Most people in California likely share the same visual representation of a drunk driving offense: one standing on the side of the road, blowing into a handheld breath testing device. This also gives rise to the familiar figure of .08 (the blood-alcohol content level that determines intoxication), which serves as an almost-universal standard.
Yet this assumption gives rise to a popular question: why would law enforcement officials measure a person’s breath in order to determine the alcohol content of their blood? Answering this question requires a basic understanding of the pathway ethanol (the type of alcohol contained in alcoholic beverages) takes in the body.
Ethanol’s path from ingestion to exhalation
Ethanol is a water-soluble compound, meaning that it can permeate membrane surfaces through passive diffusion. This allows it to work its way through the lining of the organs of the gastrointestinal tract and enter the bloodstream. Once in the blood, the veins carry the ethanol throughout the body, eventually arriving in the lungs. A portion of the ethanol then vaporizes into a gas upon coming into contact with oxygen in the lungs. That gaseous alcohol then leaves the body with each breath.
This process continues, with the ethanol content of one’s breath maintaining an equilibrium with that of their blood. According to the Alcohol Pharmacology Education Partnership, breath testing devices assume this blood-to-breath ratio to be 2100:1.
The problem with breath testing devices
The trouble with this assumption is that (in reality) one’s alcohol blood-to-breath ratio often ranges between 1500:1 to 3000:1. It may be for this very reason that the American Motorists Association reports breath testing devices may have a margin of error as high as 50%. That might call into question the reliability of their results when used as evidence supporting an alleged DUI charge.