A method or device for assaying physiological gas levels in a human, comprising: repeatedly measuring a gas in samples of breath or blood, or continuously measuring the gas through the skin or fingernail, while he or she holds his or her breath for a specified time interval (BHt) before each measurement, wherein these time intervals are selected from the group consisting of BHt=0, 4-6, 20-25 and 35-40 seconds, and recording the results to form a series of values including at least one measurement at BHt=35-40 which is treated as representing the average gas level in all the tissues of the body (T) at that time, to determine if the individual is net inhaling, net exhaling or in equilibrium with the gas.
