Can law enforcement officers influence the BAC reading?
Law enforcement officers can and do influence BAC readings. Law professor and lawyer Lawrence Taylor quotes Dr. Michael Hlastala, Professor of Physiology, Biophysics and Medicine at the University of Washington
“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%.” 1
Professor Taylor explains that “Many police officers know this. They also know that if the machine contradicts their judgment that the person they arrested is intoxicated, they won’t look good. So when they tell the arrestee to blow into the machine’s mouthpiece, they’ll yell at him, “Keep breathing! Breathe harder! Harder!” As Professor Hlastala has found, this ensures 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. With the higher alcohol concentration, the machine will give a
higher — but inaccurate - - reading.” 2
