ion hpa or lpa?

There is such a thing called LPA(low pressure air).  In the medical facility I work at, that is what we use on patients that cannot breathe on their own.(what you are talking about is 100% oxygen, which is held in tanks, and regulated down to a pressure that the patients are able to breathe)  High Pressure Air would suffocate them.(it wouldn't suffocate them, most likely if you put 4500psi into thier lungs they would explode)  The air we breathe is not measured in PSI. Yes, It is. The normal air pressure on earth is 14.7 psi. It decreases as you reach a higher altitiude though) If that was the case then we would be walking around with tank packs that compresses air for us to breathe.  The air around us is not compressed.  You can say HPA(technically anything compressed more than 14.7psi can be comsideder compreesed air) and LPA(yet again, but sung the term LPA you are suggesting that the air is in a vacuum, or is at less than atpmosheric preesure which is 14.7psi) is compressed air, and does not always mean it is for "breathing".  Medical, diving, and other areas where oxygen is needed for prolonged time is what you can properly call "breathing" air.  "Breathing " air is very critical and it has to be filtered and purified before going into a tank or apparatus.  The air that they fill up in our tanks at the paintball fields aren't always "breathing" air but  compressed air that was used with a compressor.  Who would want to go to diving or be laying on a gurney in a hospital with air that is contaminated with fuel or other gases ?  You are correct here about the filters, I was making a generalization based on the fact that if you are standing right next to a paintball compressor and breathing, you are beathing the same air that is in your tank.