How Do DUI Breath Tests Work?

The breath test is another testing option used by law enforcement to determine if there is alcohol in your system and to measure your Breath Alcohol Concentration. Although we commonly think of alcohol as being in the bloodstream, the body metabolizes and eliminates alcohol in a variety of ways, one of which is through breathing. The lungs take in air during inhalation, then exhale a variety of gases that pass out of the blood and through the lungs into the air, such as carbon dioxide and, in cases of drinking, alcohol molecules. If required to do a breath test, you will be asked by a police officer to blow into a breathalyzer or intoxilyzer, which will then provide the operator with an alcohol concentration reading.

In Arizona, the most common instrument used is the Intoxilyzer 8000. These machines are supposed to measure the alcohol concentration in the air that is exhaled from a person’s lungs. The basic premise is this:

(1) The Intoxilyzer 8000 emits a specific quantity of infrared light in the chamber where a person exhales, and there is a sensor to measure this infrared light;
(2) Alcohol molecules absorb infrared light;
(3) As a person exhales, the sensor will measure the remaining amount of infrared light in the chamber;
(4) The greater the amount of alcohol in one’s exhaled breath, the more the alcohol will absorb the infrared light, and the less infrared light will be detected in the chamber.

The Intoxilyzer 8000 will then use that detected amount of remaining infrared light to calculate a person’s blood alcohol concentration using a predetermined number called a partition ratio. The partition ratio is simply the ratio of the amount of alcohol in the blood as compared to the amount of alcohol in the exhaled breath. However, this ratio is very individualized, yet the Intoxilyzer 8000 uses the same partition ratio for every single test.

It is in this process, where the air for the machine is supposed to come from and the conversion from breath to blood, that are amongst the biggest culprits for errors in the breath testing process.


DUI Breath Testing Inaccuracies

As mentioned above, these machines provide results based on the partition ratio, a number that makes assumptions for various physiological factors, including a certain body temperature, the temperature of the breath at the time of testing, and presuming the person to be in a post-absorptive state, meaning that it is assumed that the subject’s body is no longer absorbing any more alcohol and is in the process of eliminating it. These machines produce results that are supposed to reflect someone’s blood alcohol content, within plus or minus 10%. However, an individual’s true values for these factors often fluctuate outside of these margins of error—this means that the alcohol concentration obtained may be inaccurate by up to 30% greater than the standard +/-10% allotted. Complicating this result is the fact that hyperventilation, rapid, shallow breathing, will generally result in an inaccurately lowered reading, whereas holding one’s breath prior to testing will result in an artificially increased reading. This can add yet another 10-15% variation to the “results” obtained.

For certain DUI charges in Arizona, you are allowed to introduce evidence to dispute the standard partition ratio used by the machine when it converts the breath reading to the blood reading. State v. Cooperman, 232 Ariz. 347, 306 P.3d 4 (2013).   If an individual’s partition ratio is low, and is not accounted for by the machine, the final result could be off by an additional 25% according to studies on this topic.  These partition ratios are not static. They vary from person to person, and even from moment to moment for the same person.


Breath Testing Instrument Procedures and Maintenance

Arizona law requires that the machine used to capture your breath be properly calibrated and serviced with Arizona Department of Public Safety.


Combating DUI Breath Test Results in Arizona

Your DUI attorney must be able to review and evaluate the breath test results, evaluate the machine and calibration records, and police operator records to determine how to proceed in defending your case. And of course your DUI attorney should be able to ask you the right questions to determine if the purported reading accurately and reliably reflects the alcohol consumed.  Only when your DUI attorney investigates every aspect of your DUI breath case can you begin to reach conclusions about the strengths and weaknesses of your case.

Need to add update on new Arizona and SCOTUS case law ruling that breath test is not invasive, so warrantless testing is not a violation of the 4th Amendment.