A very common misconception among those who are unfamiliar with SCUBA is that the diver’s tanks are full of oxygen. While it is true that the air in the tank is partly comprised of oxygen, it is not usually more than is naturally occurring in the air we all breathe, which is about 21%. The majority of that air is nitrogen, coming in at about 78%, and the remainder is a mix of argon, carbon dioxide, neon, and helium, to name a few. Generally speaking, this is the makeup of the air that fills a diver’s tank; the only difference is that the air in the tank is compressed, allowing underwater breathing for a prescribed amount of time.
Because our body needs it for its metabolic function, the air in a SCUBA tank cannot be devoid of oxygen. But oxygen does have toxic effects on the body when it is subjected to higher levels of pressure, specifically the central nervous system, pulmonary function, and ocular function. On the opposite side, nitrogen can cause a well-known ailment called decompression sickness. When nitrogen is absorbed into the bloodstream underwater, it is under a great deal more pressure than at the surface level. Therefore, when ascending to the surface, if adequate time is not given for the nitrogen to dissolve from the bloodstream, the result is formation of nitrogen bubbles in joints and tissue that can cause extreme pain and an extended time to decompress.
There are other mixtures that divers use for different purposes to compensate for additional pressure when going for a longer period of time than a typical recreational user. The most common of these is called nitrox. Nitrox is a very specific mixture of nitrogen and oxygen, with oxygen levels being higher than regular air, usually between 32% and 36%. This ratio allows for more oxygen than nitrogen to be absorbed into the bloodstream, which lessens the likelihood and/or the severity of decompression sickness. However, some divers mistakenly think that this will allow them to disregard dive tables, which is a universal system that tells the diver how long they can be at what depth, and the rate at which they ascend to avoid decompression sickness. This can very easily lead to oxygen toxicity, which is no better than the alternative. There is also the added responsibility of making sure your tank has the exact mixture that is appropriate for your dive. Not all dive shops are keen to nitrox quite yet, so there can be an element of risk when choosing nitrox as your air source. When done properly, there should be markings on the tank indicating the percentage of oxygen in the nitrox and the maximum depth at which you can safely dive.
The most important aspect of diving is making sure you understand and practice safe techniques, regardless of the air you use. Always do your research before trying new methods .