Understanding How Alcohol Gets on One’s Breath

Were one to ask most people in Texas to describe the first image that comes to mind when thinking of a DWI, the most likely image they might conjure up would be that of a person standing on the side of the road blowing into a handheld breath testing device.

Indeed, the breath test is the near-universal symbol for a DWI. Yet this may prompt a reasonable question among many: why would law enforcement officials rely on a breath test to measure one’s apparent drunkenness?

The Pathway of Alcohol from Blood to Breath

After all, the .08 blood-alcohol content measurement that most states follow (including Texas) to determine intoxication measures just that: one’s blood. The alcohol that people ingest with drinks (ethanol, per the Alcohol Pharmacology Education Partnership) makes its way into the bloodstream via passive diffusion (the process enabling water-soluble molecules to pass through membrane surfaces). In this case, the ethanol permeates the lining of the gastrointestinal tract and enters the bloodstream, where the veins then carry it throughout the body (eventually arriving at the heart).

From there, the right ventricle pumps that oxygenated blood into the lungs. Upon coming into contact with the oxygen in the lungs, some of the ethanol vaporizes into a gas, which then leaves the body when one breathes. As more ethanol in the blood vaporizes, the concentration in the blood remains in equilibrium. Breath testing devices assume a blood-to-breath ratio of 2100:1 to make BAC estimates.

The Breathalyzer Margin of Error

The problem with relying on a breath test to determine intoxication is that (in reality), one’s BAC can actually range from 1500:1 to 3000:1. This may contribute to the 50% margin of error that the American Motorists Association reports breath test devices to have.