Hazardous Area Classification Explained: Zones, Risks, and Equipment Selection

Industrial facilities that handle flammable gases, vapors, mists, or combustible dusts must assess where explosive atmospheres may occur. This process is known as hazardous area classification.

Hazardous area classification defines the extent of areas where an explosive atmosphere may be present and provides an engineering basis for selecting suitable explosion-protected equipment.

For electrical installations, the classification is closely connected with equipment selection, installation, inspection, and maintenance requirements.

In this guide, we explain how hazardous areas are classified, what Zone 0, Zone 1, Zone 2, Zone 20, Zone 21, and Zone 22 mean, which factors influence classification, and how the classification affects Ex equipment selection.

What Is Hazardous Area Classification?

Hazardous area classification is the process of identifying locations where an explosive atmosphere may occur and determining the extent and classification of those locations.

The classification depends on factors such as:

• The type of flammable substance

• The characteristics of the release source

• The frequency and duration of a release

• Ventilation conditions

• Operating and process conditions

• The physical characteristics of the installation

For gas and vapor atmospheres, IEC 60079-10-1 provides the basis for classification of areas where flammable gas or vapor hazards may arise. The resulting classification can then support the selection and installation of suitable equipment. :chatgpt-content-reference{index="3"}

For combustible dust atmospheres, the corresponding classification standard is IEC 60079-10-2. The current 2026 edition addresses explosive dust atmospheres and combustible dust layers. :chatgpt-content-reference{index="4"}

If you are new to hazardous locations, you may also want to read our guide on What Is Explosion Proof Equipment?

What Is an Explosive Atmosphere?

An explosive atmosphere is a mixture of air and flammable substances in the form of gases, vapors, mists, or dusts that, after ignition, can propagate combustion through the mixture.

Examples of materials that can create explosive atmospheres include:

• Natural gas

• Hydrogen

• Propane

• Gasoline vapors

• Ethanol vapors

• Grain dust

• Coal dust

• Other combustible process dusts

The presence of a flammable substance does not automatically mean that the entire facility is a hazardous area. Classification determines where and under what conditions an explosive atmosphere can occur.

Why Is Hazardous Area Classification Important?

Area classification provides an engineering basis for determining where special explosion-protection measures are required.

The classification can affect:

• Equipment selection

• Protection concepts

• Equipment Protection Level requirements

• Gas or dust group requirements

• Temperature limitations

• Electrical installation requirements

• Inspection and maintenance requirements

IEC 60079-14:2024 specifically covers the design of electrical systems, selection and installation of Ex equipment, and initial inspection of electrical installations associated with explosive atmospheres. :chatgpt-content-reference{index="5"}

It is therefore important to understand that area classification is not itself a form of explosion protection. It establishes the conditions that equipment and installations must be designed to address.

Gas and Vapor Zone Classification

For explosive gas atmospheres, IEC 60079 uses three principal zone classifications: Zone 0, Zone 1, and Zone 2.

Zone 0

Zone 0 is an area in which an explosive gas atmosphere is present continuously, for long periods, or frequently.

Typical examples can include certain internal spaces of process equipment or vessels where a flammable atmosphere is continuously or frequently present.

Zone 1

Zone 1 is an area in which an explosive gas atmosphere is likely to occur during normal operation.

Examples can include areas near certain process release sources such as pump seals, sampling points, vents, or connections where releases may occur during normal operation.

Zone 2

Zone 2 is an area in which an explosive gas atmosphere is not likely to occur during normal operation but, if it does occur, will exist only for a short period.

Zone 2 is still a hazardous area. It should not be treated as a non-hazardous or "safe" area simply because the probability of an explosive atmosphere is lower.

For a more detailed comparison, see Zone 0 vs Zone 1 vs Zone 2: What's the Difference?

Dust Zone Classification

Combustible dust atmospheres use a different zone numbering system.

Zone 20

Zone 20 is an area in which an explosive dust atmosphere is present continuously, for long periods, or frequently.

Zone 21

Zone 21 is an area in which an explosive dust atmosphere is likely to occur occasionally during normal operation.

Zone 22

Zone 22 is an area in which an explosive dust atmosphere is not likely to occur during normal operation but, if it does occur, will exist only for a short period.

Dust classification also has to consider combustible dust layers. IEC 60079-10-2:2026 specifically addresses both explosive dust atmospheres and combustible dust layers because accumulated dust can contribute to the formation of explosive dust clouds. :chatgpt-content-reference{index="6"}

Gas Zones vs Dust Zones

The numbering systems should not be mixed. A Zone 1 gas area and a Zone 21 dust area describe different hazardous-area conditions and require equipment appropriate to the relevant hazard.

What Determines the Extent of a Hazardous Area?

Hazardous area classification is not determined by zone definitions alone. Engineers must evaluate the conditions that can produce and disperse an explosive atmosphere.

Source of Release

A release source is a point or location from which a flammable substance can be released into the surrounding atmosphere.

Examples include:

• Flanges and connections

• Pump and compressor seals

• Valves

• Drains and vents

• Sampling points

• Process equipment openings

Grade of Release

The expected frequency and duration of releases are important factors in determining the resulting zone classification.

The classification process considers whether a release can occur continuously, during normal operation, or only under abnormal conditions.

Ventilation

Ventilation influences how quickly a released flammable substance is dispersed.

Natural and artificial ventilation can therefore affect both the probability that an explosive atmosphere will persist and the extent of the classified area.

Physical and Process Conditions

Pressure, temperature, release rate, physical properties of the substance, equipment geometry, and operating conditions can all influence the classification assessment.

This is why a hazardous area cannot normally be classified correctly from the equipment name or process description alone.

How Hazardous Area Classification Affects Equipment Selection

Once the area has been classified, the classification becomes one of the inputs used to select suitable equipment.

However, zone alone is not enough.

A typical equipment selection process may involve checking:

1. Area classification – Determine the applicable gas or dust zone.

2. Hazardous substance – Identify the gas, vapor, mist, or combustible dust involved.

3. Equipment Protection Level – Determine the required EPL for the application.

4. Protection concept – Select an appropriate method such as Ex d, Ex e, Ex i, Ex p, or another applicable protection concept.

5. Gas or dust group – Verify compatibility with the hazardous substance.

6. Temperature requirement – Ensure the equipment's maximum surface temperature is suitable.

7. Certificate and marking – Verify that the equipment certification and complete Ex marking cover the intended application.

8. Installation requirements – Check cable entries, glands, wiring, earthing, environmental conditions, and other installation requirements.

IEC 60079-14 provides the requirements for electrical Ex equipment selection and installation in explosive atmospheres, in addition to the requirements applicable to ordinary electrical installations. :chatgpt-content-reference{index="7"}

Common Protection Concepts Used in Hazardous Areas

Different hazardous-area applications may use different explosion protection concepts.

Ex d – Flameproof

Uses a flameproof enclosure designed to contain an internal ignition and prevent flame propagation to the surrounding atmosphere.

Ex e – Increased Safety

Uses construction and design measures to reduce the possibility of ignition sources such as arcs, sparks, and excessive temperatures under specified operating conditions.

Ex i – Intrinsic Safety

Limits electrical and thermal energy so that ignition of the surrounding explosive atmosphere is prevented under specified conditions.

Ex p – Pressurization

Uses protective gas and controlled pressure to prevent the surrounding explosive atmosphere from entering the protected enclosure.

The appropriate protection concept depends on the equipment, zone, hazardous substance, EPL, certification, and installation requirements.

Does a Higher Zone Number Mean Lower Overall Risk?

Not necessarily.

The zone classification primarily describes the likelihood and duration of an explosive atmosphere being present. It does not represent a complete assessment of the consequences of an explosion or the overall process risk.

For example, a Zone 2 area may still contain highly hazardous substances and significant process hazards.

Equipment selection must therefore consider the complete hazardous-area specification rather than treating the zone number as a standalone risk rating.

Common Mistakes in Hazardous Area Classification

• Assuming Zone 2 is a non-hazardous area.

• Using gas-zone terminology for combustible dust applications.

• Classifying an area without identifying the actual release sources.

• Ignoring the effect of ventilation.

• Assuming the zone alone determines the equipment specification.

• Ignoring gas group, dust group, or temperature requirements.

• Selecting equipment based only on the words "explosion proof."

• Failing to update the classification after significant process or equipment changes.

• Ignoring installation requirements after selecting the certified equipment.

Frequently Asked Questions

What standard is used for hazardous area classification?

For explosive gas atmospheres, IEC 60079-10-1 is the relevant classification standard. For explosive dust atmospheres, IEC 60079-10-2 applies. :chatgpt-content-reference{index="8"}

What is the difference between Zone 0, Zone 1, and Zone 2?

The zones describe the expected frequency and duration of an explosive gas atmosphere. Zone 0 represents continuous, frequent, or long-duration presence; Zone 1 represents likely occurrence during normal operation; and Zone 2 represents an atmosphere that is not likely during normal operation and, if it occurs, exists only for a short period.

What are Zones 20, 21, and 22?

Zones 20, 21, and 22 are used for combustible dust atmospheres and follow a similar frequency-and-duration concept to Zones 0, 1, and 2 for gases.

Can Zone 2 become Zone 1?

Yes. A change in release conditions, process operation, ventilation, equipment arrangement, or other relevant conditions can change the resulting area classification. Classification should therefore be reviewed when significant changes occur.

Does the zone determine which Ex equipment can be used?

The zone is an important input, but it is not the only requirement. Equipment selection also depends on the required EPL or category, protection concept, gas or dust group, temperature requirements, certification, and installation conditions.

Can ordinary electrical equipment be installed in a hazardous area?

Equipment installed in a classified area must be suitable for the applicable hazardous-area conditions. Ordinary equipment that has not been assessed or certified for the applicable conditions should not be selected simply because the area is Zone 2 or another lower-frequency zone.

How often should hazardous area classification be reviewed?

There is no single universal interval that applies to every facility. The classification should be reviewed when significant changes occur to the process, equipment, release sources, ventilation, or operating conditions, and according to the site's applicable safety-management and inspection requirements.

Hazardous Area Classification Summary

Conclusion

Hazardous area classification is the engineering process used to identify locations where explosive atmospheres may occur and to determine the extent and classification of those areas.

For gas and vapor hazards, the main classifications are Zone 0, Zone 1, and Zone 2. For combustible dust, the corresponding classifications are Zone 20, Zone 21, and Zone 22.

The classification is influenced by the hazardous substance, release sources, release frequency, ventilation, process conditions, and physical arrangement of the installation.

Most importantly, area classification is only the starting point for equipment selection. Engineers must also consider EPL or category, protection concept, gas or dust group, temperature requirements, certification, and installation conditions.

A correct hazardous area classification therefore provides the basis for selecting and installing Ex equipment appropriate to the actual conditions of the facility.

References

• IEC 60079-10-1 – Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres

• IEC 60079-10-2 – Explosive atmospheres – Part 10-2: Classification of areas – Explosive dust atmospheres

• IEC 60079-14 – Explosive atmospheres – Part 14: Electrical installation design, selection and installation of equipment, including initial inspection

• IECEx System