Flammable gas

A gas only burns when its concentration in air sits between a lower and an upper flammable limit. Below the lower limit the mixture is too lean; above the upper limit it is too rich. Some gases have a wide flammable range and are easier to ignite than others, which is why the numbers matter in a leak assessment.

Weight relative to air affects where a leak collects. Hydrogen rises quickly and disperses outdoors, whereas propane is heavier than air and can pool in low areas, trenches and basements. Natural gas mostly rises. Ventilation design and gas detector placement follow from these differences.

Most flammable gases are stored in cylinders or tanks under pressure, so a leak or tipped cylinder also carries physical hazards. Cylinders must be secured upright, protected from damage and kept away from heat. Fuel gas and oxygen cylinders are stored apart from each other.

If a leaking flammable gas is burning, the usual advice is not to extinguish the flame unless the leak can be stopped safely, since unburned gas may build up and re-ignite or explode. Emergency plans should say who makes that decision.