A breath alcohol test measures the concentration of alcohol in the breath. This relates directly to the alcohol concentration in the blood. The more alcohol a person has in their blood, the more alcohol there will be in their breath.
The scientific explanation for the relationship of breath alcohol and blood alcohol comes from a law of physics called Henry’s Law. Henry’s Law predicts how much of a volatile substance (alcohol) will evaporate from a liquid (blood). The higher the concentration of alcohol in a person’s blood, the higher the concentration of alcohol there will be in that person’s breath.