If asked to visualize a drunk driving offense, you might likely describe a person standing on the side of one of Missouri’s many roads, blowing into a hand-held breath testing device. The details of such a scene may seem of little import to you until you actually find yourself in it.

Facing the potential of a charge for driving while intoxicated, you may question how is it that law enforcement officials are able to use the results of a test measuring your breath to arrive at an assumption of the alcohol content of your blood.

How alcohol gets on your breath

According to the Alcohol Pharmacology Education Partnership, the specific type of alcohol used in drinks is ethanol. This is a water-soluble compound whose composition allows it to permeate membrane surfaces (such as those lining the organs of your gastrointestinal tract) through a process known as “passive diffusion.” That ethanol that escapes your digestive tract through diffusion ends up in the bloodstream, where the veins then carry it throughout your body, eventually arriving in the heart. Once in the heart, it mixes into oxygenated blood and then moves into your lungs. Coming in contact with oxygen causes a small portion of the ethanol to vaporize into a gas. That gaseous ethanol then escapes the body when you breathe.

The blood-to-breath ratio

Breath testing devices measure this exhaled alcohol, assuming that its concentration is in equilibrium with the concentration in your blood. To come up with a measurement, these devices assume a blood-to-breath ratio of 2100:1. In reality, however, your actual ratio may vary between 1500:1 and 3000:1. Many factors influence this variance, such as gender, genetics and body composition. Whatever the influence may be, however, this wide range of variance may lend credence to your claims that your breath test result may be unreliable.