Introducing Energy States
Around an atom, there are 7 different electron shells1. When the electrons receive the necessary amount of energy, they jump from one state to another (called "ground state" and "excited state"), as shown in the image below:
The figure (generated by POV-Ray) shows excitations of copper 3d orbitals on the CuO2-plane of a high Tc ...
I tried it out on my pulse oximeter. I placed it on my thumb where it read 97%. I then held my breath and it gradually dropped down over the course of a minute to 93% before I had to give up.
I suggest you check your device on other people in case it's defective.