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What About Mouth Alcohol Detectors

What About Mouth Alcohol Detectors

Information courtesy of Lawrence Taylor - DUIblog

In my previous post, I discussed the problem of mouth alcohol -- that is, falsely high breathalyzer readings caused by alcohol samples coming from the mouth rather than from the lungs. And the response from some of our more sophisticated readers (law enforcement and/or lawyers?) has been predictable: What about mouth alcohol detectors?

Some breath machines have what is called a slope detector, commonly referred to as a "mouth alcohol detector". This is an electronic circuit designed to detect the presence of mouth alcohol as the breath is being captured by the machine. It does this by detecting any pronounced negative slope in the alcohol intake curve, since alcohol content from the mouth or throat will decline more rapidly than alcohol from the lungs. In theory, the presence of mouth alcohol will cause the test to abort.

Unfortunately, these "detectors" are simply unreliable, due primarily to a design flaw. Rather than try to explain the technical defects, I will let Dr. Michael Hlastala, Professor of Physiology, Biophysics and Medicine at the University of Washington School of Medicine, explain:

The slope detector is problematic for all breath instruments and has been misrepresented by the manufacturers.

When a subject with alcohol in the blood, with no extra alcohol in the breath, exhales, the breath alcohol continues to increase during exhalation. It does not reach a "plateau" until the end of airflow. It continues to rise, giving a positive slope. If you swish a little alcohol in the mouth (and have no alcohol in the blood), wait awhile and exhale, the breath alcohol will rise until a peak is reached about 1/3 of the way into the exhalation, and then decline gradually. It is the declining breath alcohol (negative slope) that triggers the slope detector to register the breath as having mouth alcohol.

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