BAC stands for Blood Alcohol Concentration and is a percent (%) measure of alcohol in the blood; it is also correct to use g/dL (grams per deciliter) as a more formal unit of measure instead of %BAC. A breathalyzer is able to calculate this percentage using only a breath sample because of the precise relationship between alcohol in the blood and alcohol metabolised into the lungs from the bloodstream. This is also why it is important to make sure the mouth is clear of contaminant (including alcohol) so that the breath sample contains pure lung air, or as close as possible.