What is Gas? Flammability, Toxicity and Asphyxiation

Gas surrounds us all the time and comes from the ancient Greek word "khaos" or empty space. Gases surround us all the time and make up the air we breath (Figure 1). Gases will fill any available volume and due to the high speed at which they move the mix rapidly in any atmosphere in which they are released.

Approximately 78% of the air around is us is Nitrogen and 21% Oxygen which is required by the Human Body to live with the balance 1% being made up of a mixture of other gases. Some of the most common gases beyond those spoken about include Natural Gas (Methane) which is used in many homes for heating and cooking, Carbon Dioxide, Carbon Monoxide and Nitrogen Oxides which are by products of combustion processes often from vehicles and Hydrogen Sulphide which is found in crude oil and is also produced by decomposing proteins.

Gases can be lighter, heavier or the same density as air. Most gases are colourless and odourless so can not be sensed by the 5 human senses but this doesn't mean they are not constantly present around us. As such there are three main hazards associated with gas:


Risk of fire and/ or explosion e.g. Methane, Butane, Propane

Toxic Risk of poisoning e.g. Carbon Monoxide, Chlorine, Ammonia Asphyxiant Risk of Suffocation e.g. lack of Oxygen. Oxygen can be consumed by a reaction or displaced by other gases

Flammable Gas Hazards Combustion is the act or process of burning something. This is usually a rapid chemical process in which heat and light are created using oxygen and a fuel source which can be gas, liquid or solid. The process of combustion is represented by the Fire Triangle (Figure 2).

In order for the process of combustion to occur three factors are always required:

1. A Source of Ignition

2. Oxygen

3. Fuel

In any fire protection system the aim is to always prevent at least one of these three conditions.

There is only a limited band of gas to air concentrations which will produce a combustible gas mixture. Every gas differs in the quantities required but this band is bordered by the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL).

Below the LEL there is not enough gas to produce an explosion i.e. the mixture is to lean to burn. There is not enough fuel for any ignition to take place above this is found the Flammable range which is the area in which there is the volume of gas required to ignite with a source of ignition. The flammable range ends at the UEL as beyond this volume the gas is too rich which means there is not enough Oxygen for the gas to ignite. As Oxygen is always required for explosions to occur without Oxygen the fire can not take place. These values are not set in stone for any gas and can be effected by pressure and temperature of the vapours.

Management of flammability risk can be managed in a number of ways and we will discuss fire/ explosion prevention in further help articles.

Flammable Gases also have a temperature where ignition will take place without an external ignition source, this is known as the ignition temperature. Anything used in an area where hazardous gases are present must not have a surface temperature that exceeds the gases Ignition Temperature. This is marked on such apparatus as a T Rating providing the units maximum Surface temperature. Some examples of T Ratings and Ignition temperatures can be seen below in Figure 4:

The last thing to consider with Flammables is the Flash Point or the lowest temperature at which vapours above a volatile combustible substance ignite in air when exposed to flame. This should not be confused with the ignition temperature as previously discussed as the two are very different, Figure 5:

Toxic Gases

Some gases are poisonous and can be dangerous to life and health at extremely low concentrations. Some gases have distinctive smells like the 'rotten eggs' of Hydrogen Sulphide but often they have no colour or odour. Measurements often used for Toxic Gases are in Parts Per Million (PPM) or Parts Per Billion (PPB). These extremely small values can be immediately fatal or cause long term health issues.

1 Part Per Million (PPM) can be equated to 1 second in 12 days! More people die worldwide from Toxic Gas Exposure than from explosions caused by the ignition of flammable gas. There is a large group of gases which are both combustible and toxic, so even Toxic Gas monitors sometimes have to carry hazardous areas approvals. With toxic substances the effect is usually not immediately apparent and very low concentrations inhaled, ingested or absorbed through the skin can cause extreme adverse long term effects and in some cases are carcinogenic (cancer causing). Occupational Exposure limits (OELs) apply to both marketed products and by-products of production processes and limit workers exposure to substances. OELs protect workers against health effects but do not address safety issues such as explosive risk or asphyxiation. In the UK the standards which are enforced are called EH40 and function under the Control of Substances Hazardous to Health (COSHH) which require that the employees exposure is either prevented or brought as low as reasonable possible (ALARP).

At Safety Monitors we have provided a searchable version of EH40 so you can see the current limit values for specific substances figure 6:

Exposure limits are referred to by Short Term Exposure Limits (STEL) and Time Weighted Averages (TWA). These values are the averages over a set period:

STEL - 15 Minutes

TWA - 8 Hours

Although we often focus on the Average exposures to small quantities of gases some Toxic Gases can be immediately dangerous to life and health an example of which (Figure 7) is shown below for Hydrogen Sulphide:


The human body requires Oxygen to survive and in normal ambient condition we have plenty at around 20.9% volume however when the level of Oxygen in the atmosphere dips below 19.5% this can be considered deficient and below 16% is considered unsafe for Humans.

Oxygen depletion can be caused by:

Displacement - Other gases enter the atmosphere and replace the Oxygen

Combustion - The Oxygen is used by the process of combustion

Oxidation - Oxygen is used by the chemical process of Oxidation

Chemical Reaction - Oxygen reacts with another substance

Although by far the main focus of Oxygen is on asphyxiation risks via Oxygen depletion it is often forgotten Oxygen enrichment can also pose a risk. At increased Oxygen levels the flammability of gases increases. At levels of 24% items such as clothing can spontaneously combust. Sensors have to be specially certified for use in Oxygen enriched atmospheres.


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