A pyrophoric substance (from Greek πυροφόρος, pyrophoros, "fire-bearing") ignites spontaneously in air at or below 55 °C (130 °F). Examples are iron sulfide and many reactive metals including uranium, when powdered or thinly sliced. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or (with a few exceptions) nitrogen. Most pyrophoric fires should be extinguished with a Class D fire extinguisher for burning metals.