For most in Winston-Salem, the de-facto representation of a criminal charge for drunk driving is a person standing on the side of the road blowing into a hand-held breath testing device. This prompts the question of why law enforcement authorities would use such a device to measure the alcohol content of one’s blood. Simple logistics certainly plays a role, as it is unreasonable to expect a person to be subjected to a roadside blood test. Yet does measuring one’s breath truly offer an accurate indication of their alleged degree of intoxication?

Answering this question requires that one understand how hand-held breath testing devices work. The alcohol that one ingests eventually works its way into their bloodstream, being carried on a circuitous route around their body which includes a stop in the lungs. Upon coming into contact with oxygen in the lungs, some of the alcohol is vaporized and expelled when one breathes. As this happens, the body must work to maintain a consistent ratio between the amount of gaseous alcohol in one’s lungs to the concentration in their blood. This blood-to-breath ratio is used to estimate levels of intoxication. According to information shared by the Alcohol Pharmacology Education Partnership, breath testing devices assume this ratio to be 2100:1 (one milliliter of blood containing 2100 times more alcohol than one milliliter of exhaled air).

The trouble with this assumption is that in actuality, one’s blood-to-breath ratio can actually vary between 1500:1 to 3000:1 depending on a number of different biological characteristics. This no doubt contributes to the findings shared by the National Motorists Association indicating the breath testing devices can have a margin of error as high as 50 percent. Such information may support one’s challenge that the breath test results being used against them might be unreliable.