DUI Breath Tests
The breath test for drunk driving involves the "measuring" the amount of alcohol in a sample of exhaled air. While the test is simple to perform, the use of breath tests to determine blood alcohol is an "indirect method" and thus subject to variability.
Temperature, pressure, the chemical composition of your blood, your physical activity, and even hyperventilation can lead to varying results.
The term breathalyzer refers to the machine used by law enforcement to obtain evidence against you that presumably indicates that you were impaired when you blew into the machine (if you were above the magic .08). But exactly how does a breath machine calculate you level of impairment if the machine is testing your breath? You drank the alcohol you did not breath it. To better understand this lets start with your body. After you consume alcohol, be it wine, beer or otherwise, it gets absorbed from the mouth, throat, stomach and mostly from the intestines into your bloodstream. As the blood travels through your body it (obviously) travels to your brain which is why you feel the effects. It also travels through you liver where it immediately begins the process of removing it from you blood stream. In addition, the blood travels through your lungs, where some of the alcohol molecules move across the membranes of the lung into the air using the same process that allows you to breath in the oxygen you need to live.
At this point your lungs contain alcohol molecules along with molecules of oxygen etc. and the concentration of alcohol molecules in your lungs is related to the concentration of the alcohol in your blood. The government asserts that the alcohol in your breath is related to the alcohol in your blood at the ratio of 2,100:1. Meaning that 2,100 milliliters (ml) of air will contain the same amount of alcohol as 1 ml of blood. But this ratio is standard number and who is to say that this ratio is applicable to you. Now that we know how the alcohol passes through your body we can examine what this machine does. The breathalyzer uses infrared (IR) technology to presumably identify alcohol molecules based on the way they absorb light and the machine uses this to calculate the final result that you will see on your arrest report.
When the Intoxilyzer breath machine is calibrated (once a month), if the temperature of the testing solution is off by as little as one-fifth of one degree (.02), the calibration is not considered accurate. This may dramatically affect the read out in your case, especially if you consider that your body temperatures may vary several degrees depending on health, physical activity or even the time of the day.
The underlying physical principle of the Intoxilyzer is known as Henry's Law, which states that the concentration of a volatile chemical in vapor above a solution is proportional to the concentration of the chemical in the solution. This is true in the laboratory, but in humans, the proportions can vary greatly. This means that the Intoxilyzer can overstate your blood alcohol. Intoxilyzers operate on a principle known as "Lambert-Beer Law," which states, basically, that the amount of infrared light absorbed by your breath sample is proportional to the amount of alcohol in the sample. In other words, the more alcohol there is in your breath sample, the less infrared light that gets through the chamber to the detector. However, there are other compounds, called "interferents," which can also block the infrared and the Intoxilyzer cannot tell them apart.
With the proliferation of mouth jewelry it was just a matter of time before studies would be conducted to see what, if any, effect they may have on breath testing. In some jurisdictions the protocols for breath testing include mandatory removal of non permanent oral works. In order to see if failure to have individuals remove tongue studs would effect the breath testing, a very limited study, 2 people, was conducted in Washington. The study, which involved studs only, found that having such jewelry did not effect either the elimination rate of mouth alcohol nor the overall BrAC readings. However, the study did not include people with actual alcohol in their system. This is a flaw repeatedly pointed out by experts in the methodology of all mouth alcohol experiments. For more information please refer to: Lack of Effect of Tongue Piercing on an Evidential Breath Test. Journal of Forensic Sciences 1998, Logan and Gullberg.