True hemoglobin saturation isn't possible at .21 PPo2, it's close, but the curve to true hemoglobin saturation is pretty nuts. I have several papers I keep for one of my hobbies, look up Oxygen Window, it goes into detail of hemoglobin saturation and a little thing called "the oxygen window". One trick divers do to speed the evacuation of dissolved inert gases is to attempt to maximize the pressure delta between arterial and venous sides by going after 100% hemoglobin saturation (Co2 is highly soluble and can create a large pressure delta to "vacuum" inert gases from tissues). 98%, 99%, 99.5%, that's easy. 100% is not.
A patient on a demand regulator breathing 100% oxygen will not obtain 100% saturation, that requires positive pressure mask that covers the face and nose (I forgot the name of it)
I've repeatedly pressurized my body to several atmospheres of pressure using gas mixes that contain less than 21% oxygen and significant amounts of helium, so while I do not carry a PHD in the affects of inert gas on the body, it's in my best interest to learn all that I can (I'm still alive for starters).
Funny enough I purchased theses gases for "research purposes"
And FYI: Oxygen is toxic and WILL kill you, there is a unit called Oxygen Toxicity Units, 1 ATA of o2 for one minute = 1 OTU, if memory serves the limit is 1440 OTU per day, exceeding oxygen doses will cause Pulmonary oxygen toxicity, the first symptoms start with irritation in the chest, and can lead to coughing, pain and wheezing (been there done that, too many oxygen decompression's back to back) and CNS toxicity, convulsions, muscle twitches etc. I've been close, I've had my legs and lips twitching. Scary shit.