A fire and gas detector occurs in four distinct phases. In the first, or incipient, phase, warming causes the emission of invisible but detectable gases. In the second phase, smoldering, smoke is formed, so smoke detectors can be used. In the third phase, when the ignition temperature has been reached, flames are present and therefore their emitted radiation (infrared [IR] and ultraviolet [UV]) can be detected. In the fourth and last stage of the fire, heat is released; the temperature of the space starts to rise.