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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, whether estimated by breath, urine or blood samples, provide an objective way to identify levels of impairment, because alcohol concentration in the body is directly correlated 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. Breath testing is the primary and easiest method used by law enforcement agencies for measuring BACs. At the time of the first face-to-face contact with a suspected impaired driver, techniques for detecting whether alcohol is present or absent can be performed easily by law enforcement officers during roadside stops using hand-held passive alcohol sensors. Use of these devices is non-invasive and can even be performed while the person is still in his or her vehicle.