My male MC has encountered an alien fog that's caused an allergic reaction, seriously blistering his lungs. He's coughing, wheezing, hacking up blood, etc. The doctor checks his O2 sats, and it's a dangerously low number. Not low enough to kill him or cause permanent brain damage, but low enough that she wants to evacuate him immediately, despite the risks involved (such as the alien fog that's still lurking outside). She puts him on oxygen and they head off to the hospital, where he stays for at least a week before recovering.

My question is, what's a good number to use for his O2 sats? I tried doing research on how low it could go without killing you or causing brain damage, but the Google results I found were confusing. I arbitrarily chose 75 as the magic number, but I honestly have no idea if it's too low to be realistic.

Any doctors or nurses out there able to help?