Flammable Gas

Hazard Description 

When flammable or combustible substances are mixed with air, they can create an explosive atmosphere (ATEX = ATmosphere EXplosive).


A flammable or combustible substance could be in gas or liquid form. To be noticed that for liquids, the vapours are the flammable part (the “flash point” is the temperature at which a particular organic compound gives off sufficient vapour to ignite in air, with a source of ignition).

Mixtures of flammable or combustible substances with air will burn only if the fuel concentration lies within the following limits:

  • Lower Explosive Limit (LEL), which is the lower percentage of gas/vapour in air capable of creating an explosive atmosphere;
  • Upper Explosive Limit (UEL), which is the upper percentage of gas/vapour in air capable of creating an explosive atmosphere.



The main flammable gas hazards are:

  • Fire
  • Explosion.

The ATEX directive has the following objective:

  • Avoid the creation of explosive atmosphere;
  • Avoid the presence of ignition sources;
  • Decrease the effects of an explosion.

For this reason, if needed, the areas where flammable gases/liquids are present can be classified as ATEX hazardous area zoned as following:

  • 0 – if an explosive atmosphere is permanently present;
  • 1 – if an explosive atmosphere is likely to occur in normal operation;
  • 2 – if an explosive atmosphere is not likely to occur in normal operation.

The point of entry to an ATEX area must be indicated with the following panel:



CERN Safety Rules

The CERN safety documents concerning the risk from explosive atmospheres are the following:


In case a gas detection system is installed, the safety procedure EDMS 2053721 (which describes the actions to take if one of the detectors is triggered), should be followed.

Role of the EP Flammable Gas Safety Officer (FGSO)

We provide information and expertise on the use of flammable gas, which must comply with the CERN Safety Rule GSI-C-2 (see above). 

Prior to the use of flammable gases, and in order to comply with the requirements of this rule, an explosion risk assessment shall be carried out and the hazardous areas shall be classified when required. 

The following safety forms should be completed prior to the usage of flammable gas:

Any purchase of flammable gases is subject to prior authorization by the EP FGSO.


Guidelines for the usage of flammable gas


Training Courses

On the CERN Learning Hub, it is possible to increase the awareness of the flammable gas hazards and associated risks by following the training courses:

  • ATEX Habilitation Level 1 (classroom)
  • ATEX Habilitation Level 2 (classroom)
  • Chemical Safety – Awareness (online)


For further information about flammable gas hazards and risks, please contact: