ATEX Zone Classification: Zones 0, 1, 2, 20, 21 and 22

Last updated: September 2026 · Based on Directive 1999/92/EC, IEC 60079-10-1:2020 and IEC 60079-10-2:2026

ATEX zone classification divides a workplace into zones according to how often, and for how long, an explosive atmosphere is expected to be present. Gas, vapor and mist hazards use Zone 0, Zone 1 and Zone 2; combustible dust uses Zone 20, Zone 21 and Zone 22. The zone sets the minimum equipment category (ATEX) or Equipment Protection Level (EPL) for anything installed there, so it determines what type of Ex-protected equipment is required in each location.

The zone definitions come from Annex I of the ATEX Workplace Directive 1999/92/EC (reproduced in Schedule 2 of the UK's DSEAR). The engineering method for drawing the zones is IEC 60079-10-1:2020 for gases and vapors and IEC 60079-10-2 for dusts (third edition, published June 2026, which replaced the 2015 edition; many existing classifications still refer to the 2015 edition). Classification is the employer's duty and is recorded in the explosion protection document.

ATEX zones at a glance

Categories are those required by Directive 1999/92/EC, Annex II, part B (equipment certified under Directive 2014/34/EU). EPLs are the IEC 60079-14 equivalents: category 1 corresponds to EPL a, 2 to b and 3 to c. Higher-protection equipment may always be used in a lower-risk zone, never the other way round. See ATEX equipment categories and Equipment Protection Levels for the details.

Not sure whether a site needs zoning at all? Start with Does it need ATEX zoning?, or step through the interactive zone classification tool.

Gas and vapor zones (IEC 60079-10-1)

Gas zones cover explosive atmospheres formed by flammable gases, vapors and mists mixed with air. The zone follows mainly from the grade of release of each source and the ventilation around it.

Neither the directive nor IEC 60079-10-1 sets hours-per-year limits for the zones. The standard notes that "long" and "frequently" need not be quantified, and that indications of frequency and duration can be taken from industry codes (for example EI 15, API RP 505 or NFPA 497). Treat any hour figures you see as code-specific guidance, not as the definition.

ATEX Zone 0

Zone 0 is a place where an explosive atmosphere of gas, vapor or mist is present continuously, for long periods or frequently (Directive 1999/92/EC, Annex I; IEC 60079-10-1:2020, 3.3.4). In practice it is almost always inside process equipment, such as the vapor space of a tank holding a flammable liquid above its flash point. Only ATEX category 1G equipment (EPL Ga) may be installed in Zone 0.

Zone 0 examples

• Inside storage tanks containing flammable liquids above their flash point (the vapor space above the liquid)
• Inside process vessels, reactors and mixers where vapor and air mix during normal production
• Inside vent pipes, sumps and interceptors that continuously hold flammable vapor

A pipe or vessel that is always completely full of gas or liquid, with no air inside, is not an explosive atmosphere. Its interior becomes a zone only where air can get in, for example during filling, emptying or purging.

ATEX Zone 1

Zone 1 is a place where an explosive gas atmosphere is likely to occur occasionally in normal operation (1999/92/EC, Annex I; IEC 60079-10-1:2020, 3.3.5). It typically surrounds primary grade sources of release, such as tank vents, sample points and open filling operations, and often surrounds a Zone 0. Equipment must be category 1G or 2G (EPL Ga or Gb).

Zone 1 examples

• Area surrounding Zone 0 (e.g., around tank vents)
• Areas near equipment that frequently releases flammable substances
• Pump and compressor seal areas, where the seal type is expected to release in normal operation
• Sample points and drainage systems
• Loading/unloading areas for flammable liquids

ATEX Zone 2

Zone 2 is a place where an explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, will persist for a short period only (1999/92/EC, Annex I; IEC 60079-10-1:2020, 3.3.6). It usually comes from secondary grade releases, such as flanges, valve stems and fittings that leak only on failure, in an adequately ventilated area. Equipment must be category 1G, 2G or 3G (EPL Ga, Gb or Gc).

Zone 2 examples

• Areas surrounding Zone 1
• Areas where release only occurs during maintenance, breakdown, or abnormal conditions
• Flange connections in good condition
• Valve stems under normal conditions
• Outdoor process areas with good natural ventilation around secondary grade sources

Zone NE (negligible extent)

IEC 60079-10-1:2020 defines a Zone NE as a zone of negligible extent, so small that if ignition did occur it would have negligible consequences (3.3.8). It is written as Zone 0 NE, Zone 1 NE or Zone 2 NE. In practice "Zone 2 NE" is the one met most often: a small secondary release in a well-ventilated space where the potential cloud is tiny.

An area whose only zone is NE can be treated as non-hazardous for equipment selection, which is why clause 4.4.2 of the standard sets specific conditions that must be met and documented. These cover the size and concentration of the potential gas cloud and the consequences of ignition, which for pressurized releases include the risk of a jet fire. NE is a documented conclusion of the classification, not a label to apply to poorly ventilated rooms. The 2026 edition of IEC 60079-10-2 added a zone of negligible extent for dust on the same basis.

Dust zones: Zone 20, 21 and 22 (IEC 60079-10-2)

Dust zones cover explosive atmospheres formed by a cloud of combustible dust in air. IEC 60079-10-2 also applies its principles to combustible flyings (fibers, Group IIIA). Where flammable gas or vapor is present with the dust (a hybrid mixture), the gas part is classified under IEC 60079-10-1 and the two are assessed together; the 2026 edition gives guidance on hybrid mixtures in Annex D.

Dust layers: Directive 1999/92/EC requires layers, deposits and heaps of combustible dust to be considered like any other source that can form an explosive atmosphere. A disturbed layer can raise a cloud, and a layer on hot equipment can ignite and act as an ignition source. The examples in IEC 60079-10-2 assume effective housekeeping; where layers are allowed to build up, the classification has to include clouds raised from them, and the zones get larger or more severe. Annex C of the 2026 edition covers dust layer hazards and levels of housekeeping.

ATEX Zone 20

Zone 20 is a place where an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods or frequently (1999/92/EC, Annex I; IEC 60079-10-2). It is normally the inside of dust containment, such as silos, hoppers, cyclones, the dirty side of filters and mills. Only category 1D equipment (EPL Da) may be installed in Zone 20.

Zone 20 examples

• Inside silos, hoppers, and cyclone filters
• Inside pneumatic conveying lines
• Inside mixers and blenders processing combustible dusts

ATEX Zone 21

Zone 21 is a place where an explosive dust cloud is likely to occur occasionally in normal operation (1999/92/EC, Annex I; IEC 60079-10-2). Typical locations are just outside containment where dust escapes during normal work, such as bag-emptying stations, filling points and containment doors that are opened regularly. Equipment must be category 1D or 2D (EPL Da or Db).

Zone 21 examples

• Areas around dust filling/emptying points
• Near conveyor transfer points with frequent spillage
• Around inspection hatches and doors on dust-handling equipment that are opened in normal operation

ATEX Zone 22

Zone 22 is a place where an explosive dust cloud is not likely to occur in normal operation and, if it does occur, will persist for a short period only (1999/92/EC, Annex I; IEC 60079-10-2). It usually surrounds secondary grade releases, such as equipment that leaks dust only on failure, and depends heavily on housekeeping. Equipment must be category 1D, 2D or 3D (EPL Da, Db or Dc).

Zone 22 examples

• Areas surrounding Zone 21 with adequate housekeeping
• Near equipment that rarely releases dust
• Warehouse areas storing bags of combustible powder, where a bag can split during handling

Is there an ATEX zone for CO2?

No, not for carbon dioxide on its own. CO2 is not flammable, so it cannot form an explosive atmosphere and does not create a Zone 0, 1 or 2. Its hazards (asphyxiation, toxic effects at high concentration, cold and high pressure) are controlled with gas detection, ventilation and confined-space procedures, not zoning.

A zone can still be needed near CO2 where other substances are present, for example ethanol vapor in a brewery or distillery, or flammable components carried in a CO2 stream. Where CO2 is used to inert a vessel, the classification has to consider what happens if inerting is lost. See Does CO2 need an ATEX zone? for common applications, inerting and the gas-detection side.

North American Classification (NEC/CEC)

The US (NEC Articles 500 to 506) and Canada (CEC Section 18) use a parallel system. The mapping below is approximate; classification is done under the national code, not by conversion.

NEC Article 505 recognizes the gas Zone system (Zones 0, 1, 2) directly, and Article 506 does the same for Zones 20, 21 and 22 covering combustible dusts and ignitible fibers/flyings. The Division system is less granular: Division 1 covers both Zone 0 and Zone 1.

Class III covers ignitible fibers and flyings (textile mills, woodworking). In the Zone system these fall under the dust zones as Group IIIA. See NEC 500 vs ATEX/IEC for the full comparison.

Zone Classification Methodology

The employer is responsible for classifying places where explosive atmospheres may occur (Directive 1999/92/EC, Article 7; DSEAR regulation 7 in Great Britain). The work is typically done by a team including:

• Process engineers (source and release characterization)
• Safety/HSE engineers
• Electrical and instrument engineers
• Operations and maintenance personnel

IEC 60079-10-1 requires the people doing the classification to be competent (clause 4.6). For a step-by-step engineering walk-through, see Hazardous Area Classification.

Key Factors

1. Source of release: where flammable material can escape (flanges, valves, seals, vents, open surfaces, dust containment openings).
2. Grade of release:
   • Continuous (continuous, frequent or long periods) → normally forms Zone 0/20
   • Primary (expected periodically or occasionally in normal operation) → normally forms Zone 1/21
   • Secondary (not expected in normal operation; infrequent and short if it occurs) → normally forms Zone 2/22
3. Ventilation (gas and vapor): the degree of dilution and the availability of the ventilation affect both the extent and the type of zone.
   • High dilution with good availability reduces zone size and can lead to a less severe zone type (down to Zone NE)
   • Low dilution or poor availability extends the zone and can lead to a more severe zone type
   • Enclosed spaces with little or no air movement: the zone can extend to fill the whole space
4. Relative density of the gas or vapor: lighter-than-air gases (hydrogen, methane) rise and can collect under roofs; heavier-than-air vapors (propane, gasoline vapor) spread at ground level and into pits and trenches.
5. Housekeeping (dust): how quickly dust layers are removed changes the zones around dust-handling plant.

Zone Extent

The physical size of a zone depends on:

• Release rate and velocity
• Ventilation rate and effectiveness
• Relative density of the flammable substance
• Flash point and vapor pressure
• Lower flammable limit (LFL) of the substance

IEC 60079-10-1:2020 gives informative methods for estimating release rates (Annex B), ventilation and dilution (Annex C) and the hazardous distance (Annex D), with worked examples in Annex E. Industry codes listed in its Annex K may be used instead where they fit the installation. IEC 60079-10-2:2026 gives dust examples in Annex B.

Zone Documentation

The output of a zone classification study is:

1. Hazardous Area Classification drawings: plans and sections showing zone boundaries
2. Schedule of release sources: tabulated list of all potential sources
3. Data sheets: substance properties, release characteristics
4. Zone classification report: methodology, assumptions, results

These documents feed the explosion protection document required by Article 8 of Directive 1999/92/EC (in Great Britain, the DSEAR risk assessment record) and form the basis for selecting equipment with the correct EPL and protection method.

Common Pitfalls

• Forgetting that lighter-than-air gases (hydrogen, methane) accumulate at ceiling level
• Ignoring dust layers: a disturbed layer can form a dust cloud, and layers on hot surfaces can ignite
• Not reassessing when processes, substances or ventilation change (IEC 60079-10-2:2026 also recommends reviewing dust area classifications at a three-year interval)
• Confusing "Zone 2" with "non-hazardous": it still requires category 3 / EPL Gc equipment or better
• Under-estimating ventilation failures (what happens when the fan stops?)
• Labeling a zone NE without documenting the conditions of IEC 60079-10-1 clause 4.4.2
• Treating hours-per-year rules of thumb as the definition of a zone

Frequently asked questions

What is ATEX Zone 1?

Zone 1 is a place where an explosive gas, vapor or mist atmosphere is likely to occur occasionally in normal operation. It typically surrounds primary grade releases such as tank vents and sample points. Equipment must be ATEX category 1G or 2G (EPL Ga or Gb).

What is the difference between Zone 21 and Zone 22?

In Zone 21 an explosive dust cloud is likely to occur occasionally in normal operation; in Zone 22 it is not likely in normal operation and, if it occurs, persists only for a short period. Zone 21 needs category 1D or 2D equipment (EPL Da or Db), while Zone 22 also allows category 3D (EPL Dc).

What equipment category is required for Zone 2?

Directive 1999/92/EC allows category 1, 2 or 3 equipment in Zone 2, suitable for gas (G). In IEC terms that is EPL Ga, Gb or Gc.

What does Zone NE mean?

Zone NE is a zone of negligible extent defined in IEC 60079-10-1:2020: a zone so small that ignition would have negligible consequences, written for example as Zone 2 NE. It can be treated as non-hazardous for equipment selection only when the conditions of clause 4.4.2 are met and documented.

Does CO2 need an ATEX zone?

Not on its own. CO2 is not flammable and cannot form an explosive atmosphere, so zoning is only needed where flammable gases, vapors or dusts are also present.

Related Files

• Fundamentals: explosion triangle, basic concepts
• Gas Groups: how different fuels are classified
• EPL: equipment protection levels required per zone
• ATEX Equipment Categories: category 1, 2, 3 and the zones they cover
• Protection Methods: types of protection suitable per zone
• Hazardous Area Classification: the detailed engineering method
• Dust Explosion Protection: dust hazards beyond zoning
• Standards: IEC 60079-10-1 and 60079-10-2

Sources

1. EU-OSHA: Directive 1999/92/EC, risks from explosive atmospheres
2. legislation.gov.uk: DSEAR 2002, Schedule 2 (reproduces the zone definitions of Annex I of 1999/92/EC)
3. legislation.gov.uk: DSEAR 2002, Schedule 3 (equipment categories per zone)
4. IEC: IEC 60079-10-1:2020, Classification of areas, explosive gas atmospheres
5. IEC: IEC 60079-10-2:2026, Classification of areas, explosive dust atmospheres (replaces the 2015 edition)
6. IEC: IEC 60079-14:2024, Electrical installations design, selection and erection
7. Safety and Health at Work (2021): Specific process conditions for non-hazardous classification of hydrogen handling facilities (Zone 2 NE)

Related Topics

• Explosion Protection Fundamentals
• Equipment Protection Levels
• Gas Groups & Dust Groups
• Installation & Inspection