Many people who are pulled over by the police on suspicion of drunk driving are asked to take a preliminary breath test (PBT) to give the officer an estimate of the person’s blood alcohol concentration (BAC). As everyone who has taken a PBT knows, the test involves blowing into a device that then gives a reading, usually in the form of a number or a percentage. But what’s really being measured inside that little black box?
Breath samples given by blowing into a breath testing machine usually include three different types of samples. These are known as tidal breath air, reserve breath air, and alveolar breath air. Tidal breath air comes from the top of the lungs. When a person breathes normally, most of what they exhale is tidal breath air. Reserve breath air is exhaled when a person breathes while exercising. More air goes in and out in a reserve breath than in a tidal breath, but the air still stays in the lungs for only a short time. Neither of these types of breath samples gives an accurate depiction of a person’s blood alcohol concentration.
In order to work correctly, a New Hampshire breath alcohol testing machine must test a sample of alveolar breath air. This type comes from deep in the lungs, where it has been in contact with the blood circulating through the alveoli, which are lung tissues that put oxygen into the bloodstream and clean carbon dioxide out. An officer that asks a driver to blow steadily until the driver is gasping for air is trying to get a sample of alveolar breath air.