Acetylene, ammonia, hydrogen, propane, propylene and methane are all flammable gases referred to as fuel gases. When these gases are mixed with an oxidant and provided with an ignition source they will burn.
The diagram below shows the flammability concentration limits for an atmosphere comprised of air. The orange bands show the percentage range of fuel gas that represents a particular danger of fire or explosion.
As the percentage of the fuel gas increases, the risk of fire becomes greater. When the concentration has exceeded the higher value, the air becomes saturated by the gas and ignition becomes less likely.
Around any container and in confined spaces can even small quantities of escaping fuel gas can form an ignitable mixture if the conditions are favorable. There is, however, a small risk for reaching the lower ignition limits in open air or in large working areas with good natural ventilation.
Leakage of fuel gases may form ignitable mixtures with the surrounding air and lead to fire or explosions. For this reason some of them are odorized (given a smell), which means that leaks can be more easily identified.