Hypoxemia: an abnormally low concentration of oxygen in the blood.
Dyspnea: the feeling of not being able to breathe deeply enough or catch your breath.
Pulse oximetry: a test used to measure the oxygen level (oxygen saturation) of the blood.
At start of “pandemic” in some hot spots like NYC doctors reported a strange phenomenon called silent hypoxia, where the lungs were fine, but oxygen in blood was at critical levels.
The only thing I know that can cause this is mmWave frequencies which destroy oxygen bonds.