Oxygen is good for Human beings and Carbon Dioxide is not!
When I recently went to the hospital for my wife because of shortness of breath (yes when the so called pandemic started) the first thing the nurse did was place a device on her finger to measure her oxygen levels! I also have extensive experience being in ICUs and that is one of the most critical measurements.
95-100% is ok to excellent
90% or lower means help is needed.
Oxygen (O2) makes up 20.9% of our atmosphere and Carbon Dioxide (C02) is 0.04%
This is where is gets interesting – I recall from biology. When we breath we take in 21% O2, 0.04% CO2 and 79% Nitrogen.
According to Work safe Australia – (page 8, 12)
When air comes out of your mouth O2 is at 16%. Dangerous levels of CO2 are 10% and what comes out of your mouth is 4%. The University College London says –
In addition, masks trap moisture from the breath, increasing the viral load held in the mask –