 and 20) are highest risk probability, and 2-zones lowest (2 and 22).