The amount of alcohol in a person’s body is measured by the weight of the alcohol in a certain volume of blood. This is called the blood alcohol concentration or BAC. BAC measurements provide an objective way to identify levels of impairment, because alcohol concentration in the body is directly related to impairment. The BAC measurement is expressed as grams per deciliter (g/dl) of blood, and in most states a person is considered legally intoxicated if his or her BAC is .08 g/dl or greater.

Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream. Alcohol is not digested upon absorption or chemically changed in the bloodstream. As the blood goes through the lungs, a physiologically predictable amount of the alcohol will moves across the lung membranes and into the lungs themselves. Once in contact with the air in the lungs, it evaporates and is exhaled. The concentration of the alcohol in the air in the lungs is directly related to the concentration of the alcohol in the blood.