What is a gas called that ignites in air without the introduction of an ignition source?

Pyrophoric gas is defined as a substance that can ignite spontaneously in air at or below room temperature, meaning that it does not require an external ignition source to start burning. This characteristic is particularly hazardous for fire service personnel, as the mere exposure to air can initiate combustion. An example of pyrophoric gas is hydrogen. This property distinguishes it from other types of gases that may require a specific ignition source to ignite.

Flammable gases require an ignition source to ignite, usually at elevated temperatures. Explosive gases tend to form explosive mixtures in the air and require a spark or flame as an ignition source also. Combustible gases may ignite at higher temperatures, but like flammable gases, they do not ignite spontaneously under normal conditions. Therefore, pyrophoric gas stands out due to its ability to spontaneously ignite, making it a significant concern in fire safety and prevention.