Sounds good , let me know when it ' s ready <| sep |> narcosis but yes . The thing is , the effect that gasses have on humans mostly depends on the partial pressure of the gasses involved , which is what the pressure of the gas mixture such as air times the proportion of the gas in the mixture . The trouble is , every known gas becomes toxic or at least narcotic , but in a dangerous situation like diving , that ' s basically the same at high enough partial pressures , but helium is much less problematic than oxygen or nitrogen , so to maintain a safe partial pressure of those gasses , helium is added in higher proportions to lower the partial pressure of the more dangerous gasses as the total pressure goes up . This also works in reverse spacecraft life support can be built to run at a lower pressure if you increase the proportion of oxygen inside . This is good because the walls of the craft can be made thinner because they don ' t have to withstand as much mechanical pressure this way .