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Are Breathalyzer Results Affected by How You Breathe Into the Alcotest?

June 17, 2013

Breath machines are commonly used to determine the guilt of innocence in drunk driving cases. A number of scientific studies, however, have confirmed that breath tests results may vary depending on the breathing pattern of the person being tested, highlighting just another example of the unreliability of breath machines.

In one study, for example, a group of men drank moderate doses of alcohol and their blood-alcohol levels were then measured by gas chromatographic analysis of their breath. The breathing techniques were then varied.  The results indicated that holding your breath for 30 seconds before exhaling increased the blood-alcohol concentration (BAC) by 15.7%. Hyperventilating for 20 seconds immediately before the analyses of breath, on the other hand, decreased the blood-alcohol level by 10.6%. Keeping the mouth closed for five minutes and using shallow nasal breathing resulted in increasing the BAC by 7.3%, and testing after a slow, 20-second exhalation increased levels by 2%.

Dr. Michael Hlastala, Professor of Physiology, Biophysics, and Medicine at the University of Washington, has gone farther and concluded:

“By far, the most overlooked error in breath testing for alcohol is the pattern of breathing . . . The concentration of alcohol changes considerably during the breath . . . The first part of the breath, after discarding the dead space, has an alcohol concentration much lower than the equivalent BAC. Whereas, the last part of the breath has an alcohol concentration that is much higher than the equivalent BAC. The last part of the breath can be over 50% above the alcohol level . . . Thus, a breath tester reading of 0.14% taken from the last part of the breath may indicate that the blood level is only 0.09%.”

Police officers aware of these issues may encourage subjects to breath longer and harder, ensuring that the breath captured by the machine will be from the bottom of the lungs, near the alveolar sacs, which will be richest in alcohol — giving a higher (but inaccurate) reading.

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