When thinking about a drunk driving arrest, many people may imagine standing on the side of the road blowing into a hand-held breath testing device. Yet despite the common association between this device and DUI charges, drivers may find it surprising that judges may view the results of such tests to be unreliable.

Information shared by the National Motorists Association shows that breath testing devices may have a margin of error as high as 50%.

Most may think that breath test results indicating the blood alcohol content is 0.08% or higher provide irrefutable proof that a driver was drunk. However, there are valid reasons to challenge DUI charges. Doing so requires knowing the shortcomings of breath testing devices and the process of how a measurement of a driver’s breath can indicate the alcohol content of his or her blood.

How alcohol gets on the breath

Per the Alcohol Pharmacology Education Partnership, ethanol is the form of alcohol contained in alcoholic drinks. Ethanol is water-soluble, meaning that once a person ingests it, its molecules are able to pass through the lining of the organs of the gastrointestinal tract and enter the bloodstream. Once in the blood, the veins carry the ethanol to the heart, which sends it to the lungs.

In the lungs, ethanol molecules come in contact with gaseous oxygen, which can cause some of them to vaporize into a gas. The lungs then expel that gas on the breath. Breath testing devices rely on a general blood-to-breath ratio when generating their readings.

This presents two potential issues that may help a driver dispute DUI charges. First, the ratio used to calibrate breath testing devices can differ from that of the person taking the test. Second, with every breath, a driver’s BAC lowers, essentially making the process of measuring BAC through breath like an attempt to hit a moving target.