Poorly controlled release of compressed gas in chemical reaction systems can cause vessels to burst, create leaks in equipment or hoses, or produce runaway reactions. This means that hydrogen can be ignited when its concentration in the air is between 4 and 75 percent. The flammable range of any gas is widened in the presence of oxidizing gases such as oxygen or chlorine and by higher temperatures or pressures. For example, the flammable range of hydrogen in oxygen gas is 4 to 85 percent and the flammable range of hydrogen in chlorine gas is 4.1 to 89 percent. There are many possible ignition sources in most workplaces including open flames, sparks and hot surfaces.
Some gases have very low auto-ignition temperatures.
Some compressed gases, such as silane and diborane, are pyrophoric - they can ignite spontaneously in air. Flash-back can occur with flammable gases. Many flammable compressed gases are heavier than air. If a cylinder leaks in a poorly ventilated area, these gases can settle and collect in sewers, pits, trenches, basements or other low areas. The gas trail can spread far from the cylinder.