Control measure – Safe system of work: Atmospheric conditions

Control Measure Knowledge

The working atmosphere of some incidents may not be safe to operate in without appropriate respiratory equipment (RPE), even if the atmosphere is initially found to be respirable. Portable or fixed on-site equipment can be used to test the atmosphere, to establish if there is sufficient oxygen to support life or if there is a risk of irrespirable gases.

Atmospheric testing and monitoring should be carried out at incidents where there is potential for the atmosphere to be explosive (due to flammable gas), toxic, asphyxiating or hypoxic. Environments can include:

  • Confined spaces
  • Areas where hazardous materials are present
  • Areas surrounding a fire
  • Post-fire operations, such as damping down
  • Fire investigation sites

Testing should also be carried out if it is known that the atmosphere was previously contaminated and subsequently ventilated, such as an underground petrochemical tank.

Fire and rescue services should consider using atmospheric testing and monitoring equipment that will test and display the:

  • Oxygen level in the atmosphere
  • Presence of flammable gases
  • Presence of toxic gases

Exposure limits

Atmospheric monitoring equipment takes into account workplace exposure limits (WEL) that are produced by the Health and Safety Executive (HSE). WEL is the exposure of employees to hazardous conditions such as gases, dust and noise. The aim is to ensure that levels in the workplace are below the statutory limits. For further information refer to the Health and Safety Executive (HSE) publication, EH40/2005 Workplace exposure limits

Testing and monitoring the atmosphere

Before operations commence, the atmosphere should be tested. However, if there is a threat to life and it is not possible to carry out atmospheric testing in the timeframe required, immediate life-saving actions can be carried out with the use of breathing apparatus and entry control procedures.

If possible, testing should be carried out while limiting exposure to personnel. Incident commanders should consider using on-site testing equipment or using extendable equipment or lines to lower portable testing equipment inside the hazard area and withdrawing personnel. It may be necessary to request specialist advice or assistance for testing and monitoring atmospheric conditions.

Ideally the atmosphere of every hazard area should be tested prior to entering. To ensure the test is effective personnel should consider:

  • Remaining still, as this can assist with:
  • Allowing adequate time for equipment to sample, test and display the results
  • Preventing slips, trips and falls if personnel are on uneven or unstable ground, especially when reading the equipment
  • Sampling at various levels due to various densities of gases

Regular monitoring is necessary to identify any changes in the atmosphere; this can be achieved actively or passively. Active monitoring is where personnel use portable detectors, often attached to themselves to monitor the atmosphere they are currently exposed to. Passive monitoring is used to monitor a specific area, such as portable detectors positioned temporarily in one place or fixed on-site equipment.

Atmospheric testing should be carried out by competent personnel who are aware of the limitations of the equipment in use, ensuring the results are regularly recorded.


Ventilation may help to improve conditions for personnel and increase the potential for casualty survival.

The hazard area should be assessed to determine if ventilation would be appropriate and successful. The surrounding infrastructure and what will be released should be taken into consideration, as flammable gases may ignite if there is an ignition source near to the outlet vent.

Ventilation can be achieved naturally, such as by opening windows or inspection holes, or through forced ventilation, such as the use of mechanical fans. The use of fans with combustion engines should take into account their exhaust gases. For further information refer to Consider employing tactical ventilation.

Gas purging of spaces using inert gases is a ventilation technique performed in industry, often within confined spaces, to mitigate the risk of explosive atmospheres. This would be inappropriate if casualties are inside the confined space.

Removal of residues or materials

The removal of residues or materials, such as sludge or chemicals, may reduce the quantity of toxic or asphyxiate gases being released. However, this activity should be subject to a risk assessment as it may release more gases.

Monitoring equipment alarm actuation

Atmospheric monitoring equipment may detect different gases and levels of gases. Personnel should understand how the monitoring equipment functions and take appropriate action if it actuates. This can include the need for all personnel within the hazard area to withdraw to a safe area and review why the alarm actuated. Personnel should brief the incident commander, providing details of time, location and the actions being carried out when the alarm actuated.

Limited capability of atmospheric monitoring equipment

Most atmospheric monitoring equipment is calibrated to detect specific gases, as detailed in the manufacturer’s specification. This means that the equipment has limitations for detecting other gases that may be in the area. Some detectors can be changed; however, doing so requires trained personnel and specialist equipment.

If the substance is unknown, the use of a regional detection, identification and monitoring (DIM) officer or other specialist advisers can be requested through National Resilience Fire Control (NRFC). Due to the limited number of DIM suites and their geographical location, incident commanders should be aware that their attendance may be delayed.

Strategic Actions

Tactical Actions