Were someone to ask you what comes to your mind when you hear the term “drunk driving,” you likely would respond by describing a mental image of a person standing at the side of a road in Georgia, blowing into a hand-held breath testing device. That such an image has essentially become the de-facto representation of a DUI speaks to how heavily law enforcement relies on breath testing to ensnare those supposedly driving while intoxicated.

Yet while conjuring up this image, a question might inevitably enter into your mind: why would officials test your breath in order to determine the alcohol content of your blood? Furthermore, how accurate is such a measurement in determining whether or not you are actually under the influence?

How alcohol gets on your breath

According to the Alcohol Pharmacology Education Partnership, the alcohol you ingest when you have a drink ends up in your bloodstream by passing through the membranes lining the organs of your gastrointestinal tract via passive diffusion. Your veins then carry it throughout your body to your lungs, where some of the alcohol in your blood vaporizes when coming in contact with the oxygen contained therein (which you then expel when you breathe). The level of liquid alcohol still in your blood remains in equilibrium with the volume of gaseous alcohol in your lungs as your body metabolizes it.

Pinpointing the margin of error in breath testing devices

Hand-held breath testing devices assume a static blood-to-breath ratio of the alcohol in your body. This assumed number is 2100:1 (there being 2100 milliliters of alcohol in your blood for every one on your breath). Yet in reality, a person’s actual blood-to-breath ratio varies from 1500:1 to 3000:1. This wide range of variability contributes to why many experts estimate that breath testing devices may have a margin of error as high as 50%.