There is and probably always will be a lot of confusion about the gas that is inside you cylinder. For some strange reason people seem to think that scuba divers only breathe Oxygen when they are diving. I can see where they are coming from. Humans need Oxygen to breathe so ergo we only put Oxygen in our cylinders.
On the surface we breathe air. I do it, you do it, even the birds and the bees do it. Air is made up of 79% Nitrogen and approximately 21% Oxygen. There are a few other gases in there but their percentage is so small that in the grand scheme of things it’s not worth mentioning.
So why do diver’s breathe air and not Oxygen?
First of all let’s set the record straight as a recreational diver you breathe air and only air. You may breathe air with a higher percentage of Oxygen but it is not pure Oxygen. This is known as Enriched Air Nitrox and never gets higher than 40% Oxygen.
Now, we could breathe pure Oxygen but there are two problems with exposure to Oxygen. One is CNS Toxicity and the other is Pulmonary Toxicity. CNS Toxicity (Central Nervous System) occurs when the partial pressure of Oxygen goes beyond 1.4ata As the partial pressure of Oxygen increases the deeper you go, this means that you have to restrict your depth so as to avoid convulsing and drowning. It is recommended that you do not exceed an oxygen partial pressure of 1.6ata. Pulmonary Toxicity occurs from exposure to high levels of Oxygen for extended periods. This is normally a problem for technical divers and not recreational.
So let’s end the confusion, what you have in your cylinder is air. If you want Oxygen in your cylinder become a tec-diver.