Mitigating against the dangers of confined space

As defined by the 1997 Confined Space Regulations, a confined space is: “Any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk.”

With regard to gas detection, what is the relevant specified risk? A specified risk is the loss of consciousness or asphyxiation of any person at work, which arises from gas, fume, vapour or lack of oxygen.

Every employer has a requirement under the Management of Health and Safety at Work Regulations to carry out an assessment of the health and safety risks to which their employees and others may be exposed.

Assessing risks

The general condition within the confined space should be assessed to identify which gases could potentially be present, not forgetting to also assess which gases are potentially not present, such as oxygen.

The ideal environmental conditions within the confined space will be fresh air, as found in normal atmosphere. Fresh air is a complex mixture of gases, the three principal ones being oxygen, nitrogen and carbon dioxide.

By volume, their composition is generally accepted as being:

  • Oxygen – 20.93%
  • Nitrogen – 79.04%
  • Carbon Dioxide – 0.03%

The air itself is colourless, odourless and tasteless, supports combustion and is necessary for respiration. As air is a combination of gases, it is possible for individual gases to be isolated through particular processes.

Where possible, it is useful to check records of both previous entries and any work undertaken within the confined space, for information on the environmental conditions that were encountered on those occasions. Account should be taken of the previous contents of the confined space, such as toxic or flammable substances and residues from chemical substances.Furthermore, the presence of rust, should be checked as this absorbs oxygen and can be a cause of oxygen deficiency. The presence of sludge should also be checked, as it can release gases, fumes or vapour when disturbed.

Consideration should also be given to the possibility that gas contamination can occur from leaks in adjacent work areas, including:

  • The effect of acidic ground water acting on limestone, resulting in a build-up of carbon dioxide
  • Methane produced during the decay of organic matter that could enter or be released into the confined space where the work is to take place

Oxygen and confined spaces

As detailed in the following sections, there are two particular issues with regard to the presence, or lack thereof, of oxygen in a confined space.

Oxygen enrichment

This can occur when certain processes are being undertaken in or near confined spaces; for example, oxygen leaking from cylinders used in welding or as produced by chemical processes. It can also occur when other gases such as nitrogen are removed from air. The increase of oxygen content results in a considerable increase in the flammability of combustible material. This may also alter the flammable range of flammable gases.

Oxygen deficiency

This can occur when a confined space is infrequently exposed to the atmosphere and rusting has occurred. This rusting is a slow form of combustion and absorbs oxygen. Again, other processes such as burning and welding can consume the oxygen content. There are many examples of fatalities occurring due to this reduction in the oxygen content.

Unconsciousness and subsequent fatalities, can result when the oxygen content drops to around 16%. In order to provide a reasonable level of safety, the lowest acceptable level of oxygen content should not be below 19%.

What is important, however, is that variation in oxygen content should be investigated and the reasons fully appreciated and understood.

Note that it is important to be aware that the person does not have to enter the confined space to be exposed to its risks. An example of this is a meter reader, who may kneel down and put their head in the confined space to read a water meter. This can be fatal due to the possible lack of oxygen.

Cleaning chemicals

COSHH assessments for any cleaning chemicals to be used in a confined space should be fully studied and understood to ensure the assessed risk of the chemical and its impact on the person using it are acceptable.

There was recently a very disturbing example of a person who was using chemicals to clean an aviation fuel tank. There was a lack of an effective risk assessment. The persons undertaking the work were aware that the use of the chemical caused them a problem within the confined space. This was identified as making them dizzy and feeling nauseous, but nothing was done to correct this situation. One person working with the chemical found that it was so hot inside the fuel tank that he decided to unplug the light to remove this as a source of the heat. When he unplugged the light, this caused a spark, which ignited the vapour from the cleaning chemical, resulting in an explosion severely burning the person concerned. His injuries were sufficient to prevent him from ever working again.

Further investigations also revealed that the cleaning chemical was not needed – warm soapy water did as good a job of cleaning the tank. This would probably have been identified sooner had an effective risk assessment been undertaken prior to the job commencing.

Detecting gas, vapour and fume

Any confined space can, and should, be tested from the outside prior to persons entering it.

This can be achieved through the use of tubing or a probe to draw a sample to the detection device. An alternative to this is to place or lower the detection device into the space and allow it to record the conditions. Most gas monitoring devices will retain peaks and troughs of gas readings obtained from within the confined space.

Where the risk assessment indicates a requirement to test the environment within a confined space, the space should be opened and ventilated, either mechanically or naturally, and tested prior to any person entering the space. If required, it should be tested again after a predetermined period ascertained by the risk assessment.

Testing should be carried out on every occasion the confined space is re-entered to ensure nothing has changed within the space. The reason for this is that small changes in barometric pressure can alter the atmosphere within a confined space. When persons are working within the space, the atmosphere should be constantly monitored, particularly if mechanical ventilation is a requirement of the risk assessment. Constant monitoring will also be required if the work being undertaken is particularly hazardous, such as cutting or welding.

Testing equipment

Gas detection devices can be purchased in various combinations of sensors. These vary from specific one sensor devices to multi sensor devices. They may also vary depending upon the design role of the software attached to the sensor. This can be as simple as recording the readings and activating an alarm if trigger points are reached. Others can record peaks and troughs for each period of testing. Some will allow full data logging of everything that is detected by the device over a period, with the capability of downloading the information onto a computer for its retention and analysis.

The choice of device used to test the atmosphere will depend on what is known about the confined space and the potential gas, vapour or fume likely to be present – not forgetting the possibility of oxygen enrichment or deficiency. A chemical detector tube may be an appropriate device, but where possible, portable electronic devices that constantly monitor the atmosphere in which the person is working should be considered. Whatever the choice of device, it is most important that it specifically monitors and detects the gas, vapour or fumes identified by the risk assessment.

Gas monitoring devices can be purchased using two measuring scales – percentage volume (vol) and percentage lower explosion limit (LEL), the latter being the most common.

Gas monitors that use the percentage volume scale record the actual concentration of gas within the environment in which people are working. If 1% methane is present in the working area, the gas monitor will display 1% methane gas. Some of these gas monitors can have a detection range of up to 5%, while others may have detection ranges of up to 100%.

Gas monitors that use the percentage LEL scale detect the gas in the working area and compare the amount detected against that needed to saturate the gas monitor. A percentage volume gas monitor that registers up to 5% methane is comparable to a percentage LEL gas monitor detecting 100% saturation. With this in mind, some percentage LEL monitors do in fact give gas readings in excess of 100% LEL.

A methane level of 1% in the working area will not register as such on a percentage LEL monitor – it will register as 20%. Likewise, if 5% methane is present in the working area it will register on an LEL monitor as 100%. These figures are fairly accurate when detecting methane gas, but may differ when detecting other gases. You will need to consult with the appropriate manufacturer to get accurate comparison ratios.

An LEL monitor has two major benefits:

  • The detection scale is more sensitive, meaning alarm levels can be set more accurately
  • When devices alarm on percentage LEL the reading on the gas monitor is much higher than the actual gas detected, which psychologically may encourage persons to exit the work area more quickly

Order of testing

The priority gas to be ascertained as to its presence is oxygen, due to the influence that it has on both the flammability and toxicity of other gases. Flammable gases should be tested next and toxic gases, fumes and vapours should follow this. It should also be remembered that some gases, such as carbon monoxide, are both toxic and flammable. In this example the toxic risk from carbon monoxide is the greater of the risks, but there are many examples where the flammable risk from the gas will be the greater.

Who should carry out the testing?

Testing should be carried out by a competent person who knows and understands the characteristics of the gases, fumes and vapours being detected. They should also know and understand both the relevant legal requirements and standards. They must be capable of interpreting the results and implementing any necessary actions resulting from the readings.

Many people at work are exposed to gases, fumes and vapours that can, if not controlled, have a harmful effect on their health. These are called hazardous substances and must be controlled.

European occupational exposure limits

Indicative Occupational Exposure Limit Values (IOELVs) are health based limits set out by the Chemical Agents Directive (98/24/EC). The Scientific Committee on Occupational Exposure Limits (SCOEL) advises the European Commission on limits. This committee evaluates the scientific information available on hazardous substances and makes recommendations for the establishment of an IOELV. These are listed in Directives that member states are obliged to implement by introducing national limits for the substances listed.

Workplace exposure limits

Workplace exposure limits, or WELs, are the British occupational exposure limits and are set in order to help protect the health of workers. WELs are concentrations of hazardous substances in the air, averaged over a specified period of time, referred to as a time-weighted average (TWA).

Two time periods are used:

  • Long term – 8 hours
  • Short term – 15 minutes

Properties of gases

Specific gravity

The specific gravity (SG) of a gas is the weight of that gas compared with the weight of the same volume of air at the same temperature and pressure.

Air is given an SG of one. A given quantity of a gas can then be compared against the same quantity of air. Gases with a specific gravity of less than one will rise in the atmosphere; gases with a specific gravity greater than one will sink and seek regions of low elevation at the earth’s surface.

Knowing the specific gravity of gases is of great use when working in confined spaces. It will not only give you clues as to where specific gases may accumulate in the confined space, but will also provide information on how their presence may impact on safe working within a confined space.

Examples of specific gravity include:

  • Methane – the SG is 0.55, this indicates it is lighter than air and so ensure that you test high up within the space
  • Carbon dioxide – the SG is 1.53, which indicates it is heavier than air, so ensure that you test low down within the space

The diagram below indicates specific gravities of various gases within a confined space.

Barometric pressure

Barometric pressure can have an influence in confined spaces. When the barometric pressure is rising, as measured on a barometer and usually indicating that good weather is on the way, the gases in the atmosphere expand, therefore exerting more pressure on their surroundings. This may keep gases within a confined space. If the barometer is falling particularly rapidly, then not only is bad weather on the way, but the gases in the atmosphere will contract, exerting less pressure on surroundings, meaning gases within a confined space may escape.

Understanding gases

The following gases comprise fresh air and common gases.


Oxygen is colourless, odourless and tasteless; in its pure state it is heavier than air. The human body requires a constant supply of oxygen in order to survive and this is normally obtained from the atmosphere around us.


Nitrogen is colourless, odourless and tasteless. It does not support life or combustion and is slightly lighter than air. It is the main constituent of air and acts as a diluent for oxygen.

Carbon dioxide

Carbon dioxide is colourless, has a slightly pungent or acrid smell and a soda water taste. It is heavier than air. Carbon dioxide plays a major role in respiration and cerebral circulation.

In high concentrations it acts as a respiratory and central nervous system stimulant. At excessive concentrations it depresses the central nervous system producing unconsciousness, narcosis confusion, stupor, respiratory arrest and death.


Hydrogen is colourless, odourless and tasteless. It is the lightest and simplest chemical element, with an SG of 0.07. It is found in natural gas from oil wells, certain volcanoes, and is a major constituent of water. Hydrogen has an explosive range of 4-74%.


Methane is colourless, odourless and tasteless, with an SG of 0.55. It is produced by the decomposition of vegetable matter and has an explosive range of 5-15%.

Carbon monoxide

Carbon monoxide is colourless, odourless and tasteless. It has an SG of 0.97. Carbon monoxide is produced by incomplete combustion. It is highly poisonous and has an explosive range of 12.5-74%.


Nitrogen is colourless, odourless, tasteless and lighter than air. It constitutes 79% of air.

Nitrogen dioxide

Nitrogen dioxide is a reddish brown gas, which has an acrid smell and an acid taste. It has an SG of 1.16. It is produced by diesel exhausts and explosives of the nitroglycerine type. It is highly poisonous.

Hydrogen sulphide

Highly poisonous, hydrogen sulphide is colourless, has a sickly sweet taste and smells of rotten eggs, as caused by acidic mine waters acting on certain sulphides when using explosives. With an SG of 1.18 it is heavier than air. Its explosive range is 4.3-43%.

Sulphur dioxide

Sulphur dioxide is colourless, but has a pungent, suffocating, sulphurous odour and an almost intolerable acidic taste. It occurs when there are fires or spontaneous combustion in coal, when rubber is burned and from diesel exhaust fumes. Its SG of 2.26 means it is considerably heavier than air. It is highly poisonous.


Ammonia is colourless with a pungent smell. It is lighter than air, as denoted by its SG of 0.68. It is poisonous and occurs naturally from bacteria acting on urine and faeces. It is produced commercially for use in refrigeration systems, fertilisers, animal feeds and manufacturing processes to name a few. Ammonia is being used increasingly in refrigeration plant.

Flooding fire suppression gases

Banks of fire extinguishers were commonly used in transformer rooms and substations to flood areas with an inert gas to put out fires. The gases most commonly used in these types of systems were carbon dioxide and nitrogen.

Good practice

Ensure the whole area within the confined space is examined and continue to monitor the area while people are in the confined space. If the gas levels alter, ensure you know why and what the effect will be on the persons working in the confined space.

Ensure the readings are within workplace exposure limits and that workers stay within safe limits.

One other form of testing that may be useful is spot sampling, especially if the occupation of the confined space is to be ongoing and the nature of the work is high risk. In spot sampling you take a reading at a specified time period using the same device at the same point within the space. This method is useful for identifying a trend within the space, such as whether conditions are improving or deteriorating. It also indicates external influences that may affect the confined space, such as barometric pressure.

Published: 22nd Sep 2014 in Health and Safety International