Question

At what oxygen level does brain damage occur?

Answer 1

The brain survives for up to 6 minutes once your heart stops and if the heart is not restarted within 6 minutes then brain damage occurs.

So it's important to hopefully get the heart started before that 6 minutes to keep the person from having brain damage should the person survive.

Brain damage usually occurs when the oxygen level drops below 90% to 86% as an 86% oxygen level is considered severe and brain damage will almost always occur at 86%.