Gas Detection In Confined Spaces – The Risks & Detection of Flammable & Toxic Gases

Published 17 Mar 2017

  • By Chris Dodds : estimated reading time 11 minutes
Gas Detection In Confined Spaces

Flammable & Toxic Gas Detection In Confined Spaces

Detecting Gas

This article focuses on Gas Detection In Confined Spaces – the risks of entering and working in confined spaces and specifically those associated with oxygen depletion/enrichment and/or toxic and flammable gases. The aim is to educate people about the risks, understand the dangers and look at what measures can be taken to monitor and provide gas detection when working in confined working spaces.

Just last week another news article reported that two men had been seriously injured whilst working in a man hole. On entry, both men were exposed to toxic gas, one collapsed almost immediately and the other tried to escape but lost consciousness and fell causing serious injury.

The news article reported that one man was making a recovery but, “it did not look good for the other.”

Some people are oblivious to some of the risks as they can’t see the danger. Unlike working at height where the risk is blatantly obvious, toxic and flammable gases can be an “invisible killer.”

Guidelines and legislation are in place to help protect those exposed to the risk and it is the employers responsibility to carry out risk assessment and implement safe systems of work to protect their employees.

However, it is sometimes people’s attitudes towards safety and a lack of understanding about the gas risks which cause the majority of accidents.

There are many articles about poor working practices and poor attitudes towards the real hazards associated with the lack of gas detection in confined spaces, people who think because they have worked in the same confined space a hundred times, there is no need to carry out pre-entry checks.

People who have felt sick and dizzy so left the confined space until they felt better and then returned.

These problems exists in all countries and some may have a far better or worse approach to safety. It would be interesting to hear views from anyone else who has an interest in this topic.

What Is A Confined Space?

HSE definition: A confined space is a place which is substantially enclosed (though not always entirely), and where serious injury can occur from hazardous substances or conditions within the space or nearby (e.g. lack of oxygen). Some common and not so common examples are listed below:

  • Storage tanks
  • Silos
  • Reaction vessels
  • Enclosed drains
  • Sewers
  • Open-topped chambers
  • Vats
  • Combustion chambers in furnaces etc
  • Ductwork
  • Unventilated or poorly ventilated rooms
HSE Definition Of A Confined Working Space

HSE Definition Of A Confined Working Space

A confined space does not necessarily mean a fully enclosed area. Ducts, culverts, tunnels, boreholes, bored piles, manholes, shafts and excavations can all be considered as confined spaces.

The Risks & Hazards

  • Flammable Gases & Oxygen Enrichment
  • Toxic Gases
  • Oxygen Deficiency
  • Ingress of Liquid
  • Ingress of Solids
  • Excessive Heat

Other hazards may be a concern but those listed are specific to working in confined spaces. I will now go on the explore the first three in more detail.

ATEX : Gas Detection & Ventilation In Confined Spaces

It is essential to regularly check that confined spaces are free from both toxic and flammable gas and vapours where risk assessment indicates that conditions may be subject change. The stratospheric gases to be monitored are Methane (CH4), Carbon Monoxide (CO) and Hydrogen Sulfide (H2S) each of these gases can be found at different levels due to their density.

Gas Detection

Image Courtesy : SA Equip, Manufacturers Of The SA Flexiheat Portable ATEX Certified Heater For Zone 1 & Zone 2 Hazardous Area Heating

Flammable Gases & Oxygen Enrichment

A flammable gas is defined as a gas whose temperature and pressure make it flammable when air touches it at 12% by volume. Some common examples include Methane, Hydrogen, Propane and Butane.

Methane is particularly common in the petrochemical industry whilst Hydrogen is probably considered the most dangerous.

Flammable gases cause a serious risk of fire and/or explosion.

Flammable gases are produced in many industrial process, some examples of which include:-

  • Coating of articles with paint, adhesives or other
  • Solvent extraction processes;
  • Combustion of gas or oil;
  • Combined heat and power plants;
  • Battery charging.

When dealing with flammable gases it is important to understand the terminology used. LEL and UEL are both frequently referred to and mean:-

LEL: Lower Explosion Limit. This is the lowest concentration at which a gas can be ignited.

UEL: Upper Explosion Limit. The highest concentration at which a gas can be ignited.

The 3 ingredients for a fire

The 3 ingredients for a fire

Flammable gases can be present but will never ignite without two other ingredients. Air (oxygen) and a spark/flame must be present to complete the triangle and create the explosion. It is sometimes the case that a gas concentration is purposely kept above its UEL.

At this point there is not enough air (oxygen) in the environment to create an explosion.

Oxygen enrichment is also very dangerous. Approximately 21% of the air that we need to breath is made up of oxygen yet even a small increase in concentration (>24%) can create a very dangerous atmosphere. Flammability of materials increases as the concentration of oxygen increases.

The following video from YouTube demonstrates the dangers very well. If clothes become soaked in oxygen you could be in serious trouble. See what happens to the stick man at the end of the video.

Oxygen Accelerated Fire Demonstrations

Toxic Gases

The Health And Safety Executive

The Health And Safety Executive

A toxic gas is one which is poisonous and capable of causing illness, reduced quality of life or even death. They are present and can pose a risk in many industries. Some of the more common gases include carbon monoxide (CO), hydrogen sulphide (H2S) and chlorine (CI2) all of which can be very harmful even at low levels of exposure.

In confined spaces, toxic gases and fumes may be caused as a result of the previous process or previous storage. Gases can also enter from adjacent plant that has not been isolated correctly.

Contaminated ground can cause toxic fumes inside sewers and manholes. Acidic ground water acting on limestone creates carbon dioxide.

The type of work activity may also create toxic gases, for example spray painting, welding and flame cutting. Although diesel generators should never be used inside confined spaces, a recent incident occurred whereby carbon monoxide fumes from a nearby diesel generator entered a trench where men were repairing underground cables.

Fortunately the men were wearing portable gas detectors and were alerted to the danger.

In the UK, the health and safety executive stipulate work place exposure limits (WEL) for a number of toxic substances. These are occupational exposure limits averaged over:-

  • 15 minutes: Short Term Exposure Limit (STEL)
  • 8 Hours: Long Term exposure Limit(LTEL)

The limits set are usually time weighted averages (TWA) and are usually defined in parts per million. PPM. In the USA, Occupational Safety and Health Administration use a similar system to define permissible exposure limits. (PELs)

The table is taken from HSE document EH40/2005 Workplace exposure limits.

As an example it states the LTEL for Acetone is 500PPM and the STEL is 1500PPM. In contrast, Hydrogen Sulphide is considered extremely toxic and will cause nausea and headaches to those people exposed to levels between 2-5PPM.

Exposure to 1000 – 2000PPM results in instant death.

EH40 2005 Workplace Exposure Limits

Gas SubstanceCAS NumberWorkplace Exposure LimitComments
Long term exposure limit (8 hr TWA reference period)Short term exposure limit (15 minute reference period) The Carc, Sen and SK notations are not exhaustive. Notations have been applied to substances identified in IOELV directives. 
Acetic anhydride108-24-70.52.5210
Acetylsalicylic acid50-78-25
Acrylaldehyde (Acrolein)107-02-
Acrylamide79-06-010.3Carc, Sk
Acrylonitrile107-13-124.4Carc, Sk
Allyl alchohol107-18-624.849.7Sk
Aluminium alkyl compounds2
Aluminum metal7429-90-5

It must be said that the WELs are not a definitive line of what is deemed safe and unsafe levels but there to provide guidelines to be used in conjunction with COSHH.

Toxic gas detectors are used to monitor the concentration of gas in PPM and alert the worker to the danger. The STEL and LTEL can be used to determine the alarm threshold levels. (more on this later)

Oxygen Deficiency

Oxygen enrichment is indeed a hazard however the more obvious hazard is oxygen deficiency. As can be seen from the table, the “safe zone” is somewhere between 19.5% – 23%. There is some debate over the exact figures but in truth I’m not a doctor or an expert on the respiratory systems so can’t give a conclusive argument.

For those interested, you may wish to read:- Confined Spaces – Is 19.5% oxygen really safe?

So what causes Oxygen depletion? Our friends at Crowcon gas detectors offer the following explanation:-


Microbiological Action (bacteria)

Absorbtion (chemical processes)

Displacement by other gases (eg, Nitrogen, CO2)

Combustion (fires, generators etc)

Oxidation (rusting)

Work (respiration: people & animals)

Exposure to low levels of oxygen can cause giddiness, confusion, poor coordination, sickness, loss of consciousness and death. Without the use of correct monitoring and sampling methods, there are no warning signs of oxygen depletion.  Breathing one or two breaths of air containing oxygen levels below 10% can cause immediate loss of consciousness.

Minimising The Risks

There are extensive guidelines and legislation for minimising the risk such as the HSE approved code of practice for safe working in confined spaces provides. Rule No.1 is to avoid working in confined spaces if at all possible.

However, we all know there are hundreds of work activities that must be carried out in a confined space. Monitoring the atmosphere with gas detection equipment should always be part of any safe system of work for confined spaces.

Testing The Atmosphere For The Presence Of Gas Hazards

Before workers enter a confined space, the air should be tested from the outside. This can be done by lowering a portable gas detector, such as Crowcon T4, into the work area. Alternatively, a tube can be fed into the area and a sample can be drawn using a pumped gas detector.

Some gases are lighter or heavier than air and therefore it may be necessary to take samples from different heights within the confined space.

Testing should be carried out every time the area is re-entered to check there have been no changes to the atmosphere.

Baromic pressure can effect the atmosphere within a confined space. An increase will cause gas to expand putting more pressure on surroundings and keeping gas within the confined space and a decrease in Baromic pressure has the opposite effect and will force gas out of a confined space.

Continuous monitoring should also be carried out whilst workers remain in the area.

Portable Gas Detectors – Selecting The Correct Device

Gas Detection In Confined Spaces

Portable gas detectors can be used to sample and monitor confined spaces and instruments are available to detect a single gas or multiple gases.

The Crowcon clip is a single gas detector and can be used to detect Hydrogen Sulphide, Carbon Monoxide and Oxygen. This is a simple device which will activate an alarm in the event the target gas exceeds the set point.

More sophisticated detectors such as the Crowcon Gas-Pro can be used to monitor up to 5 different gases and  record various gas levels and allow for data logging which can be downloaded at a later date.

The choice of gas detector will depend on what gases/vapours have been identified as a potential risk.

When measuring flammable gases a detector usually measures the gas as a % of its lower explosion limit (LEL). When measuring toxic gases, the unit of measurement is normally PPM (parts per million).

The sensitivity of the gas detectors is normally factory set.

Regular bump testing should be carried out to ensure the detector is working correctly and the sensors have not been poisoned or contaminated. Full calibration should always  be carried out as per manufacturers recommendations.

Because an increase or decrease in oxygen levels has an effect on other flammable and toxic gases, this should be the priority gas. Second would normally be flammable gases and then toxic gases. A word of caution though, some gases are both flammable and toxic.

Crowcon Gas Pro Portable Gas Detector

Crowcon Gas Pro Portable Gas Detector

In this case you should detect whichever poses the greatest risk.

For example, Hydrogen Sulphide is a flammable gas in high concentrations but the toxic risk is much greater even at very small exposure levels. Testing should always be carried out by a competent person.


More Reading
Flame, Heat, Gas Detectors & Detection Systems

ATEX Beacon

Hazardous area industries including offshore oil/gas platforms and FPSOs, onshore oil refineries, processing plants, pipelines, storage farms and LPG/LNG plants all utilise or produce a wide range of hazardous flammable liquids and gases that can be detected using correctly specified flame and gas detectors.

Detecting toxic and flammable gases requires the detectors to be classified and certified according to the specific IECEx or ATEX standard – we distribute flame and fire detectors manufactured by Spectrex to operate in the harshest environmental conditions including self-contained stand-alone devices designed for direct connection to control and alarm systems or automatic fire extinguishing systems.

Our range of Hazardous Area Fire & Gas Detection System products also includes Explosion Proof Warning Systems & Hazardous Area Lighting – comprehensive range of intrinsically safe, flameproof and explosion proof alarm sounders, sirens, bells and horns, loudspeakers and beacons.

♦ LV Power Products | Junction Boxes & Enclosures ATEX | Plugs ATEX | Control Stations ATEX | Isolators ATEX

Detect Flame Heat Gas ATEX

Thorne & Derrick | Detecting Flame | Heat | Gas

Experts in Equipment

for Explosive Atmospheres

Experts In Equipment for Explosive Atmospheres & Hazardous Area Locations

Further Information | Lock | Door | Heater | Fan


Follow our Showcase Page on LinkedIn to receive hazardous area product innovations, industry news, whitepapers, videos, technical tips and training webinars for professionals involved in the explosive atmosphere industries.

Hazardous Area Explosive Atmospheres