Breath Alcohol Testing

When someone drinks alcohol and that alcohol is absorbed into the blood, it moves through the bloodstream. As the blood goes through the lungs, some of the alcohol moves across the lung's air sacs (alveoli) into the air, because alcohol will evaporate from a solution -- in science terms it is "volatile".

The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. As the alcohol in the alveolar air is exhaled, it can be detected by the device used for breath alcohol testing. Instead of having to draw a driver's blood to test his alcohol level, an officer can administer breath alcohol tests on the spot and instantly know if there is a reason to arrest the driver.

Because the alcohol concentration in the breath is related to the blood alcohol concentration, you can figure the BAC by measuring alcohol on the breath. The ratio of breath alcohol to blood alcohol is 2,100:1. This means that 2,100 milliliters (ml) of alveolar air will contain the same amount of alcohol as 1 ml of blood. Breath alcohol tests can be faster and less invasive than blood alcohol testing.

The Intoxilyzer, a common breath alcohol testing device, uses infrared spectroscopy which identifies molecules by the way they absorb infrared light. The absorbed wavelengths help to identify the substance as alcohol, and the amount of infrared absorption tells you how much alcohol is there.