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How do breath testing devices measure BAC?

On Behalf of | Jul 2, 2022 | DWIs

For most people in Kentucky, the common image that comes to mind when considering criminal charges for driving under the influence of drugs or alcohol is one standing at a roadside sobriety checkpoint, blowing into a breath testing device.

With this image comes the correlation with the near-universal standard for intoxication: a blood-alcohol content level of .08. Yet it also begs the question of why law enforcement officials would use a measurement of one’s breath to determine the alcohol content of their blood. Can any connection between the two arrive at a level of accuracy to justify the potential ramifications of a criminal offense?

Alcohol’s internal pathway within the body

To answer this question, one must first understand the pathway that ethanol (the form of alcohol consumed when drinking) takes in the body. After ingestion, ethanol (thanks to its water-soluble nature) permeates the linings of the organs of the gastrointestinal tract through the process of passive diffusion. It then enters the bloodstream, where the veins carry it throughout the body. It eventually arrives at the heart, entering into the right atrium, where the right ventricle then pumps it into the lungs. Once in the lungs, ethanol comes into contact with oxygen, which causes a small portion of it to vaporize into a gas (which then leaves the body when one breathes).

Breath test measurements

As this process continues (until the alcohol is completely metabolized), the amount of alcohol on one’s breath compared to their blood remains in equilibrium. Per the Alcohol Pharmacology Education Partnership, breath testing devices assign a standard blood-to-breath ratio for this concentration of 2100:1. The trouble is that in reality, one’s actual ratio can range from 1500:1 to 3000:1 (depending on a number of different biological factors). This may call the accuracy of breath test results into question.