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The National Highway Safety Administration (NHSA) maintains a list of "breathalyzer" machines approved for law enforcement agencies to use in testing drivers for intoxication. (Click here for list.) Breathalyzer-type machines are easy to use (compared to blood tests), but they are an inherently-flawed means to measure blood alcohol concentration (BAC). For example, the Intoxilyzer 5000, which uses infra red (IR) technology to measure the alcohol in the subject's breath, even when calibrated, may achieve an accuracy of only +/- 10%. Some of the newer machines, which use fuel cell as well as IR technology, are more accurate--if calibrated properly. The infamous hand-held PAS (Preliminary Alcohol Screening) device used in roadside tests is much less accurate. But, the accuracy of the various Breathalyzer machines in measuring alcohol in breath isn't even the main problem.
The main problem with any machine that attempts to calculate the BAC by analyzing the subject's breath is simple: it isn't measuring the actual alcohol content of the blood. Instead it must extrapolate from the alcohol content of the breath to arrive at the alcohol content of the blood. Proponents of the various "Breathalyzer" machines used would have the public believe that such extrapolation involves nothing more than a simple mathematical formula – the so-called "partition ratio" of 1:2100. Unfortunately, it isn't that simple.
Even measuring the BAC directly (i.e. by testing the blood) involves significant problems. But at least, blood flows through the circulatory system constantly and more or less evenly, so that the BAC of one blood sample should be much the same as that of another sample taken at the same time. With breath, however, the alcohol content can vary considerably from one breath sample to another. Any alcohol in the mouth, as caused by belching or burping, increases the alcohol content of the measured breath.
The alcohol content of the breath also varies depending on which part of the lungs it comes from. There is evidence that the alcohol does not come from the deep part of the lungs, as proponents of the machines argue, but largly from the surface tissue of the airways. Studies show that the level of alcohol in the breath rises as a person exhales and continues to exhale. Holding the breath can also charge it with alcohol. Conversely, if the subject hyperventilates before the breath test, the breath may have less alcohol in it, due to the "flushing" effect of the rapid breathing.
Not only does this mean the test is somewhat capricious with respect to anyone, it means it is inherently biased against many people – because it assumes they are average in all relevant respects. For example, a woman or smaller-than-average man forced to empty the lungs more to "blow" a breath sample into the breathalyzer machine will probably produce a higher "BAC" reading than a larger person with more lung volume. Similarly, people suffering from chronic bronchitis, asthma and other medical conditions affecting the lungs will probably produce higher BAC readings than would a healthy person. Even a person with a fever will "blow" a higher BAC than someone identically -intoxicated without a fever.
In spite of the known variances in the breath alcohol concentration of individuals with exactly the same blood alcohol levels, because of the convenience, police departments in California and around the nation continue to use "breathalyzer" machines to determine BAC. These machines blindly assume that the ratio between alcohol in the breath and alcohol in the blood is 1 to 2100 (partition ratio). In reality, the actual ratio for a given individual can range from approximately 1:1300 to 1:3000, depending on the circumstances; and the ratios for different individuals can vary even more widely. What does it mean in real life? A person whose breath has a partition ration of 1:1500 who had an actual legal BAC of .07% would "blow" an apparent BAC of .10% – i.e., legally DUI. The bottom line – measuring breath is an unreliable way to measure BAC.
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