Sorta, 19.5% and >19.5% is safe. When you deal with OSHA guidelines, they are precise to the very number, so anything lower than 19.5% is unsafe and that's where they'll get you in terms of compliance. Aberrance is the bane of science & definitely healthcare.
FAA has different guidelines, so you can't really use that for OSHA, any more than you can say OSHA governs over people who are not on the job who require certain protection(s) under certain conditions. Not all people exist under the conditions mentioned by OSHA - it's impossible.
For the healthcare workers, for example, they'll be governed by OSHA as well as CDC, NIH, AHCA, AHA, CMS and a few others and they report to the CDC as well as WHO.
I guess what I'm saying is that not all entities answer to OSHA standards for everything. This is why Drs. Fauci & Birx often refer to the CDC standards and, if you recall in the beginning of the Covid situation, it was also the WHO, when they make their recommendations.
The key here is that monitoring is done via Pulse Oximeter, a clothespin-like device you put on a finger which tells the oxygen saturation in the blood, but you probably already know this. Not many people have one of these gadgets & various conditions can affect the readings. That means they're not as reliable as a blood draw with a diagnostic to test oxygenation the way they do in cardiovascular or pulmonary surgery. I've had the personal pleasure of playing games with mine & also with my pulse rates when a case of the imp comes over me and I'm teasing the practitioner/doc, and I say that because it's possible for certain outcomes to be manipulated either on purpose or by other factors. "Other factors" play a HUGE role in Covid, the treatment of along with prevention.
Hartford Healthcare states this: "Healthy blood oxygen levels are around 97 percent. At levels below 90 percent, the brain may not get enough oxygen and people might feel confused or sluggish. If levels dip to the low 80s, vital organs could be damaged." https://midstatemedical.org/about/news-press/news-detail?articleid=25777&publicId=395 Again, I must emphasize that various factors & conditions will affect the outcome of the % of oxygen in the blood so it can't be a "one size fits all" kind of thing, can it?
Again, though, I'm not at all certain that CDC has even measured the oxygen saturation of a patient wearing a mask, including the type of mask(s), while doing various daily types of activities and with extended or prolonged use. I don't know, I haven't studied this at any great length as this isn't exactly my area of expertise. Given that the "Novel Coronavirus" is exactly that - a NOVEL or new virus - I doubt very much that any studies on continuous or extended face mask use have ever been done.
Yes, our surgeons use face masks during surgery, sometimes for several hours at a time. The longest surgery I've seen was over 12 hours. However, masks and gloves are changed at intervals and they're not the same kind that the ordinary "Joe" would wear. This is to minimize the risk of bacterial or viral transmission from the surgeon to the patient. CDC has LONG recommended the use of wearing face masks to prevent person-to-person transmission of viruses & other conditions (TB, for example). Here's fun reading from CDC in 2009 "Face Mask Use and Control of Respiratory Virus Transmission in Households" https://wwwnc.cdc.gov/eid/article/15/2/08-1167_article
Please note, there's no mention on the oxygen saturation & of such in terms of safety to the wearer after extended use! What does that mean? Probably it's something they either never thought about or haven't yet studied,
Personally, I don't like masks, find them restrictive and difficult to breathe while wearing one. In the state where I'm living, it's still voluntary to wear except in healthcare settings & some other places; I personally limit where I go including just how close I get to someone I don't know. That's just me, but I notice others aren't as cautious about it - with/without wearing a mask. "Herd mentality" - I'm all for it, if you've got kids & sent them to school, you'll recall the first couple of months of sniffles, bad colds that go through the house, etc. Covid is worse than that, but...I wonder if the principle is similar. I don't know, I'm not a scientist & don't presume to have all the facts nor know all the answers.
“When you deal with OSHA guidelines, they are precise to the very number, so anything lower than 19.5% is unsafe”
Two threads and no one has shown where OSHA says anything lower than 19.5% is unsafe!
“For the healthcare workers, for example, they’ll be governed by OSHA”
Then why doesn’t OSHA ban the use of their approved masks if they are not safe?
“so anything lower than 19.5% is unsafe”
Please ... Why then is it safe to pilot plane with an effective oxygen level of 13%?
Please ... Why then is it safe to fly passengers with an effective oxygen level of 15%?