Test principle !

Alcohol­ isn't digestible upon assimilation, nor chemically converted in the bloodstream. As the blood passes by the lungs, more or less of the alcohol goes across the membranes of the lung's air sacs (alveoli) into the air, since alcohol will vaporize from a solution -- i.e., It's evaporable. The density of the alcohol in the alveolar air is associated to the density of the alcohol in the blood. As the alcohol in the alveolar air is breathed out, it could be discovered from the breath intoxicant examining device. Rather than having to draw a driver's blood to examine his intoxicant degree, a policeman could examine the driver's breath on-site and straightaway know if there's a ground to arrest the driver.

Since the concentration of blood and breath are proportional, one can easily figure the BAC by measuring alcohol by breath. The ratio of breath alcohol to blood alcohol is 2,100:1 which implies that the amount of alcohol in 1 ml of blood is equal to that in 2100 ml of exhausted air.