There are multiple degrees of hazard:
Zone 0: explosive gas-air mixture continuously present, or present for long periods.
Zone 0/1 or Class I, Division 1: hazardous concentrations of flammable gases or vapors continuously, intermittently or periodically present under normal operating conditions.
Zone 1: explosive gas-air mixture is likely to occur in normal operation.
Zone 2: explosive gas-air mixture not likely to occur and, if it occurs, it will exist only a short time.
Zone 2 or Class I, Division 2: volatile flammable liquids or flammable gases present, but normally confined within closed containers or systems, from which they can escape only under abnormal operating or fault conditions.