It is common knowledge that the near-universal standard for determining drunkenness is a blood-alcohol content measurement of .08. Yet as you have likely noticed, law enforcement officials in Wheeling develop make their initial BAC recording using a breath testing device. This prompts the question of how can your breath offer an indication of the alcohol concentration of your blood?
Information shared by The Alcohol Pharmacology Education Partnership offers the answer. The alcohol used in beverages is called ethanol. It is a water-soluble compound which can pass through cellular membranes through a process known as "passive diffusion." After you ingest it, ethanol molecules pass through the lining of your gastrointestinal tract and enter into the bloodstream, where they are carried throughout your body (including to your brain, where they inhibit the function of neurons, causing the symptoms of intoxication).
Eventually, the blood carrying the ethanol arrives at your heart, where it is then pumped into your lungs to be oxygenated via the right ventricle. Upon reacting with the oxygen in the lungs, some of the ethanol vaporizes into a gas. That gas escapes the lungs as your breath. As ethanol escapes, more is vaporized to cause its gaseous concentration to be in equilibrium with that of the blood. The process remains fluid, slowly releasing excess ethanol over time, and lowering you BAC.
Understanding this process also can help you see some of the shortcomings inherent with breath test readings. Your BAC is technically lowering with each breath. This (combined with alcohol being metabolized in your liver) means that every reading taken subsequently will be lower, making it difficult to rely on a handheld breath tester to obtain an accurate one-time reading.