Although helium is volatile due to its light weight and physical properties, it is chemically inactive and is, therefore, not flammable. For an object to be flammable, it must be able to react with an oxidant, such as oxygen, within the presence of an ignition source.
The U.S. Department of Transportation defines a gas as flammable if it is ignitable within the presence of a less-than-13-percent mixture of air. This has to be able to occur at 68 degrees Fahrenheit and at atmospheric pressure. Because helium is chemically inert and nonflammable, it has uses in areas such as welding and pressure systems, where a gas is needed for a specific function but it is important that the gas not react with the metals in the system.