OPINION:  The vast majority of citizens arrested for drunk driving are tested for blood-alcohol on breath machines (generically referred to as "breathalyzers").  The test results from these machines will largely determine:  (1) if the suspect is booked or released; (2) whether the driver's license is automatically suspended or not; (3) what charges, if any, will be filed by the prosecution; and (4) whether the the accused will be found guilty or  not. 

Problem:  Breathalyzers don't actually measure alcohol.

Most models of breath machines used in drunk driving cases today utilize infrared analysis.  A beam of infrared light is passed through a chamber containing the DUI suspect's breath sample.  Light at the wave length used will be absorbed by the methyl group in alcohol's molecular structure; the more light that is absorbed by the methyl group, the less that reaches the sensors at the other end of the chamber -- and the higher the reading.

In other words, breath machines used in DUI investigations measure the compounds on the breath containing the methyl group -- and assumes it is alcohol.

Problem:  Thousands of chemical compounds besides alcohol will also contain the methyl group and be absorbed within the range of light used in the breathalyzers -- and falsely reported as alcohol.  Making things worse, breathalyzer readings are cumulative -- that is, multiple compounds in the breath will each absorb the infrared energy, producing an additive effect to the final reading.

Of the compounds containing the methyl group, scientific studies have found well over a hundred on the human breath.  In one study involving 28 subjects, researchers found that the "combined expired air comprises at least 102 various organic compounds of endogenous and exogenous origin".  See "Characterization of Human Expired Air", 15 Journal of Chromatographic Sciences 240. And Canadian scientists have discovered over 200 such compounds. "The Diagnostic Potential of Breath Analysis", 21(1) Clinical Chemistry 5.

So what kinds of compounds may be on a person’s breath that can cause false readings in a DUI case? 

For starters, diabetics with low blood sugar can have high levels of acetone — which is "seen" as alcohol by breathalyzers. And scientific studies have found that people on diets can have reduced blood-sugar levels, causing acetone hundreds of times higher than found in normal individuals. Frank and Flores, "The Likelihood of Acetone Interference in Breath Alcohol Measurements", 3 Alcohol, Drugs and Driving 1.

If you are a smoker, to cite another example, your breathalyzer result is likely to be higher than expected. The compound acetaldehyde is one of those reported by breathalyzers as "alcohol", and researchers have discovered that levels of acetaldehyde in the lungs can be 30 times higher in smokers than in non-smokers. Result: higher blood-alcohol readings on the machine -- and citizens falsely convicted of drunk driving.

And then there are the industrial compounds: paint, glue, gasoline, thinners, and other compounds contain the methyl group. No, you don’t have to drink the stuff: simply absorbing it through your skin or inhaling the fumes can result in significant levels of the chemical in your body for hours or even days, depending upon the "half-life" of the compound. So if you’ve painted a room or pumped some gasoline in the last day or two, don’t breath into a breathalyzer.

Yet, in what has become "trial by machine" in our courts, citizens continue to be found guilty of drunk driving -- beyond a reasonable doubt -- based upon unreliable readings. 

About the author:  Lawrence Taylor is a Los Angeles DUI attorney and the author of the standard legal textbook in the field, Drunk Driving Defense, 6th edition.