DAN Medical Frequently Asked Questions

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Breathing Gas Mixture

The particular breathing gas mixture you use, and how you use it, can play a role in the development of DCS. A mixture known as enriched air nitrox, or simply nitrox, is increasingly popular for recreational diving. The percentage of oxygen in the mix is increased, reducing the nitrogen fraction. This means that there is less nitrogen uptake at a given depth. The decompression effect of nitrox, compared to that of air, can be calculated by computing what is known as equivalent air depth (EAD). The risk of DCS when diving with nitrox to the EAD table limits is not appreciably different than diving with air to the air table limits. It is possible to achieve a decompression safety buffer by using nitrox with air table limits, since this will reduce your inert gas uptake compared to using air.

The critical caveat with nitrox is that its higher oxygen content means that a diver breathing nitrox is at risk of developing oxygen toxicity at a shallower depth than a diver breathing air. The recommended maximum partial pressure of oxygen — partial pressure being the portion of the total gas pressure represented by a single gas — is 1.4 ATA for recreational diving. When diving with air (21 percent oxygen), this level is reached at a seawater depth of 187 feet (57 meters) — beyond the usual recreational diving limit (187 feet of seawater = 6.6 ATA * 0.21 ATA oxygen in air = 1.4 ATA). When diving with a 32 percent nitrox mixture, this level is reached at a seawater depth of 111 feet (34 meters), and with 36-percent nitrox at just 95 feet (29 meters) — depths commonly reached by recreational divers.

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