Because of the problems associated with oxygen toxicity, nitrogen narcosis, and decompression sickness, the maximum safe limit for breathing air is about 200 feet (61 meters). To overcome these problems, gas mixtures other than air should be used. Perhaps the most severe and potentially deadly of the limitations is CNS oxygen toxicity. Air contains about 21% oxygen. The maximum safe PO2 limit of 1.4 ATA is exceeded with air when the ambient pressure is about 7 ATA, or 198 feet (60 meters). The nitrogen narcosis at this depth has been likened to drinking several Martinis; and, for each minute spent at this depth breathing air, about 3 to 8 minutes are required for decompression.The first step is to solve the CNS oxygen toxicity problem. This is actually relatively easy: to increase the depth at which the PO2 limit of 1.4 ATA is reached, one need only reduce the fraction of oxygen in the breathing gas. For example, a mixture containing only 10% oxygen would reach a PO2 of 1.4 ATA when the ambient pressure is 14 ATA – over 400 feet (120 meters) deep! The problem, however, is that if the removed oxygen was replaced by more nitrogen, the effects of nitrogen narcosis would be increased. Thus, to extend the maximum safe depth of diving, both the oxygen and the nitrogen must be reduced. The only was to do that is to introduce another constituent to the breathing gas mixture. That constituent is usually helium.Helium has two fundamental advantages over nitrogen for deep diving breathing mixtures. The first advantage is that it does not cause narcosis, even at very high inspired partial pressures. The second advantage is that it is a much smaller molecule, and therefore much less dense. Because gas molecules are more closely packed together under higher pressures, the density of the gas is increased. For relatively large molecules, the increased gas density can lead to a significant increase in work of breathing. Helium is less dense at 300 feet (91 meters) than nitrogen is at sea level. These two advantages make helium the gas of choice for deep diving breathing mixtures.Helium breathing mixtures generally come in two forms: Heliox – helium and oxygen without any nitrogen or other gas constituents; or Trimix – a combination of three primary gases, including helium, oxygen, and usually nitrogen. Heliox is more often used by military and commercial divers, whereas Trimix is more often used by civilian “technical” divers. Each has advantages and disadvantages, but both achieve the same basic result: reduce the concentration of oxygen, reduce or eliminate the nitrogen, and reduce the overall gas density.Unfortunately, from the perspective of decompression, helium is not an ideal gas for the sorts of dive profiles most civilian deep divers do (i.e., less than one or two hours at depth). Because of its very small molecular size, helium dissolves into the blood and tissues much faster than nitrogen does. More dissolved helium in less dive time means lower ratios of dive time to decompression time. If Heliox or Trimix were breathed for the entire duration of the dive, including the decompression, total dive times would be extremely long. The rate of decompression from deep dives using helium can be greatly increased if, during the ascent, the breathing mixture is changed to one that does not contain any helium. Because most decompression time is spent at relatively shallow depths, narcosis is not a problem, so air would be adequate.However, air is not an ideal decompression gas either, because it contains so much nitrogen. Even though the helium comes out of the body quickly when decompressing while breathing air, nitrogen is at the same time entering the blood and tissues. The amount of nitrogen added to the body can be reduced by reducing the fraction of nitrogen in the decompression breathing mixture. Because oxygen does not factor in to decompression dynamics, the nitrogen can be replaced with oxygen. Mixtures containing only nitrogen and oxygen, with more than 21% oxygen, are popularly referred to as Nitrox. More and more, recreational divers are using Nitrox for dives to moderate depths, where CNS oxygen toxicity is not a major concern, and no-decompression times can be extended. For deep diving, Nitrox is used to accelerate decompression times. While Nitrox is useful for decompression at intermediate depths, pure oxygen can be used at depths of 20 feet (6 meters) or shallower. Without any nitrogen or helium, pure oxygen maximizes the rate of decompression, cutting total decompression times down dramatically.Thus, by using different gas mixtures during different portions of the dive, limits of conventional scuba can be extended and decompression can be optimized. A great deal of additional information on these and related topics is available in a wide variety of publications, some of which are listed below. Divers who are interested in utilizing breathing gas mixtures other than air are encouraged to read as much material as possible, and to seek out proper training in mixed-gas diving techniques.