@ssssnake529
Assuming it is a serious question.
New divers generally use air which is historically the standard diving gas.
The issue of PO2 and Scuba diving relates to the fact as the ambient pressure increases (i.e. as you get deeper), the PO2 will increase.
So the first point is that, when diving air. Air is a safe gas to breath at an ambient pressure of 1 bar. So that the minimum PO2 of air (as a Scuba breathing gas), is 0.21. - Basically it is safe to breath on the surface - the human body is designed to breath air at ambient pressure.
As you get deeper the PO2 of air increases. For the PO2 of air to get to a level that would cause concern, there are a lot of more worrying adverse issues caused by the Nitrogen which makes up the balance. Hence, PO2 was historically never a concern when using air as a diving gas.
Air historically was used as the preferred breathing gas, because it is cheap, readily available and easy to compress.
In shallow depths it is ideal.
Nitrox (Enriched Air), is a relatively new diving gas for amateurs. It started to become available in the late 1990's. Previously used by the scientific community, and other diving professionals. Initially, it was the 'technical diving community' that started to embrace Nitrox.
There was initially some resistance by the main diving agencies. It was perceived as too high a risk. Toxicity issues, handling (compressing) issues, special decompression tables etc.
It rapidly became apparent that the safety aspects out weighed the perceived risks, and that the average scuba diver was more than capable of understanding and using Nitrox.
Nitrox is now the preferred diving gas for many.
The only way to have low PO2 issues with Scuba diving is to start with a gas with a low initial oxygen content. Which is a possibility when using Trimix, or Heliox, which are used for deep diving. The use of Trimix & Heliox require advanced diving qualifications.
The traditional restriction is a mix with a 16% or lower level of oxygen, i.e. a PO2 of 0.16 on the surface.
The risk comes on the surface or in shallow water.
The only other potential area of exposure to low PO2 breathing mixes is when using SCR or CCR units. In particular CCR units. CCR units are gas recycling units, if the metabolised oxygen is not replaced, eventually all O2 in the breathing look will be consumed, or it will reach a value to low to sustain consciousness.
HOWEVER
There is a risk of incorrectly identifying the breathing gas. Other than the high PO2 issue that you have already aware of. If you use the wrong decompression table for the wrong gas, then there is a risk of DCI. i.e when using air, you must use an air table (or computer set for air). You can use a table for less O2 than the O2 in the breathing mix (i.e. an air table with Nitrox 36), but you can't use a Nitrox 36 table when breathing air.
In short. Low PO2 (below 0.16), there is a risk of blackout, especially if working hard. Advanced diving gases do allow you the opportunity to have access to specially mixed gases that could have a O2 content of under 16%, which are used for deep diving. They are not safe for use in shallow water or on the surface. Additional gases are carried and used by OC scuba divers for the shallow sections of the dive.
Traditional a travel gas, (say 32% O2), for the start of the dive. Then a transition to the deep gas (say 10% O2 +He +N) at around 30m. Then on the ascent, a switch to travel gas at or around 30m, and finally a switch to the decompression gas (80-100% O2).
The other point about low O2 gases, is that they are poor gases to decompress on, and shorten the bottom time.
Gareth
