Minimum and Maximum Gas Levels in Confined Spaces

29th January 2024
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When working in confined spaces it is possible that the air around you may be quite different from the air outside. In such environments, understanding and monitoring gas levels becomes paramount for ensuring the safety and well-being of workers.

In this article, we explore the importance of assessing gas levels in confined spaces, the common gases that pose risks, and the measures that can be taken to mitigate potential dangers. Read on and stay safe!

What does the normal atmosphere consist of?

The air around us is also known as the atmosphere and comprises approximately:

  • 21% Oxygen
  • 78% Nitrogen
  • 1% Argon and other gases
Gases within normal atmosphere

What causes dangerous gas levels in confined spaces?

When working in confined spaces, several factors can introduce dangerous gas levels into the local atmosphere, including:

  • Use of cutting, burning and welding equipment
  • Use of diesel powered vehicles or equipment
  • Leaking oxygen cylinders
  • Oxidisation of metals
  • Changes in weather conditions

Gases are a commonly acknowledged hazard when entering/working in confined spaces.

They can mix with air to form a mixture that can explode, irritate, poison or asphyxiate workers.

Should you check gas levels before entering a confined space?

Where the environment within the confined space cannot be guaranteed to be safe, no person should enter without there being prior and continuous atmospheric monitoring which, as a minimum, monitors for:

  • Low oxygen
  • Flammable gas
  • Toxic gas

There will be some instances where the nature for which the space was designed and used means that there is a requirement for other types of monitoring (e.g. carbon monoxide, chlorine, hydrogen sulphide etc). The risk assessment should determine the requirement for this type of monitoring and plan to put the monitoring and controls in place.

What is a safe level of oxygen in a confined space?

An oxygen deficient atmosphere can occur when the oxygen content falls to less than 19.5% (this is normally the pre-set alarm level for environmental monitors). This may lead to asphyxiation (suffocation) and death.

The following outlines the effects that a reduced oxygen content may have on persons working within it:

  • 21-19% - Fit for respiration (as long as no other contaminants are present)
  • 16% - Dizziness, shortness of breath, increased heart and respiration rate, diminished concentration and reasoning, ability and awareness to make recovery decisions highly impaired (i.e. the decision to exit the confined space)
  • 10% - Nausea, vomiting, muscular incapacity
  • 6% - Rapid loss of consciousness and death


What does the Specific Gravity of gases mean?

The specific gravity of a gas simply means its weight compared to the same volume of air. It is measured using a numerical scale, with air serving as the benchmark and assigned a value of 1. 

Any gas with a specific gravity higher than 1 will fall towards the Earth’s surface, whilst any gas with a specific gravity lower than 1 will rise in the atmosphere. But why is this important?

Well, if you know the specific gravities of particular gases, you will understand where they might accumulate in confined spaces. Gases can be heavier or lighter than air, so you need to ensure that environmental monitors are placed in the correct position within the confined space to detect the gas.

Additionally, you’ll have a better estimation of how the presence of specific gases may affect safe working in a confined space.

Behaviour of gases


How many different types of gas are there in a confined space?

As well as those gases listed above, be aware that gases present in a confined space may also include:

  • Asphyxiant - A gas or vapour that can cause unconsciousness or death by suffocation due to lack of oxygen
  • Flammable - Gases which are easily ignited and capable of burning rapidly. A flammable gas is one which burns and, when mixed with air between certain limits, will propagate a flame away from the source of ignition. These limits are sometimes referred to as the explosive limits because pressure is often associated with the flame propagation
  • Toxic - The characteristic of a chemical substance to produce injury once it reaches a susceptible site in or on the body. The effects may be acute or chronic, local or systematic
  • Explosive - This is where a flammable gas is present in air within certain concentrations that, if an ignition source is introduced, results in extremely fast burning of the whole volume in the atmosphere – better known as an explosion

Can gas levels in a confined space be regulated?

The HSE has published a document called EH40 “Workplace Exposure Limits”, which sets out standards for the amount of a substance that an individual may be exposed to over time. These are:

  • WEL – this is known as a working exposure limit and it is believed that people can be exposed to this amount without any adverse effects over an 8-hour period
  • STEL – this is known as short term exposure limit and it is believed that people can be exposed to this amount without any adverse effects over a 15-minute period

Most environmental monitors are set to alarm once a WEL has been reached, although your employer can have these settings altered.

Combustible gases are usually measured in percentages, and each flammable gas has its own explosive range. For example, methane has an explosive range of 5 -15%:

  • If there is less than 5% methane in the atmosphere, the gas will burn if a flame is introduced.
  • If there is between 5 -15% methane in the atmosphere, the gas will “explode” if a flame is introduced.
  • If there is more than 15% methane in the atmosphere, the gas will burn if a flame is introduced (however, should the % of methane drop into the 5 -15% explosive range, the gas may “explode” if still exposed to a flame).

The following table sets out some of the gases that may be encountered during confined space work activities. It sets out where gas is likely to be found, its safe working limits for 8 hours and 15 minutes, its explosive range and its principal danger to health.

A screenshot of a chart

Description automatically generated

  • Workplace exposure limit
  • ** Short term exposure limit
  • *** Lower explosive limit – upper explosive limit

What equipment do you need to test gas levels in a confined space?

Although there are many ways in which to monitor the environment, portable multi-gas monitors are widely used for confined space gas detection.

Without using environmental monitors, you may not know you are in a life threatening atmosphere, as dangerous, colourless, odourless and tasteless gases may be present.

Improve confined space safety with MRS Training and Rescue

MRS Training and Rescue offers multiple specialist confined space safety training programmes. With nationwide, state of the art facilities, we can provide hands-on, practical confined space training in a safe and controlled environment.

Our confined space training includes low risk, medium risk and high risk programmes, as well as several rescue and refresher courses to ensure the highest level of safety when working in these challenging environments.

Final thoughts

Environmental monitoring is an important part of confined space work. Without using environmental monitors to ensure safe gas levels in a confined space, you may not know you are in a life threatening atmosphere — as colourless, odourless and tasteless gases may be present.

Contact MRS today to learn more about working in confined spaces, or to book confined space training.

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