Another disadvantage of helium is that it conducts heat about six times faster than nitrogen, so divers get colder than with air diving. Professional divers can use voice unscramblers to make their speech intelligible. Overall, helium offers no advantage for recreational divers. Diving with heliox is strictly for technical and professional divers. Trimix is a mixture of oxygen, helium and nitrogen.
Nitrogen slows down nerve conduction.
Trimix is used for the deepest scuba dives, usually greater than 400 feet. These mixtures are all in the realm of technical and experimental diving. The goal with any non-air mixture, of course, is to dive deeper or longer than can safely be accomplished with compressed air. Unless your computer is designed to incorporate nitrox diving, you must follow the standards for air diving. Less risk of arterial gas embolism is not afforded by nitrox.