Peter Walsh, Health & Safety Laboratory, UK
- 1 Introduction
- 2 General gas properties and gas dispersion
- 3 Safety
- 4 Toxic effects
- 5 Gases in the workplace
- 6 Risk management
- 7 Emerging risks
- 8 Summary
- 9 References
- 10 Links for further reading
Gases and vapours are commonly encountered in the workplace at normal atmospheric pressure or elevated pressure. They arise from very many industrial processes. Generally, vapours are produced through evaporation of volatile liquids. The risks they present can be flammable and explosive, toxic, and asphyxiating. Furthermore, there are additional risks associated with the use of pressurised gas systems. In this article, the properties of gases/vapours, their type and typical occurrences in the workplace, and the methods used to control risk and mitigate effects are discussed.
Gases and vapours are, henceforth, referred to collectively as gases in this article unless specifically stated.
General gas properties and gas dispersion
At typical ambient pressures and temperatures encountered in the ambient workplace atmosphere, gases remain in the gaseous state, whereas a vapour can also co-exist with its liquid at certain pressures or temperatures. The concentration of a vapour can be lowered by condensation due to increased pressure or reduced temperature. The maximum vapour concentration above the liquid at a given temperature is known as the saturated vapour concentration. Some gases and vapours can react chemically with each other upon mixing.
The density of a released gas affects its buoyancy: a less dense gas than the surrounding air will rise initially while gases heavier than air will fall and tend to flow into low spots, e.g. pits, trenches. The temperature of the released gas also plays a part: if a source of release and the surrounding air is substantially warmer than the ambient air, the released mixture can rise initially, even though the relative density of the mixture, at the ambient temperature, is higher than that of the air. The converse applies where the release is colder than the ambient temperature.
All gases and vapours will fully mix with each other if stirred, e.g. by convection, natural or mechanical ventilation, or by diffusion (the latter typically occurring over a longer period of time than the former). Once gases and vapours are mixed they will remain mixed, unless a component is removed, e.g. by chemical reaction or absorption.
The extent and rate of build-up of both flammable and toxic releases can be affected by properties of the released gas and the process or location of the release. These can be largely characterised by the following chemical and physical factors:
- Release rate of gas or vapour
- Flammable, exposure limits
- Relative density of the released gas
- Temperature, pressure of the released gas
- Climatic conditions
- Site topography and congestion
Mathematical models can be employed to predict the movement of gas and they find use in, for example, zoning of hazardous areas derived from calculation of flammable gas cloud size, estimating the exposure over an area resulting from a toxic gas release, optimising the location of flammable/toxic gas detectors.
Fire and explosion risks
A gas mixture is only capable of supporting combustion in air, resulting in a fire or explosion, when an ignition source is present, e.g. arc, flame, heat, between certain concentration limits. These limits are known as the Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL), which are expressed as a percentage of the gas in air. Lists of Lower Flammable Limits and other flammability-related data can be found in various reference sources, e.g. EN 60079-20-1.
It is therefore necessary when using flammable gases to ensure that control measures are adopted to minimise the risk of fire and explosion. This can be achieved in various ways by ensuring that (a) there are no ignition sources, although any leakage of gas may result in a health risk (see later); (b) the concentration of any flammable gas leaking into the workplace atmosphere is kept well below the Lower Flammable Limit, e.g. by using ventilation, although, again, there may be a health risk; (c) the gas mixture inside the receptacle or reactor is always non-flammable by reducing the oxygen content..
Oxygen at elevated levels becomes a fire risk (normal levels at 20.9% volume to volume ratio - v/v) e.g oxy-gas welding. Even an increase in the oxygen level in the air to 24% can create a dangerous situation where it becomes easier to start a fire, which will then burn hotter and more fiercely than in normal air; it may be almost impossible to put the fire out. Oxygen is also very reactive: pure oxygen, at high pressure such as from a cylinder, can react violently with common materials, e.g. oil and grease. Other materials may catch fire spontaneously and nearly all materials, including textiles, rubber and even metals, will burn vigorously in oxygen.
For a substantial number of gases, particularly hydrocarbons, ill health effects can occur at concentrations much lower than their Lower Flammable Limits.
Risks associated with pressurised gases
The main hazards associated with pressurised gases are:
- impact from the blast of a gas cylinder explosion or rapid release of compressed gas;
- impact from parts of gas cylinders or valves that fail, or any flying debris;
- contact with the released gas or fluid (such as chlorine), e.g. cooling and freezing of body parts with liquefied gases such as nitrogen, helium, carbon dioxide;
- fire resulting from the escape of flammable gases or fluids (such as liquefied petroleum gas);
- impact from falling cylinders;
- manual handling injuries;
The main causes of accidents with pressured gases are:
- inadequate training and supervision;
- poor installation;
- poor examination and maintenance;
- faulty equipment and / or design (e.g. badly fitted or incorrect valves and regulators);
- poor handling;
- poor storage;
- inadequately ventilated working conditions;
- incorrect filling procedures;
- hidden damage.
Gases in cylinders are classified as Dangerous Goods and as such their transport is governed by European legislation. Industrial gases are subject to colour coding: labelling information identifies the hazards associated with the gas. Appropriate safety measures must be taken when handling, storing and transporting pressurised gas containers to prevent loss of containment and other accidents
Exposure to toxic gases in the workplace can result in, under some circumstances, a harmful effect on workers’ health. If exposure to the toxic gas is not properly controlled it may cause harm by being taken into the body through breathing; being absorbed through the skin, e.g. high exposures to bromomethane, hydrogen cyanide; or acting directly on the body at the point of contact, e.g. acid gases on the skin. Some illnesses caused by exposure to hazardous gases in the workplace may not appear until a long time after the first exposure, e.g. carcinogens such as benzene. Other gases, depending on their concentration, may have a rapid effect, e.g. irritants, carbon monoxide which reduces blood oxygen, and sensitisers such as isocyanates.
Certain gases, while not toxic themselves, can, at high concentrations, reduce the oxygen content of the atmosphere to cause asphyxiation. Catastrophic leaks of inert gases such as nitrogen and argon have resulted in many fatalities. Note that carbon dioxide does not act solely as an asphyxiant. It does have a Limit Value and health effects could occur at concentrations much lower than those relating to oxygen deficiency (less than 19.5% oxygen).
Some gases can act as anaesthetics, for example, hydrocarbons which have sufficient vapour pressure exhibit central nervous system (CNS) toxicity. This occurs at lower concentrations than those associated with asphyxiation. Depending on the substance, the exposure concentration and the period of exposure, symptoms range from subtle behavioural effects, through headache, 'intoxication' to unconsciousness. These effects are often referred to as ‘narcosis’.
Finally, radon is a radioactive gas, which emits alpha particles. It is naturally occurring and used in medical environments. Radon is a carcinogen, causing lung cancer.
Gases in the workplace
Gases are a subset of chemical agents, and are found in very many industries. Gases may be conveniently classified into inorganic and organic gases in order to highlight their major usage in industry. Generally, for inorganic gases, the toxic and asphyxiation risk is more prevalent. A few inorganic gases, however, additionally present a flammable/explosive hazard, if allowed to reach concentrations above the Lower Flammable Limit. The number of organic gases found in the workplace is enormous and can be found together in simple or complex mixtures in the gaseous or liquid state. Organic gases are used in the petrochemical and chemical industries as feedstock, products or by-products. Additionally, the use of organic liquids as solvents widens the range of gases to which workers are exposed to and which may form flammable atmospheres.
Furthermore, toxic gases can occur ‘indirectly’ in the workplace as products of combustion, e.g. use of fuel-powered electricity generators, heating or cooking appliances.
The transport of materials in, for example, cargo containers when loaded or unloaded can result in the exposure of workers to fumigant gases such as phosphine, and methyl bromide, in addition to industrial gases emitted by the cargo itself.
Gases in their compressed state, particularly compressed air, steam, natural gas and liquid petroleum gas (LPG), are virtually indispensable to a wide range of industries. Pressure receptacles, which include gas cylinders, are a convenient way to transport and store gases under pressure. These gases are used for many different purposes including:
- chemical processes;
- welding and flame cutting;
- breathing (e.g. diving, emergency rescue);
- medical and laboratory uses;
- dispensing beverages;
- fuel for vehicles (e.g. fork-lift trucks);
- extinguishing fires;
- heating and cooking;
- water treatment.
Chemical reactors which use gases as feedstock under pressurised conditions are also widespread throughout industry, particularly in the chemical, petrochemical and manufacturing industries.
The use of pressurised gas containers can introduce additional risks other than those due to the hazardous properties of the gas itself from a leak, e.g. physical injuries due to high pressure and/or extremely low temperature of the escaping gas, and accidents from improper use and transport of gas cylinders.
Gases are particularly associated with the special risks of working in confined spaces. The lack of understanding or care when in such an environment has resulted in many fatalities, injuries and ill-health due to asphyxiation, toxic gas exposure and/or fire/explosion. Examples of gases and the typical processes where they are released in confined spaces include:
- Consumption of oxygen and release of carbon dioxide, methane and other gases by living organisms – fermentation including brewing, sewage systems, waste management
- Loss of oxygen or liberation of other gases by chemical reactions - rusting, oxidation, outgassing
- Sudden release of high concentrations of solvent vapours, process gases, refrigerants - tank cleaning, sludge removal, cold room repairs, etc.
- Deliberate creation of high concentrations of gases/vapours, leading to oxygen deficiency (e.g. inert gases, argon, nitrogen, carbon dioxide, solvent vapours) - purging, gas freeing, inerting, solvent degreasing, etc.
- Build-up of gases directly caused by the work process (e.g. welding fume, carbon dioxide, carbon monoxide, welding shield gases, solvents, isocyanates, refrigerant gases) - welding, spraying, pipe freezing etc.
Examples of the more common inorganic gases encountered in the workplace, which are predominantly toxic, although some also present a flammable risk, with some of their principal uses listed, are shown in Table 1:
|Ammonia||Toxic/flammable||Fertilisers or from bacterial action on organic material, refrigeration (see, for example ), synthesis|
|Carbon dioxide||Toxic/asphyxiant||Refrigeration, beverages, synthesis, combustion product|
|Carbon monoxide||Toxic/flammable||Synthesis, fuels, combustion product|
|Chlorine||Toxic||Synthesis, food processing, water treatment|
|Fluorine||Toxic||Synthesis, water treatment|
|Hydrogen chloride||Toxic||Synthesis, chemicals and food processing|
|Hydrogen fluoride||Toxic||Synthesis, aluminium production, metals/glass processing|
|Hydrogen sulphide||Toxic||Chemical processing, oil by-product, bacterial action by-product|
|Mercury||Toxic||Electrical/electronics, chemicals manufacture|
|Nickel carbonyl||Toxic/flammable||Nickel processing|
|Nitrogen monoxide||Toxic||Nitric acid synthesis, process and combustion product, medical|
|Nitrogen dioxide||Toxic||Nitric acid, fertilisers, explosives, synthesis, materials|
|Nitrous oxide||Toxic/asphyxiant||Anaesthetic, specialist fuel|
|Ozone||Toxic||Water treatment, bleaching|
|Phosgene||Toxic||Synthesis, by-product from chlorinated hydrocarbons processing|
|Radon||Toxic/radioactive||Medicine, naturally occurring|
|Sulphur dioxide||Toxic||Synthesis, bleaching, food, fertilisers, catalyst|
Source: adapted from
The presence of asphyxiating gases in the atmosphere reduces the oxygen concentration (partial pressure) although typically the atmospheric pressure is around 1 bar. Altitude, other hypobaric (low pressure)  and hyperbaric conditions also change the partial pressure of oxygen by affecting the atmospheric pressure.
Inorganic gases which could present an asphyxiation risk include:
- Argon – used in welding and metal treatment
- Carbon dioxide (both toxic and asphyxiating) - fruit storage
- Nitrogen – inerting flammable atmospheres
- Helium – cryogenics, specialist use
- Water vapour (steam), only at elevated temperatures - inerting flammable atmospheres, process industries
Organic gases can present a flammable, toxic and asphyxiation risk. Although the number of organic gases in industrial use is vast, many of these gases arise from the petrochemical and chemical industries. Examples of products from ethylene synthesis include: ethylene oxide, vinyl acetate, ethylene dichloride, acrylonitrile, styrene, butadiene, olefins, and ethanol.
The hydrocarbons methane, liquid petroleum gas (LPG) and acetylene are commonly used as controllable fuel sources. Methane is the principal component of (liquid) natural gas and is a major industrial and domestic fuel. LPG is also used in a range of applications including in heating and cooking appliances, industrial applications, in vehicles and as a propellant and refrigerant. LPG can be obtained primarily as propane, butane or a mixture of the two. Acetylene is used in cutting and welding tools. It is an extremely flammable and unstable gas; under certain conditions it can decompose explosively into its constituent elements, carbon and hydrogen.
Solvents, usually a product or by-product of petrochemical processing, are an extremely important group of industrial chemicals. Their improper use can lead to significant vapour concentrations in the atmosphere which can result in ill health and, at greater levels, fire and explosion. The common solvent classes are:
- Aliphatic, e.g. heptane, white spirit
- Aromatic, e.g. toluene
- Oxygenated, e.g. methyl ethyl ketone
- Halogenated, e.g. dichloromethane
- Nitrogen and sulphur-containing, e.g. triethylamine
- Green solvents, e.g. supercritical carbon dioxide, methanol, ethanol.
Exposure to hydrocarbon gases and their often complex mixtures, commonly found in the petrochemical industry including offshore installations, can result in acute toxic effects. Examples are: gaseous hydrocarbons C1 – C4 (lowest acute toxicity); aromatics (most acutely toxic of the hydrocarbons series with both narcotic and irritant properties at relatively low levels of exposure); C5 to C9 hydrocarbons (acute narcotic effects at relatively low exposures); hydrocarbons above about C12 (acute inhalational toxicity increases with the number of carbons but is counterbalanced by a reduction in volatility).
Control and mitigation measures
The hazards associated with gases should be dealt with using the hierarchy of control. If their elimination or substitution is not possible, then hazardous gases require appropriate controls to minimise risk and mitigation measures to minimise any detrimental effects from their release. For flammable risks, the following directives apply: Directive 99/92/EC (also known as the 'ATEX Workplace Directive') on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres (see guidance in ); and Directive 94/9/EC (also known as 'the ATEX Equipment Directive') concerning equipment and protective systems intended for use in potentially explosive atmospheres. For health related risks, the relevant Directives are 98/24/EC Protection of the health and safety of workers from the risks related to chemical agents at work and 2004/37/EC Carcinogens or mutagens at work .
Control techniques for fire/explosion gaseous hazards are covered in various references and include:
- the avoidance of ignition sources;
- the avoidance of the propagation of fires or explosions;
- the provision of explosion pressure relief;
- the provision of explosion suppression equipment;
- the provision of plant able to withstand the pressure likely to be produced by an explosion; and
- the provision of suitable personal protective equipment (PPE).
Mitigation techniques for flammable/explosive gases include:
- reduction of the quantity of gas to a minimum;
- avoidance or minimising of the release of gas;
- control of the release of gas at source;
- prevention of the formation of an explosive atmosphere, including the application of appropriate ventilation;
- ensuring that any release of gas which may give rise to risk is safely contained, removed to a safe place, or otherwise rendered safe, as appropriate;
- avoidance of ignition sources and adverse conditions;
- segregation of incompatible dangerous substances.
Control and mitigation techniques for toxic gases and asphyxiants include control of the release, e.g. through engineering controls; respiratory protective equipment (RPE); and provision of suitable ventilation. If respiratory protective equipment is to be used then an appropriate filter is required to remove gaseous contaminants effectively from workplace air. They should not be used for protection in situations with reduced oxygen levels. In certain, more dangerous situations, breathing apparatus has to be used which requires a supply of breathing quality air from an air cylinder or air compressor.
Exposure to toxic gases can be reduced by the use of suitable ventilation, particularly local exhaust ventilation (LEV) by confining the gases to a specified area, e.g. fume cupboards, or removing the gas before it can build up to Limit Values, e.g. extraction hoods.
For gases which are both flammable and toxic, the implementation of adequate controls to control the toxicity risk will also control the flammable risk because exposure Limit Values are much lower than Lower Flammable Limits.
Assessing exposure to toxic gases
Exposure is principally via the inhalation route although, at sufficiently high concentrations, skin absorption or tissue damage may occur.. Factors which influence exposure to gases, in addition to those listed in Section 2.2, include:
- the number of sources from which the gas is released;
- arrangement of the process and design of the control measures;
- variations in the process, job and tasks being carried out.
Also, the workers can also influence the level and pattern of their exposure, for example:
- type and position of each source relative to the worker;
- length of time the worker spends in the vicinity of the source;
- whether the worker has direct control over the task or process and an appreciation of the risks involved;
- type, maintenance and suitability of respiratory protective equipment (RPE).
Also, for certain tasks, e.g. welding, where a variety of hazardous gases (and fume) are generated, there could be unintended co-exposure.
Exposure measurement for gases may involve air sampling or possibly biological monitoring for certain gases which have biological monitoring guidance values (BMGVs) to check whether controls are adequate.
Monitoring is used to ensure compliance with appropriate legislation, e.g. Directive 99/92/EC (explosive atmospheres) and Directive 98/24/EC (chemical agents); and to evaluate control measures. There are various approaches to [[ measuring gases in air ranging from simple passive sampling techniques to sophisticated remote sensing devices.
Gas detection provides warning of a deteriorating atmosphere and can help mitigate the effects of a gas release. Detectors, typically with alarms, are used for personal, portable and fixed monitoring of flammable and toxic gases, and oxygen deficiency. Multi-gas detectors can monitor several gases in the same instrument which is particularly useful in confined spaces and other situations where there are multiple hazards.
Emerging risks arising from gases in new technologies include:
- Development of newer energy sources or more commercialisation of previous small-scale sources. Examples of such technologies are: advanced coal and carbon abatement; use of biomass and biomass co-fired with coal; pyrolysis to improve energy yield from biomass; production of biofuels by fermentation or chemical extraction and biogas from anaerobic digestion; and hydrogen-based energy technologies.
- Recycling materials: release of inorganic and organic gases from various materials during processing.
- Advanced materials: polymer composites are increasingly being used for primary and secondary structural components. Manufacture of polymer composites and their decomposition in fires can generate a complex mixture of hazardous gases and organic vapors (e.g. isocyanates, styrene).
- Hypoxic environments: where inert gases are used to reduce the oxygen concentration for fire suppression in, for example, data centres.
- Homecare service providers: use of oxygen therapy for patients at home.
While the technologies may be relatively new, the gases generated as part of these processes are not particularly novel. They typically include those flammable, toxic and asphyxiating gases mentioned above.
Gases and vapours occur across a wide range of industries. Uncontrolled releases can cause harm to personnel and damage to premises/equipment from fire/explosion and from the effects of toxicity and asphyxiation. It is therefore important to ensure that the risks from gases are understood, safe practices are adopted when transporting and handling pressurised containers, adequate control is always maintained and mitigation techniques are available.
- Webber, D. M., Ivings, M. J. & Santon, R. C. ‘Ventilation theory and dispersion modelling applied to hazardous area classification’, Journal of Loss Prevention in the Process Industries, Vol. 24, 2011, pp. 612-21.
- Tielemans, E., Schneider, T., Goede, H., Tischer, M., Warren, N., Kromhout, H., Van Tongeren, M., Van Hemmen, J., Cherrie, J.W., ‘Conceptual Model for Assessment of Inhalation Exposure: Defining Modifying Factors’, Annals of Occupational Hygiene, Vol. 52, No. 7, 2008, pp. 577–586
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- Gardner, R., ‘Use of the reciprocal calculation procedure for setting workplace emergency action levels for hydrocarbon mixtures and their relationship to lower explosive limits, Annals of Occupational Hygiene, Vol. 56, No 3, 2012, pp. 326-339.
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- Cheremisinoff, M, P., Industrial solvents handbook. Marcel Dekker, New York, 2003.
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- European Standard BS EN 60079-29-2 Explosive atmospheres. Part 29-2: Gas detectors - Selection, installation, use and maintenance of detectors for flammable gases and oxygen, 2007.
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Links for further reading
Brosseau, L. M., & Lungu, C. T., ‘The nature and properties of workplace airborne contaminants’ Occupational Hygiene, Blackwell, Oxford, 2005, Ch 9.
Goodfellow, H. & Tähti, E., Industrial Ventilation Design Guidebook, Academic Press, New York, 2001.
HSE – Health and Safety Executive, Controlling airborne contaminants at work A guide to local exhaust ventilation (LEV), 2nd ed., HSE Books, 2011.
Walsh, P., Evans, P., Lewis, S., Old, R., Greenham, L., Gorce, J.-P., Simpson, P. & Tylee, B., BOHS Technical Guide Series No. 15 (3rd Edition) Technical guide on direct-reading devices for airborne and surface chemical contaminants, 2012. Available at: 
Yaws, C. L., Matheson Gas Data Book, McGraw-Hill, New York, 2001.