I received a PM, and I apologize that not everyone understands all the terms used.
PpO2 has to do with the partial pressure of Oxygen in your gas at depth. You'll learn this in a Nitrox class.
In your OW class you know that 99 feet is 4 ATMs. There's a law, I forgot which (Dalton's?) that explains how the partial pressures of the mix is effected by the total pressure.
So therefore if you dive a gas that is 21% O2, then it has an O2 partial pressure of .21 ATM at the surface. Therefore at 99 feet you have a partial pressure of .21 * 4, or .84.
In recreational diving, you'll never exceed a PPO2 of 1.04 [the PPO2 of air at 130 feet].
When you use nitrox though, you have to be aware of PP02. For example, a mix of 32% o2 will have a partial pressure of 1.4 at 111 feet, which is considered a safe PPO2 for recreational dives. When you start pushing towards 1.6, you have to start paying attention to the Oxygen Toxicity clock, which states at 45 minute exposure with an O2 partial pressure of 1.6.
So a partial pressure of 1.3 is considered fairly conservative.
However when you start playing with medication that is designed to increase oxygen flow through your respiratory system, like ephedrine for example, you're body is now taking in more oxygen then normal, and thus the experimentally gathered 'oxygen clock' theories go out the window.
So because of this, anything that is designed to open your airway, or increase your breathing, is potentially harmful.
How harmful? Who knows. But one of my biggest soap boxes is to make sure Pete's death was not in vein.
