LPG (propane or butane) is supplied in liquid form under pressure in the cylinders, which readily evaporates into a gas. It has no smell, although it will normally have an odour added to help detect leaks.

When mixed with air in a proportion between 2% and 10% LPG forms a flammable vapour/air mixture. If then ignited in a confined space the mixture will explode.

Even small quantities of LPG particularly in liquid form can give rise to large volumes of vapour/air mixture and thus produce a considerable hazard.

As it is heavier than air, if there is a leak the gas will tend to fall to a low level and flow along the ground into drains, cellars and basements and accumulate in these and any other low lying areas. This can result in the buildup of an inflammable mixture some distance from the point of leakage and the flame can flash back to the source of the leak.