(4) Trimix is a breathing gas,consisting of oxygen, heliumand nitrogen, and is often usedin deep commercial diving. Thehelium is included as a substitutefor some of the nitrogen,to reduce the narcotic effectof the breathing gas atdepth. It helps to reduce nitrogennarcosis and to avoidthe dangers of oxygen toxicity.With a mixture of threegases, it is possible to createmixes suitable for differentdepths or purposes by adjustingthe proportions of eachgas.