Zone 0, 1, and 2 are classifications for hazardous areas based on the likelihood of an explosive atmosphere being present. 

Zone 0 is the most hazardous, where an explosive atmosphere is present continuously or for long periods. 

Zone 1 is an area where an explosive atmosphere is likely to occur in normal operation. 

Zone 2 is the least hazardous, where an explosive atmosphere is not likely to occur in normal operation and will only exist for a short time if it does.