Hyperoxia results from breathing nitrox mixes which have an elevated PO2 level greater than 1.6 (1.4 for a working diver). CNS toxicity becomes a potential problem as the submerged diver's PO2 exceeds 1.6ATM resulting in a grand mal type seizure. This can occur very suddenly without warning signs or symptoms. During the first phase of this malady the diver stops breathing followed by unconsciousness and a rigid body. In the second phase convulsions occur for about one minute followed by a return of consciousness only if the diver has been able to maintain their regulator second stage. In most cases, drowning occurs due to an expelled regulator during the seizure. The best way to avoid oxygen toxicity is through conservative dive practices, monitoring PO2, oxygen clock, proper dive planning and lots of training.

To avoid oxygen toxicity a diver must both stay within the maximum depth limitations of their mix and the maximum time limits for their PO2. The table below illustrates the depth and time limits for a diver breathing .36% nitrox. The maximum operating depth (MOD) is 113 fsw at a PO2 of 1.6 ATA and at this maxed PO2 the diver can spend only 45 minutes. If you need a bottom time greater than 45 minutes make sure to plan the dive with a PO2 of 1.5 or less.