70% briefly is survivable. I don’t know how many bouts of 70% in x time are survivable, but when my son, at age 4, had an attack of croup that had him gasping for breath, which left pinpoint hemorrhages all over his chest-- we called 911, and they were there in about 3 minutes. His initial pulse ox was 71%. They put a mask on him, and it went up to 97%. They gave him steroids, and put the mask back on, and in a couple of minutes, he was up to 100%, and his breathing was no longer in gasps.
Two hours, more steroids, cough suppressant, an antibiotic, and an ear flush later, he was watching Cars on TV, and drinking juice. They decided to admit him because he had rales with a stethoscope, but his color was good, and he wasn’t coughing.
Daddy stayed all night with him, while I went home to let the dog out, put away the food we’d left on the stove, try to get a few hours sleep, and go back to the hospital with changes of clothes for everybody.
When I got back, the boychik was sitting on his bed, happily eating a pizza for breakfast, and watching How to Train Your Dragon.
This was six years ago, and he has shown no residua. No even a second bout of croup. The only residua is that I panic if he has a cough that even sounds slightly croupy.
TL;DR: my son had croup once, and his pulse ox reading dropped to 70%, but he’s perfectly fine now. You can survive a few minutes of hypoxia of 70% with no affects.