Normal air is 21% oxygen. Breathing air with less than19.5% oxygen is considered bad. A mask reduces the 21% to 18-18.5%. From memory, can't remember the source. There was a video where a guy was measuring the level under the mask.
Normal air is 21% oxygen. Breathing air with less than19.5% oxygen is considered bad. A mask reduces the 21% to 18-18.5%. From memory, can't remember the source. There was a video where a guy was measuring the level under the mask.