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Why Breathalyzers Don't Measure Alcohol

Why Breathalyzers Dont Measure Alcohol

Information courtesy of Lawrence Taylor - DUIblog

Thats right: Breathalyzers dont actually measure alcohol. What they actually detect and measure is any chemical compund that contains the methyl group in its molecular structure. There are thousands of such compounds -- including quite a few which can be found on the human breath. And this machine that determines a persons guilt or innocence will "see" all of those chemicals as alcohol -- and report a falsely high "blood-alcohol" concentration (BAC).

Most breath machines used in DUI cases by law enforcement today employ a technology called "infrared spectroscopy". The DUI suspect breathes through a tube connected to the machine and a breath sample is captured in a small "sample chamber" inside the machine. Then beams of infrared energy are shot through the captured breath sample. If there are any compounds containing the methyl group, they will absorb some of this energy; the more of the chemical compound in the breath sample, the more energy is absorbed. The more energy that is absorbed, the less infrared energy that reaches sensors at the other end of the sample chamber. And the less energy that is detected by the sensors, the higher the "blood-alcohol" reading.

Problem: the machine is designed to simply assume that the chemical compound absorbing the energy is alcohol.

If a person has any of these other compounds on his breath, called "interferents" by the engineers, he will get a falsely high BAC test result. And if there are two or three such compounds on his breath, the machine will read a cumulative result: it will add them up and falsely report the total as the blood-alcohol level.

So what kinds of compounds may be on a persons breath that can cause false BAC readings in a DUI case? In one study of eight men, 69 different compounds containing the methyl group were discovered. "Trace Composition of Human Respiratory Gas", 30 Archives of Environmental Health 290. In another study invoviing 28 subjects, researchers found that teh "combined expired air comprises at least 102 various organic compounds of

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