Gases have different volumes at different temperatures and pressures, as we know from the Ideal Gas Law , and the pressures and volumes of gases in a mixture will depend on the amount of water vapor in the mixture.
So it would be pointless to talk comparatively about volumes of gas unless we had a standard framework of temperature, pressure, and water satruation in which to work.
To this end, we have two standards that we'll use in this class:
BTPS is useful for biomedical engineers and other medical types, because it uses body temperature (37 degrees C), Barometric Pressure (whatever the ambient pressure is; if it's not stated, assume 760 mm Hg), and assumes that the gas is saturated with water vapor .
That is: