Scuba diving involves breathing compressed gases and withstanding high pressures that increase with depth. These are two things the human body was not meant to do, and so diving brings with it certain health hazards. To cope with these hazards, divers sometimes change the gas contents of their scuba tanks.
Most recreational scuba diving is done on compressed air. Like the air in the atmosphere, this is composed of 78 percent nitrogen, 20 percent oxygen and a small mixture of trace gases. Most of the time compressed air is adequate for breathing, but the high nitrogen content becomes a problem for deep dives, long dives, repeated dives or some combination of the three.
Under high pressure, the body absorbs more nitrogen than it would under normal circumstances. If there is too much nitrogen in the body and the water pressure is suddenly reduced, as happens in rapid ascents to the surface, the nitrogen fizzes in the bloodstream much as carbon dioxide bubbles out of a soft drink. This is the cause of decompression sickness, which can cause injury or even death.
Another problem with nitrogen is familiar to anyone who has had nitrous oxide at the dentist: nitrogen narcosis. Too much nitrogen in the brain has a narcotic effect, impairing sense perceptions and judgment. Because of the twin dangers of decompression sickness and nitrogen narcosis, divers who dive deep, stay underwater for long periods or make multiple dives in a day often use a gas mixture with less nitrogen.
Nitrox, or oxygen-enriched gas, is designed to mitigate the problems of nitrogen for long-duration and repeat divers. This gas mix increases the amount of oxygen in the tank to 32 or 36 percent, with a corresponding decrease in the amount of nitrogen.
The problem with nitrox is oxygen toxicity. The body absorbs more oxygen under pressure as well, and too much oxygen can be toxic and potentially lethal. As a rule, nitrox is safe for use within the customary recreational diving limit of 130 feet. Below that, it becomes more hazardous and regular air is safer.
To combat the dual hazards of nitrogen and oxygen on dives beyond 130 feet, and especially beyond 165 feet, divers use a gas mixture called trimix. Trimix works by substituting helium for both nitrogen and oxygen. Helium has very little effect on the body and is less dense than oxygen and nitrogen, making it easier to breathe under extreme pressure.
The specific blend of gas depends on how deep a dive is planned; deeper dives demand more helium. Helitrox is a type of trimix suitable for dives to 145 feet, with a mixture of 26 percent oxygen, 57 percent nitrogen and 17 percent helium. A dive to 300 feet might have a mixture of only 10 percent oxygen, 20 percent nitrogen and 70 percent helium.
- diving tank image by Marcin Wasilewski from Fotolia.com