Explosive atmospheres, a subject that is sometimes poorly understood
- Marc Duvollet
- Aug 24
- 7 min read
What is an explosive atmosphere?
An explosive atmosphere , or ATEX , refers to a mixture of flammable substances (gases, vapors, mists or dusts) with air, under normal atmospheric conditions, which can ignite in the presence of an ignition source.
A simple electrical switch, a spark or a hot surface can be enough to trigger an explosion if these substances are present in critical quantities.
A real danger… and frequent
ATEX can occur in many industrial and construction environments. Here are some concrete examples :
Grain silo : In 2020, an explosion blew up an agricultural silo in Germany, caused by the accumulation of flammable grain dust.
Gas station : Poorly planned maintenance caused an explosion following insufficient degassing.
Automotive paint shop : Non-ATEX-certified lighting generated a spark, causing solvent vapors to ignite.
Shipyard : Welding work carried out in a poorly degassed tank led to an explosion of hydrocarbon vapors.
The European regulatory framework: the ATEX directives
Several directives govern ATEX in the EU:
Social directives for employers
- Directive 89/391/EEC Implementation of measures to promote the improvement of the safety and health of workers (in particular the nine general principles of prevention).
- Directive 1999/92/EC Applicable since July 2006 “protection in matters of health and safety of workers likely to be exposed to the risk of explosive atmospheres”
- Classification, ATEX zoning and signage
- drafting of the DRPCE
Following this directive, two decrees were published, dated July 8, 2003
These orders concern in particular:
- The definition of locations where ATEX can form.
- Minimum requirements aimed at improving the health and safety of workers exposed to explosion risks.
- The selection criteria for devices and protection systems used in locations where ATEX may occur.
- The sign indicating dangerous locations.
A decree of July 28, 2003 sets the conditions for installing electrical equipment in locations where ATEX may occur.
Technical guidelines for manufacturers and distributors of ATEX equipment
- Directive 94/9/EC ATEX 95 Approximation of the laws of the Member States for protective equipment and systems intended for use in ATEX
- Directive 2014/34/EU Declaration of conformity and EU type certification instead of CE
Classification of ATEX zones
Here is a reminder of the zones, with real examples:
Medium | Area | Risk | Example |
Gas | Zone 0 | Continuous presence | The inside of a fuel tank |
Zone 1 | Probable presence | Area around the pressure relief valves | |
Zone 2 | Accidental presence | Poorly ventilated underground parking | |
Dust | Zone 20 | Permanent presence | Interior of a flour silo |
Zone 21 | Probable presence | Pneumatic sawdust conveying system | |
Zone 22 | Occasional presence | Technical room for sanding workshop |
The scientific bases: the ATEX hexagon
The explosion is only possible if 6 elements are combined at the same time:
A fuel : wood, paper, aluminum, flammable chemical, gasoline, etc.
The fuel must be suspended in the air. E.g., flammable gas, wood dust, cotton wool, flour, methane, flammable chemical vapor or gasoline above the flash point temperature)
An oxidizer (usually oxygen)
A source of ignition (e.g. non-ATEX tool, electric arc, spark, flame, etc.)
A confined environment guaranteeing poor evacuation of suspended fuel (e.g. underground gallery, tank, deep trench, room without ventilation, etc.)
The explosive range : An air-fuel mixture is only explosive if it is between two thresholds :
Lower explosive limit (LEL) : minimum fuel concentration below which the mixture is too lean to explode.
Upper explosive limit (UEL) : maximum concentration beyond which the mixture is too rich to explode (lack of oxygen).
Between the LEL and the UEL , the mixture is explosive : this is called the explosive range . Dust does not have a LEL/UEL expressed in %, but a minimum concentration threshold in g/m³ .
Example : On a renovation site, an uncertified industrial vacuum cleaner caused a discharge in a cloud of plaster dust, triggering an explosion.
Focus on the construction sector
In the construction sector (building and public works) , ATEX risks are often underestimated , even though they can be fatal . They mainly concern interventions in closed or contaminated environments , or the use of flammable products . Here are the main risks and the associated means of prevention and protection :
Solvent or resin vapors
Sources : paint, glues, epoxy resins, bitumen, waterproofers.
Examples of situations :
Application of resin in a cellar or poorly ventilated room.
Cleaning with solvents in tanks or cisterns.
Risk : formation of an explosive cloud in the air → explosion in the presence of a halogen lamp, an electric tool, or even a static spark.
Hot work in a confined atmosphere
Sources : welding, grinding, thermal cutting.
Examples :
Repair of pipes or tanks containing residual hydrocarbons.
Demolition of structures that contained flammable products.
Risk : ignition of residual vapors or dust.
Combustible suspended dust
Sources : wood, plaster, cement, flour, granules.
Examples :
Sanding or sawing materials indoors.
Cleaning of silos or technical ducts.
Risk : suspended dust + ignition source = explosion.
Presence of gas or vapors in the ground or buried networks
Sources : old gas pipes, polluted groundwater, industrial leaks.
Examples :
Earthworks in an industrial zone.
Digging trenches near gas networks.
Risk : explosion by accidental ignition (electric tool, flame, damaged cable).
Means of prevention and protection used in the construction industry
ATEX assessment in the prevention plan / PPSPS
Identification of risk areas.
Planning of risk interventions.
Efficient ventilation
Natural or assisted ventilation (blower).
Maintain vapor concentrations < LEL (lower explosive limit).
Atmospheric monitoring
Portable or fixed gas detectors.
Continuous detection in confined areas.
Use of ATEX certified equipment
Vacuum cleaners, lighting, power tools.
Equipment marked ATEX zone 1, 2, 21, 22 as appropriate.
Degassing and pre-cleaning
Obligation to neutralize any flammable atmosphere before work (e.g.: rinsing, purging, inerting with nitrogen).
Real case : In 2018, in a disused factory converted into housing, workers were seriously burned following the ignition of residual hydrocarbon vapors.
ATEX risk prevention and protection measures
Prevent the formation of an explosive atmosphere
To do this, it is necessary to act on the products and processes which can cause the formation of an ATEX.
Act on fuels
replace the combustible product with another non-combustible or less combustible one,
increase the particle size (move from powder to granules),
adding inert solids to combustible dusts,
control the process parameters (temperature, pressure, etc.) according to the physicochemical characteristics of the products,
maintain the concentration of the fuel outside its explosive range (capture of vapors or dust at the source, dilution, regular cleaning by suction of the layers of deposited dust, etc.).
Act on the oxidant
The main measure consists of introducing an inert gas (nitrogen, argon, etc.) in sufficient proportions into an atmosphere laden with combustible substances, leading to its oxygen depletion and therefore making ignition impossible. However, beware of the risk of hypoxia (reduction in the supply of oxygen to the body's tissues) if an employee is present in the area concerned.
Avoid sources of ignition
This preventive action firstly focuses on removing equipment that does not need to be there from the ATEX zone. It is then necessary to eliminate all potential sources of ignition, including flames and open fires, hot surfaces, sparks of mechanical, electrical or electrostatic origin, heating due to mechanical friction, electrical appliances or thermal engines. To achieve this, various measures can be implemented.
Act on the processes
These include the implementation of:
safety measures following exceeding the thresholds defined for certain safety parameters (temperature, pressure, oxygen level, etc.),
cooling systems to control, for example, a chemical reaction or heating due to gas compression,
magnetic, gravity separators (wedge boxes) to remove elements that could cause sparks or carry hot surfaces into ventilation networks.
Carry out checks
This involves checking that certain parameters do not exceed the thresholds beyond which inflammation is effective. For this, different detection systems exist:
temperature and pressure rise detectors, etc.
infrared thermography (detection of hot spots on electrical networks),
carbon monoxide detectors (detection of the start of fermentation with release of heat),
systems for controlling the speed of movement and/or offset of conveyor belts, jamming, rotation (limitation of friction, heating and electrostatic charges generated during the operation of this equipment).
Act on devices
mobile tools that do not cause sparks,
equipotentiality and earthing of the entire installation,
correctly sized and regularly checked electrical installation.
Implement organizational measures
operating procedure for carrying out tasks,
fire permit for all hot spot work,
work permit or intervention order for any operation in an ATEX zone,
limitation of the number of workers entering an ATEX zone,
consultation with external companies and management of co-activity (prevention plan, presence of a representative for external companies),
establishment of “smoking areas” to enforce the smoking ban in places where ATEX may occur,
wearing appropriate work clothes made of materials that facilitate the flow of electrostatic charges, regular cleaning by vacuuming for dust,
employee training.
All measures taken must be realistic and rigorous, in order to provide effective and appropriate prevention solutions.
Limit the effects of an explosion
Since it is difficult to fully control ignition sources, in the event that the formation of an explosive atmosphere cannot be avoided, protective measures must be adopted to mitigate the harmful effects of an explosion. The actions to be taken are specific to each work situation, process or installation:
actions on containment (explosion vents),
triggered fire extinguishers (explosion suppressors),
explosion overpressure resistant devices ,
technical decoupling systems (system which prevents an explosion from spreading to the rest of the installation: flame arresters, rotary valves, quick-closing valves, “Ventex” valves, triggered extinguishers, explosion diverter, clearance chimney, etc.),
actions on the configuration and design of the premises : compartmentalization, resistance of materials (glass, roofing in fragile materials, etc.), design and construction of the premises (choice of suitable and fire-resistant materials, premises resistant to the possible collapse of the building), grouping of personnel in dedicated locations so that they are not victims of falling structural elements.
Such technical means (vents, technical decoupling systems, etc.) are explosion protection systems under ATEX regulations and must therefore be recognized and certified as compliant with them.
Risk culture: the key to security
Strong commitment from Management on this subject
Mandatory training adapted for all those working in ATEX zones.
Prevention plans and work permits in ATEX zones drawn up with external companies.
Clear signage of premises and machines and compliance with procedures.
Regular safety drills .
Example : In a waste treatment company, raising awareness among operators about handling solvents helped prevent several major incidents.
Explosive atmospheres are invisible but dangerous. Thanks to European directives, certified equipment, and proper staff training, it is possible to control this risk. In the construction industry, as elsewhere, prevention requires analysis, rigor, and anticipation.
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