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How Police Officers Influence Breathalyzer Results in San Francisco

Knowingly or unknowingly, police officers often influence the results of breathalyzer tests, resulting in inaccurately high readings. To understand how, we have to review some facts about how breathalyzers work.

One of the most important factors in determining how high the breathalyzer registers someone’s blood alcohol level—almost as important as the amount of alcohol consumed—is breathing pattern. One study showed that holding your breath for 30 seconds before blowing into the breathalyzer increases the result a whopping 15.7 percent. Hyperventilating for 20 seconds, on the other hand, decreases it by 10.6 percent. (“How Breathing Techniques Can Influence the Results of Breath-Alcohol Analyses”, 22(4) Medical Science and the Law 275). Another study showed that running up a flight of stairs, like hyperventilating, decreased the result by 11-14 percent. Running up two flights of stairs, as you might expect, lowered it by double the percent: 22-25 percent.

Yet another way in which breathing pattern can affect the results of a breathalyzer test has to do with what part of the breath the machine is exposed to. In other words, different parts of the exhalation will give different blood alcohol readings. According to 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%." (Hlastula, M. Physiological errors associated with alcohol breath tests. The Champion, 1985, 9(6))

DUI/DWI lawyer and law professor Lawrence Taylor, speaking about this fact about breathalyzers, 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.” (Taylor, Lawrence. How to Fool the Breathalyzer. (http://www.duiblog.com/2004/11/27)

A police officer may not necessarily do so intentionally. A police officer may simply have been instructed by a superior or fellow officer to ensure that a subject blows long and forcefully into the machine. Nonetheless, it is staggering to consider how many falsely high breathalyzer results have been obtained—and how many people have been arrested and convicted as a result—because of these instructions, and the device’s potential for inaccuracy.