What are Hazardous Areas?
Simply put, a hazardous area is a three-dimensional space where ignitable concentrations of flammable gases, vapors, mists, dusts, or fibers are, or may be expected to be, present in quantities sufficient to require special precautions for the construction, installation, and use of equipment. Think of areas around fuel storage tanks, chemical processing plants, woodworking facilities, or grain silos.

The Role of Substances: Fueling the Hazard
The very existence and characteristics of the hazardous substance are the primary drivers for defining and categorizing these areas. Different substances possess varying degrees of flammability and explosibility. Key properties that dictate the level of hazard include:
- Lower Explosive Limit (LEL) / Lower Flammable Limit (LFL): The minimum concentration of a flammable substance in air below which an explosion or propagation of flame will not occur in the presence of an ignition source. A lower LEL indicates a higher risk, as less substance is needed to form an ignitable mixture.
- Upper Explosive Limit (UEL) / Upper Flammable Limit (UFL): The maximum concentration of a flammable substance in air above which an explosion or propagation of flame will not occur.
- Auto-ignition Temperature (AIT): The minimum temperature at which a flammable substance in air will spontaneously ignite without the presence of an external ignition source. Lower AITs signify a greater risk of accidental ignition.
- Maximum Experimental Safe Gap (MESG): For gases and vapors, this represents the maximum gap width of a joint in a flameproof enclosure that, under specified test conditions, will prevent an internal explosion from propagating to the surrounding explosive atmosphere. This is crucial for selecting appropriately rated equipment.
- Minimum Ignition Energy (MIE): For dusts and fibers, this is the lowest electrical energy stored in a capacitor that is discharged as a spark capable of igniting the most easily ignitable mixture of the substance with air. Lower MIE values indicate a higher ignition sensitivity.

Categorization: Zoning the Danger
Based on the likelihood and persistence of the hazardous atmosphere, areas are classified into different zones (for gases, vapors, and mists) or divisions (in some systems, particularly in North America) and zones (for dusts and fibers). These categories dictate the type of equipment and installation practices required to ensure safety.
For Gases, Vapors, and Mists:
- Zone 0 / Division 1: An area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is present continuously, for long periods, or frequently. Examples include the vapor space above flammable liquids in open tanks or areas with frequent equipment leaks.
- Zone 1 / Division 1: An area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is likely to occur in normal operation occasionally. This could include areas near filling connections or where releases might occur during maintenance.
- Zone 2 / Division 2: An area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is not likely to occur in normal operation but, if it does occur, will persist for only a short period. This might include areas with contained systems where releases are infrequent and accidental.
For Dusts and Fibers:
- Zone 20 / Division 1 (Dust): An area in which a cloud of combustible dust in air is present continuously, or frequently, or for long periods. Examples include inside dust collectors or areas with continuous powder processing.
- Zone 21 / Division 1 (Dust): An area in which a cloud of combustible dust in air is likely to occur in normal operation occasionally. This could be near bag filling stations or where dust layers can easily be dispersed.
- Zone 22 / Division 2 (Dust): An area in which a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for only a short period. This might include areas where dust accumulation is minimal and infrequent.

The Interplay: Substance Properties Drive Categorization
The crucial link here is that the properties of the hazardous substance directly influence the extent and category of the hazardous area. For instance:
- A substance with a very low LEL will likely result in larger and more severe hazardous zones because only a small leak can create an explosive atmosphere over a significant area.
- Substances with low auto-ignition temperatures demand stricter temperature control and equipment ratings across all zones.
- The MESG of a gas or vapor dictates the type of flameproof enclosure required for equipment used in specific zones.
