What is the difference between natural and artificial ventilation in area classification?
Abstract
Natural ventilation is air movement from wind and temperature differences through openings; artificial ventilation uses fans, either diluting a whole room or extracting at the source. IEC 60079-10-1 judges both by degree of dilution and availability.
Natural ventilation is air movement caused by the wind and by temperature differences, entering and leaving through openings. Artificial ventilation is air movement produced by fans, either as general ventilation that dilutes releases throughout a room or as local extraction that captures a release at its source. IEC 60079-10-1 assesses both in the same way, by the degree of dilution they achieve (high, medium or low) and by their availability (good, fair or poor). The difference lies in how strong, how predictable and how reliable each one is.
Short answer
- Natural: wind outdoors; wind pressure and buoyancy (stack effect) through openings in buildings. No power needed, but it varies with the weather and can be blocked by closed doors, new walls or cladding.
- Artificial: fans and ducts. Flow is known and can be measured, but it stops when the fan or its power fails.
- General artificial ventilation dilutes the whole room; local extraction captures the release close to the source with much smaller air flows.
- Both are judged by dilution and availability. Open-air natural ventilation is usually good availability. A correctly placed fan system that is running is usually rated fair; an automatic standby fan, or flow monitoring that stops the release when the flow is lost, can make it good.
- Ventilation never changes the grade of release. It changes the zone type and the zone extent.
How the two are assessed
The definitions in IEC 60079-10-1 treat ventilation simply as the movement of air and its replacement with fresh air, whether by wind, by temperature gradients or by artificial means such as fans or extractors. The 2020 edition gives an annex on natural ventilation of buildings, covering wind-driven, buoyancy-driven and combined cases, and examples of artificial arrangements: supply only, supply with extraction, and local extraction.
In both cases the assessor needs a ventilation velocity (outdoors a wind speed, indoors derived from the air flow through the room), compares it with the release characteristic of each source on the standard's dilution chart, and checks indoors that the background concentration stays below a critical value, most often 25 % of the lower flammable limit (LFL). The answer is a degree of dilution. Availability is then judged separately.
| Aspect | Natural ventilation | Artificial ventilation |
|---|---|---|
| Driving force | wind pressure, buoyancy from temperature difference | fans |
| Flow rate | varies with weather, season, wind direction and time of day | set by design, can be measured and monitored |
| Typical availability | good in open air, except in places with restricted air movement such as pits; indoors, judged at the least favourable conditions, counting only openings that cannot be closed | usually fair; good with automatic standby fans or flow monitoring that stops the release |
| Main weakness | calm, hot weather; wind opposing the stack effect; blocked openings | fan, power or control failure; poor air distribution |
| Design basis | the least favourable credible conditions | the duty flow, plus what happens when it is lost |
| Extra sources created | none | exhaust outlet is a source of release; fans handling flammable air must suit the atmosphere |
Natural ventilation: free but variable
Outdoors, the wind does the work, and an unobstructed open-air location is usually treated as good availability. Indoors, air enters and leaves through openings, driven by wind pressure on the building and by buoyancy when the inside is warmer than the outside. French guidance from Ineris (2022) stresses that natural flow rates are by definition highly variable, that wind can stop the buoyancy flow, and that natural ventilation cannot be counted as a reliable safety back-up for a mechanical system. For enclosed spaces it applies the approach of the standard: base the flow on the least favourable conditions. HSE adds a practical warning: new walls or buildings can block the natural ventilation of nearby plant and so change its classification.
For light gases such as methane and hydrogen, high-level openings or a ridge vent let the gas out; for heavy vapours, low-level openings matter. A shelter open on several sides can sometimes be treated close to open-air conditions, but that needs justification.
Artificial ventilation: controllable but dependent
Fans give a known flow, so the dilution can be calculated with more confidence. Two forms are used:
- General ventilation replaces the room air. It covers releases anywhere in the room but needs large flows for large releases, and real mixing is never perfect. Ineris advises against sizing it by air change rate alone.
- Local extraction (hoods, extraction points at a filling station, spray booth extraction) captures the release at the source and is much more efficient for known, fixed sources.
HSE notes that mechanical ventilation can reduce hazardous area extents where large releases are possible, for example in compressor houses, provided the design is verified at commissioning and the effect of a loss of power is considered. That last point is the price of artificial ventilation: the classification must cover what happens when the fan stops.
Example: A small gas metering kiosk has louvres at high and low level. On a calm, warm day the natural flow may be close to zero, so the assessor uses a conservative low air speed and finds low dilution: Zone 1 inside. Adding an extract fan with a flow switch, and closing the gas inlet valve automatically on loss of flow, gives medium or high dilution with good availability, and the inside can become Zone 2 or even Zone 2 NE, depending on the release rates.
Common mistakes
- Treating "indoors with windows" as well ventilated. Windows and doors that can be closed cannot be relied on; count only permanent openings.
- Counting a single fan as good availability. Without a standby fan or an automatic response that stops the release, it is usually fair.
- Ignoring the exhaust point. Extracted air carries the gas somewhere, and that outlet needs its own zone.
- Sizing by air changes alone. The required flow follows the release rate and the gas, not a fixed number of air changes.
Related on excodex
- Ventilation in area classification: degree of dilution and availability
- What happens to the zone classification if the ventilation fan fails?
- How many air changes per hour make a room adequately ventilated?
- Can ventilation make a Zone 2 area non-hazardous?
- Standards space: IEC 60079-10-1
Standards and further reading
- IEC 60079-10-1:2020 (edition 3.0), classification of areas, explosive gas atmospheres, with annexes on ventilation, natural ventilation of buildings and examples of artificial ventilation: preview; the 2008 edition preview gives the general definition of ventilation: preview
- Ineris, Principes de ventilation en zone ATEX (in French), 2022: ineris.fr
- HSE, Hazardous area classification and control of ignition sources: hse.gov.uk
- Choi and Byeon, Safety and Health at Work, 2021, reproducing the zone table of IEC 60079-10-1 edition 2 (2015), which the 2020 edition keeps as Table D.1: koreascience.or.kr
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.