Editing Trimix (breathing gas) (section)

== Function of the helium ==
The main reason for adding helium to the breathing mix is to reduce the proportions of nitrogen and oxygen below those of air, to allow the gas mix to be breathed safely on deep dives.<ref name="Brubakk" /> A lower proportion of nitrogen is required to reduce [[nitrogen narcosis]] and other physiological effects of the gas at depth. Helium has very little narcotic effect.<ref name="palautz97" /> A lower proportion of oxygen reduces the risk of [[oxygen toxicity]] on deep dives.

The lower density of helium reduces [[Work of breathing|breathing resistance]] at depth.<ref name="Brubakk" /><ref name="palautz97" /> Work of breathing can limit the use of breathing gas mixtures in underwater breathing apparatus, as with increasing depth a point may be reached where work of breathing exceeds the available effort from the diver. Beyond this point accumulation of carbon dioxide will eventually result in severe and debilitating [[hypercapnia]], which, if not corrected quickly, will cause the diver to attempt to breathe faster, exacerbating the work of breathing, which will lead to loss of consciousness and a high risk of drowning.<ref name="Mitchell et al 2007" />

Because of its low molecular weight, helium [[Graham's law|enters and leaves tissues by diffusion more rapidly]] than nitrogen as the pressure is increased or reduced (this is called on-gassing and off-gassing). Because of its lower solubility, helium does not load tissues as heavily as nitrogen, but at the same time the tissues can not support as high an amount of helium when super-saturated. In effect, helium is a faster gas to saturate and desaturate, which is a distinct advantage in [[saturation diving]], but less so in bounce diving, where the increased rate of off-gassing is largely counterbalanced by the equivalently increased rate of on-gassing.

Some divers suffer from [[compression arthralgia]] during deep descent, and trimix has been shown to help avoid or delay the symptoms of compression arthralgia.<ref name="VannVorosmarti2002" /><ref name="Bennett1974" />