The independent is the one you change. So the size of the gap in the window could be your independent variable
The one that you control could be use one room with window open and a room with the window shut
Variables Kept constant. Make sure you have the same people in the room as respiration will effect the Blood Oxygen saturation
Other Variable could be that you control what ever you are doing when the window is open or shut...That is- if youre running around in the class when the window is shut then you must also run around when the window is open
And ABG and Pulse Oximeter is what you use.