Sunday, 01 April 2001 00:00

What benefit does Nitrox have over air?

By Richard Taylor

There have been many statements made about the benefit of Nitrox, mainly that it is 'safer' for the diver, 'extends dive time', makes you 'feel' better and that it 'will become the gas of choice in the 18 msw / 60 fsw to 30 msw / 100 fsw depth range. These may all be true (the last one is a matter of individual opinion), but to understand the benefit, we need to understand why we can benefit.

Nitrox is also termed 'Oxygen Enriched Air' (often shortened to EAN) or DNA, for Denitrogenated Air, if produced via the DNAx filter system. It is a breathing gas which contains a higher portion of oxygen in it than air (air has basically 21% oxygen and 79% nitrogen). Every day our body metabolises oxygen in the air we breathe, without which we would quickly die (we only use about 4% oxygen, but this subject alone covers many different physiological functions). The nitrogen though is generally not used but absorbed into our body and then expelled again as we breathe out.

When we go diving it is this nitrogen being absorbed into our body which determines the no decompression limits for our diving. As it is not used, the old adage of "what goes in must come out" is very pertinent. In our Open Water course we learned about the relationship between depth, pressure, volume & density. As we descend the pressure increases and the density of the air is increased. Therefore we breathe more air at depth than on the surface which in effect means that when we dive we breathe in more oxygen & nitrogen molecules with each breath.

If we then use Nitrox, (more oxygen, less nitrogen) what happens is that the amount of nitrogen we absorb is decreased. This then means that our no decompression limits can be extended, our dives become longer. And just as our surface interval for repetitive dives is based upon the amount of residual nitrogen we have after our first dive, if we lessen the amount of nitrogen we have absorbed, we can also lessen our surface intervals. We get longer dives and shorter surface times using Nitrox than when using air (great on those live aboard holidays we take!).

As we know that you should never dive to the limits of the tables, we can also decide to use Nitrox but dive to our air tables or computers. This gives us additional conservation on these tables and significantly reduces the risk of decompression sickness as we will absorb less nitrogen than calculated for by the tables (note: the risk of DCI can ONLY be reduced, there still remains a chance of other factors adding to the risk such as temperature, fitness, weight, breathing rate, alcohol consumption, etc).

Aside from the nitrox reducing nitrogen absorption for decompression it will also reduce narcosis. Narcosis, or 'raptures of the deep' as it was once known is described as "..a state of impending general anaesthesia" ('Diving and Subaquatice Medicine' 3rd Ed. by Drs Carl Edmunds1 & Christopher Lowry and John Pennefather). As in the above, the deeper we go, the denser the air we breathe and greater the amount of nitrogen. This nitrogen effects our mental judgement (some say they each 15 msw / 50 fsw is equal to drinking one martini, Martini's Law). Therefore the less nitrogen in what we breathe, the less the effect of the narcosis (again, this will vary from individual to individual).

Aside from the reduced nitrogen in nitrox we can also benefit from the increased oxygen. Oxygen under pressure is used in hyperbaric chambers not only to help in cases of DCI, but also to assist in cell repair after surgery or many cases of tissue damage. It is accepted that the increased oxygen in Nitrox does offer the diver the same benefit, but to what degree has yet to be established (as we do not use the high levels of oxygen in diving that are used in the hyperbaric chambers this benefit will probably be minor).

Does it make you feel better? Possibly it does, but then so can a nice relaxing dive in good conditions. The above factors no doubt help to produce a psychological wellbeing as well.

Is it safer? In as much as a diver follows good diving practices of ascent rates, table conservation and the correct attitude, yes. However, Nitrox is not the panacea for bad diving practices, staying within the limits for any type of diving is probably the best safety practice.

(note 1. Dr Carl Edmunds is a consultant in Underwater Medicine to the Royal Australian Navy as well as past president of the South Pacific Underwater Medicine Society. He is the Director of the Diving Medical Centre in Sydney and can be contacted on +61 (02) 9437 6681.)