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When these gases are mixed with an oxidant and provided with an ignition source, they will burn.
Flammable gas hazards
Fuel gases all have an associated asphyxiation hazard but they also present the more important hazards of fire and explosion.
The diagram displays the flammability concentration limits for an atmosphere of air.
The orange bands show the percentage range of fuel gas that present particular danger of fire and explosion.
As the percentage of fuel gas increases, the risk of fire becomes greater.
After the concentration has exceeded the upper value, the air becomes saturated by the gas and ignition becomes less likely.
If a container is kept inside or in a confined space, even small quantities of escaping fuel gas are sufficient, under certain conditions, to form an ignitable mixture.
There is only a small risk that the lower ignition limits will be reached in large working areas that have good natural ventilation or are in the open air.
Leaking fuel gases may form ignitable mixtures with the surrounding air and lead to fire or explosions.
For this reason many of them are 'stenched' (given a smell, e.g. rotting fish) which means that leaks can be more readily identified.