Can ventilation make a Zone 2 area non-hazardous?

Yes, in some cases. In IEC 60079-10-1, a secondary release (one not expected in normal operation) that meets high dilution with good or fair availability of ventilation produces only a zone of negligible extent, written Zone 2 NE, and the place can be treated as non-hazardous. This has to be shown by a documented assessment of each release and of the ventilation, and it only works for small releases and ventilation that is reliably present.

Short answer

• Ventilation does not change the grade of release, but it changes how large and how persistent the flammable cloud is.
• Secondary release + high dilution + good or fair availability gives non-hazardous (Zone 2 NE). With poor availability, the same release gives Zone 2.
• Medium dilution gives Zone 2 whatever the availability, so ventilation alone cannot remove the zone.
• Primary releases need high dilution with good availability to reach negligible extent.
• The NE designation means that ignition of the small cloud would have negligible consequences. That has to be justified, and needs particular care where even a small flash fire could reach people.
• Making a room non-hazardous with fans usually also needs flow monitoring, and either a standby fan or an interlock that stops the release when ventilation fails.

How IEC 60079-10-1 decides

The standard combines three things: the grade of release, the degree of dilution (high, medium or low, from the release rate, the gas properties and the air velocity) and the availability of ventilation (good, fair or poor). For secondary releases the combinations work out as follows:

The table is a summary of the standard's zone table as reproduced in published research; check the current edition and its footnotes before relying on it. Indoors, the background concentration of gas in the room must also stay below the critical value chosen for the assessment, most often 25 % of the lower flammable limit (LFL). If it does not, the room as a whole is not diluting the release, and a negligible-extent result cannot be claimed whatever the local air velocity.

The standard defines a zone of negligible extent (Zone NE) as one so small that ignition would have negligible consequences. It is a classification result, not a separate zone type: the place is treated as non-hazardous for equipment selection. UK guidance on DSEAR (HSE L138) says the same in its own words: ventilation can in some circumstances be sufficient for the hazardous area to be treated as Zone 2 NE, because a sufficiently small explosive atmosphere would have insignificant overpressure or thermal effects if ignited.

What it takes in practice

• Know every release. List each flange, valve, seal and connection, with a credible hole size and pressure. One large potential release can defeat the whole room.
• Calculate the dilution. Use the release characteristic and the ventilation velocity, plus the background concentration check indoors. "Well ventilated" without numbers is not an assessment.
• Make the ventilation reliable. Before rating mechanical ventilation as good, assessors usually look for a standby fan that starts automatically on low flow, or an interlock that isolates the gas supply when ventilation fails. A single fan with only a flow alarm is often rated lower. UK guidance (HSE L138) expects mechanical ventilation to be monitored, with flow failure detection and alarm.
• Get the air pattern right. Extract at high level for lighter-than-air gases such as methane and hydrogen, at low level for heavier vapours. Dead corners and obstructions reduce the real dilution.
• Maintain and manage change. Filters, dampers, fan belts and louvres need inspection. New equipment, partitions or closed openings change the result.

Sector codes apply the same idea with their own criteria. For natural gas at up to 10 bar, the UK code IGEM/SR/25, for example, accepts negligible extent where its calculated hypothetical gas volume is below 0.1 m³ and ventilation availability is good.

Where ventilation cannot do it

• Large or high-pressure releases. The required air flow becomes impractical, or the background concentration check fails.
• Continuous or frequent releases, such as open vessels or regular venting: ventilation reduces the extent, but the source still needs a zone or local extraction and containment.
• Confined or occupied spaces where even a small flash fire would injure someone.
• Unreliable natural ventilation, for example through doors and windows that can be closed.

Where the release itself cannot be made small, the other route is a pressurised or artificially ventilated room built to IEC 60079-13, which keeps an outside atmosphere out or dilutes internal releases under monitored conditions.

Common mistakes

• Rating a single fan as good availability. Without automatic standby or an interlock that stops the release, it is usually rated fair or poor.
• Using air changes per hour instead of a release-based calculation. The required flow depends on the release, not on the room volume.
• Forgetting the exhaust. The discharge point of the extract system is itself a source of release, to be assessed outside.
• Treating gas detection as ventilation. Detection does not dilute; it can only trigger actions such as isolation or extra ventilation.

Related on excodex

• Ventilation in area classification: degree of dilution and availability
• Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
• What does 'Zone 2 NE' (negligible extent) mean?
• What happens to the zone classification if the ventilation fan fails?
• Standards spaces: IEC 60079-10-1, IEC 60079-13

Standards and further reading

• IEC 60079-10-1:2020, Classification of areas, explosive gas atmospheres (definition of Zone NE, ventilation and dilution, Annex C ventilation guidance): iteh.ai sample
• IEC 60079-13:2017, Equipment protection by pressurized room "p" and artificially ventilated room "v".
• Choi and Byeon, Specific process conditions for non-hazardous classification of hydrogen handling facilities, Safety and Health at Work 12 (2021), table of zones for grade of release, dilution and availability: koreascience.or.kr
• HSE, L138 Dangerous substances and explosive atmospheres, guidance on ventilation and Zone 2 NE: hse.gov.uk
• HSE, Hazardous area classification and control of ignition sources: hse.gov.uk
• IGEM/SR/25 Edition 2, Hazardous area classification of natural gas installations: igem.org.uk

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.