Daily Archives: March 31, 2005

What About “Mouth Alcohol Detectors”?

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. If the subject has alcohol in the blood as well as the mouth, then the normal rising breath alcohol curve will add to the declining mouth alcohol curve to produce what is often a level curve.  Thus, the slope detector is unable to detect the presence of mouth alcohol when some is present in the mouth, yet breath alcohol concentration will be higher than it should be.  The slope detector cannot detect false mouth alcohol under this circumstance.

To make matters worse, the slope/mouth alcohol detectors are never calibrated by the police, as this has to be done at the factory.  The only thing police technicians do is simply rinse their mouth with alcohol and then breath into the machine: if the detector is triggered, it is assumed to be working.  But as Professor Hlastala has observed:

Whenever the slope detector is checked, it is done with alcohol in the mouth, but not in the blood.  Therefore, the slope detector serves no purpose and mouth alcohol frequently affects the breath alcohol reading.

Bottom line:  Despite the claims of manufacturers anxious to sell their machines, these detectors are unreliable and mouth alcohol remains a serious problem in breath alcohol analysis.