[die_sekte]

HPNS is a problem beyond 500 feet when using conventional gas mixtures (Nitrox, Heliox, ...); however, there have been simulated dives to 3500 feet on Hydrox (Hydrogen/Oxygen), which seems to prevent HPNS (I can't seem to find anything more specific about the cognitive abilities at 3500 feet). Hydrox isn't used often.

[storborg]

Diving with nitrox (nitrogen/oxygen) beyond 500 feet is very rarely done, and HPNS isn't really an issue because the overwhelming narcotic effect of the nitrogen would be devastating (think 20 tequila shots in a row). HPNS seems to be caused by rapid changes in the partial pressure of helium, so it can be alleviated somewhat by using less helium and replacing it with nitrogen.

COMEX (a French commercial diving company) has done successful working dives to beyond 1,700 feet on hydroheliox, a mixture of 49% hydrogen, 50% helium, and 1% oxygen. However, there are major practical issues with breathing a hydrogen mixture--namely, if you have hydrogen and oxygen together at high pressure with more than a tiny fraction of oxygen, it will explode. This makes it only practical to use a hydrogen mixture at a depth great enough where 1% oxygen is a high enough partial pressure to sustain consciousness.

Also, hydrogen at high partial pressures is said to have a narcotic effect similar to LSD, but permanent.