When assessing the danger in a hazardous area, an Ex zoning assessment is conducted. Flash point, auto-ignition temperature, and flammability limits are the key explosion risk parameters in this process.

The ATEX directive (short for “ATmosphères EXplosibles”) sets the rules for classifying these hazardous zones and ensuring the appropriate safety measures and equipment are in place.

In this article you will find a brief explanation of all Explosion Risk Parameters in hazardous Gas environments (Ex zone 0, 1 and 2).

Important Explosion Risk Parameter: The Flash point

The flash point of a liquid is the minimum temperature at which it emits enough vapour to form an ignitable mixture with air near its surface.

At this temperature, an external ignition source, like a spark or flame, can cause the vapour to ignite. However, this ignition is usually temporary unless the liquid is continuously heated.

Flash point is one of the most important properties in evaluating the explosion risk of flammable liquid, as it gives an indication of both volatility and flammability.

Flash point applies to all liquids and liquified gases, but for permanent gases like methane and hydrogen flash point is irrelevant, because these can form flammable mixtures at any temperature.

The flash point helps determine how a liquid should be stored, transported, and handled safely.

Liquids with lower flash points are more volatile and ignite more easily, while those with higher flash points are generally less volatile but can still pose a fire risk. This is why liquids are classified into categories that indicate their level of danger.

Oil Fluid Classes of Flash Point

For instance, gasoline, a Class I liquid, has a flash point below 21°C, making it highly volatile and easily ignitable even at ambient temperatures.

On the other hand, diesel—a Class III liquid—has a flash point above 55°C, making it relatively safer at lower temperatures but still potentially hazardous if heated.

The flash point serves as a lower boundary in flammability assessment and helps with categorizing materials and defining storage standards.

Gas Group Classifications: IAA, IIB, IIC

The gas group classification system categorizes gases by their ignition energy, which varies depending on conditions and should be seen as approximate.

While not used directly in area classification, these groups help in selecting equipment suited to the specific gases in a workspace.

Equipment rated for Group IIC can also be used for IIB and IIA gases, but not the reverse.

Oil Fluid Categories

While the flash point indicates how easily a liquid may ignite, oil fluid categories describe how these liquids behave when released. These categories are as follows:

• Category A: Highly flammable liquids that rapidly vaporize upon release, increasing the risk of ignition and explosion. For example, solvents like acetone belong to this category and can quickly form ignitable vapour clouds.
• Category B: Liquids handled at elevated temperatures that may boil upon release, but don’t fall into category A. Industrial oils and lubricants, often heated in processes, can fall into this category when exposed to air.
• Category C: Liquids with a flash point above ambient temperature which, when released, can form flammable mists or sprays that ignite readily. Diesel and certain hydraulic fluids fall into this category, as they may produce combustible mists under high-pressure releases.

Similar to the flash point, Hydrogen  and Methane are classified into separate categories., G(i) and G(ii), respectively.

Auto-Ignition Temperature, Ignition Without a Spark

The auto-ignition temperature (Tᵢg) defines the temperature at which a substance will ignite on its own, without needing a flame or spark.

For example, hydrogen has an auto-ignition temperature of 560°C, meaning any equipment used around hydrogen must not exceed this temperature to avoid spontaneous combustion.

Temperature Class Codes T1 – T6

Equipment must be classified based on its maximum surface temperature to prevent auto-ignition. Temperature class codes (T1 to T6) indicate the highest temperature that equipment can reach.

For instance, if you’re working with acetylene, which has a Tᵢg of 305°C, you would need equipment rated for T2 or better (T3 – T6) to maintain safe operating conditions.

The Armadex OZC 3 ATEX camera is rated T6, with ATEX marking II 3G Ex h ic IIC T6 Gc, making it suitable for gases with a Tᵢg above 85°C.

Flammability Limits (LEL & UEL)

Flammability limits are another important characteristic for explosion risk assessment, they describe the concentration of a gas mixed with air that can sustain combustion.

The Lower Explosive Limit (LEL) is the minimum concentration required for ignition, while the Upper Explosive Limit (UEL) is the maximum concentration at which a flame can still propagate. The LEL is especially important in ATEX zone classification.

For example, methane has an LEL of 5% and a UEL of 15%, meaning it can only ignite within this range. This means methane and air mixtures below 5% or above 15% will not ignite.

Ventilation is an effective strategy to reduce the mixture below the LEL. Environmental factors such as temperature and pressure can alter these limits, which is why continous monitoring and safety margins are important.

Vapour Density, Gas Dispersion

Vapour density—the ratio of a gas’s weight to the weight of an equal volume of air—helps predict how gases behave upon release.

This risk parameter for hazardous area classification reveals whether gases are likely to rise, sink, or diffuse with air currents, which in turn influences zone shapes and layering risks.

Gases with densities near 1 will mix with air more readily, while heavier gases tend to settle, accumulating in low areas. Lighter gases, significantly less dense than air, rise and may collect at high points.

Additionally, release conditions, like temperature, can alter vapour density and impact these behaviors.

Understanding these behaviors supports effective ventilation and evacuation planning for safe management in confined or low-lying areas.

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