Cautions when using Nitrox - Know your Maximum Depth Allowance!

Few advances in diving have had a more profound impact during your lifetime than the widespread availability of Enriched Air Nitrox.  Nitrox and air divers alike are typically aware of the longer bottom times allowable while diving with Nitrox.  But, I’d like to remind all of you about the danger of oxygen toxicity associated with diving with Nitrox.

Any percentage of oxygen can cause toxicity at a high enough pressure. But, when diving on air, you would have to dive in excess of 220 feet, to experience acute oxygen toxicity. The danger of nitrox is that it brings the possibility of oxygen toxicity within recreational diving depths.

In order to avoid exceeding the safe depth limitations, nitrox divers are taught to use a table or complete a maximum operating depth (MOD) formula before each dive: (PPO2 / O2 percentage = MOD). Using this formula, a diver breathing Nitrox 35 will have a maximum operating depth of 99 feet, or 4 atm, when using the recreational limit of 1.4 PPO2: 1.4 / 0.35 = 4 atm, 33 feet x (4 atm - 1 atm--the one surrounding the earth) = 99 feet.  The same calculation for 40% Nitrox yields: 1.4 / 0.40 = 3.5 atm, 33 feet x (3.5 atm - 1 atm) = 82 1/2 feet.  

Both the formula and the nitrox tables are easy with proper training, and an easily programmable nitrox dive computer can eliminate the need for calculations altogether.

Nitrox divers should recognize that air divers may not be aware of the MOD issue associated with using a cylinder containing Nitrox and could get into trouble if they inadvertently use a Nitrox cylinder.  

All divers would benefit from knowing that oxygen toxicity manifests as symptoms such as visual changes (such as tunnel vision), ringing in the ears (tinnitus), nausea, twitching (especially of the face), behavioural changes (irritability, anxiety, confusion), and dizziness.  If you see such symptoms manifest in a dive buddy while at depth you may be able to alleviate the symptoms by buddy breathing from your air source or assisting the affected individual in a controlled ascent.