As we alluded earlier, your body may appreciate the use of Nitrox. As you dive, your tissues absorb nitrogen while processing the oxygen. Your tissues load or absorb nitrogen in their cells, hence we use the term “tissue loading.” As you ascend, the nitrogen stored in the tissues is removed from the body as a part of the normal respiration cycle. By reducing the amount of nitrogen in the breathing mixture, you expose your tissues to less nitrogen. Since the quantity of nitrogen in the cells is reduced, you get less tissue fatigue and increased safety.

Looking at this concept in more detail, the decompression obligation is dependent on the quantity of nitrogen absorbed by the body during the course of a dive. Both the rate of the nitrogen absorption and the total quantity of nitrogen which can be absorbed by the body are determined by the partial pressure of nitrogen in the breathing gas mixture. When you reduce the nitrogen content of the gas mixture and replace it with oxygen, which the body can use in metabolism, you reduce the absorption of nitrogen in the cell tissues.

Figure 1-2: Comparison of the no-stop dive time limits using the IANTD Dive Tables for the appropriate mixtures.

The dive tables and computer models use mathematical models to determine the amount of nitrogen absorbed by the body’s tissues. As Figure 1-1 illustrates, at any time during a dive with Nitrox, the nitrogen absorbed by the body is less than for air.

Taking the safety factor further, it stands to reason, if we are reducing the nitrogen absorbed in the body, we should be able to reduce the interval between our last dive and the time we can fly. While there are some methods for calculating this reduced period, IANTD still recommends a twelve to twenty-four hour surface interval before flying. You should also be aware that this surface interval can increase to as much as forty-eight hours after some very long decompression dives.

A third benefit of using Nitrox is that you can use Nitrox to extend your bottom time by equating the amount of nitrogen in your breathing mixture to that of air. By using this ratio, your bottom time will increase.

The relative time increases are shown in Figure 1-2. We will examine how to do this in the dive planning section using the Equivalent Nitrogen (Air) Depth formula, and the IANTD EAD/MOD Table.

Figure 1-3: Oxygen limits and physiological implications.

Figure 1-4: Depth, pressure and patial pressure of oxygen relationships for various EANx mixtures.