Confined Space Entry: Know Your Gas Detection Requirements

Oct 22, 2019

Confined Space Entry: Know Your Gas Detection Requirements

by | Oct 22, 2019

Confined space entry and working in confined spaces poses dangers such as asphyxiation, explosion, fire and toxic substances. Read our guide to understand the risks of working in and around confined spaces.


The HSE Confined Space Regulations 1997 define a confined space as:

“A space of an enclosed nature where there is a risk of death or serious injury from hazardous substances or dangerous conditions”

Whereas some confined spaces are easy to identify, others are less obvious and often disregarded. Examples of this include; open-topped / combustion chambers, roof spaces, road tankers, and refrigerated storerooms.

It is also important to know that a confined space is not necessarily restricted to industrial applications; a confined space can range from cellars to enclosed sewers and drains and to canals.

A spae is confined if it is large enough for someone to enter and work in, but;

  • Has limited restriction of entry/exit
  • A volume of less than 100 m3
  • Has no means of ventilation
  • Below two metres in length, width and height



Under the Management of Health and Safety at Work Regulations 1999, Regulation 3, you must carry out a sufficient risk assessment for all work activities to decide what measures are necessary for safety. For work in confined spaces, this means identifying the hazards present, assessing the risks, and determining what precautions to take. In most cases, the assessment will include consideration of:

  • the task
  • the working environment
  • working materials and tools
  • the suitability of those carrying out the task
  • arrangements for emergency rescue.

Once an assessment has been carried out the worker must ask themselves:

  1. Can this work be carried out from an outside point?
  2. Can it be altered so that entry isn’t necessary?

If the answer to both of these questions is no and all options have been exhausted and the confined space must be entered, then the necessary action and equipment should be put into place for personnel protection.


No matter what the application or space, there is always the risk of extra hazards that arise as a result of work being carried out in a confined space;

  • Limited volume in a confined space is extremely harmful, as it can lead to a build-up of gases. Inadequate air leads to an oxygen deficiency and a replacement of harmful gases such as H2S or CH4 (depending on the application).
  • Fires and explosions become a lot more volatile in smaller spaces and, as a result, have a larger impact, the results of which are disastrous to personnel.
  • Free-flowing liquids and gases can be disturbed as a result of using heavy-duty machinery or even carrying out basic tests. Extra care must be taken to ensure all physical breaks in pipework and electrical have been isolated.
  • A build-up of temperature; a less apparent risk, but if space is gradually overheated, the worker’s body temperature is at risk of rising to unnatural levels.
  • The use of machinery poses a threat. Have all electronics been isolated by locking off isolating switches or removing fuses? Extra precautions should be taken, such as dust extraction or precautions against electric shocks.


If you cannot avoid entering the confined space, a confined space strategy must be implemented;

  • First things first; appoint a job supervisor to establish all items on the list are checked and the process runs as smoothly as possible
  • Secondly, a signed permit – a permit is more than a box-ticking exercise; not only does it provide traceability, but a permit will identify the correct procedure for work in a confined space.
  • Thirdly, ensure the workspace is mechanically & electrically isolated
  • Next, review employee training – this should address the company’s policy and confined space program; recognising the confined space warning and identification signs; changes in use or configuration of non-permit spaces that could require the space to be reclassified as a permit-required space.
  • Remember to plan an escape route! Although it seems obvious, it can be overlooked, remember confined spaces don’t necessarily share the same entrance/exit
  • Then, check the size of the space, taking care to note: have the entry/exit points been tried and tested? Can the staff be rescued in the case of an emergency?
  • Finally, pre-entry atmospheric tests of LELs in the air. Testing should be carried out by a competent person using a suitable gas detector that is correctly calibrated. Be aware that continuous monitoring of the air may be needed.


The atmosphere in confined spaces can very rapidly become extremely hazardous because of the lack of natural air and ventilation. Although the gases below are common gases, the build-up in a small space is deadly. Many processes such as welding not only consume oxygen but replace the oxygen in the air around to create gaseous containments.

Flammable Gases

Flammable gas arises from fumes, gases, vapours, and dust. The threat of fire or explosion arises from confined space contents or from the materials being used to clean the confined space. For example, a flammable liquid base or propellant gases of aerosol sprays.

In addition, debris and discarded materials from repair operations in confined spaces can be ignited by hot work operations. Where there is a possibility of flammable substances being present, then electrical equipment will have to be approved to eliminate the risk of a spark or ignition source.

Static electricity can also be a source of ignition in confined space operations where flammable gases and vapours are present.

Toxic Gases

Toxic gases, fumes and vapours can be identified using correctly specified confined space testing equipment.

Consideration should be given to the possibility of gases trapped within residues and sludge. For example, wastewater treatment plants, which may not have been identified by initial atmospheric testing, and can be disturbed and released by someone having recently entered into the confined space.

Where work in excavations is taking place, the contamination can come from hazardous substances previously deposited in the ground from natural sources. Hydrogen Sulphide often results from the bacterial breakdown of organic matter in the absence of oxygen gas, such as in swamps.

Oxygen Depletion & Enrichment

The normal oxygen level in the air is 20.9%, yet enriched levels above 23.5% can increase the risk of fire.

Oxygen can be diminished by the presence of rust or by operations such as welding or burning. Materials such as wet grain can also deplete oxygen.

Oxygen depletion has serious effects, more so in confined spaces. 19.5% is the minimum legal working level, below which harm can occur to anyone in the confined space. Oxygen depletion will quickly affect brain function and reduce the affected person’s ability to respond to their environment.

When oxygen depletes to concentrations of about 10% or less, the worker will begin to experience nausea and vomiting, leading to a loss of consciousness. At levels <6% the user will struggle with respiration and heart action will cease.

Download the Full Confined Space Guide