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You may already be somewhat familiar with the breathalyzer test police officers commonly use with suspected drunk drivers, but most people don’t fully understand how a breathalyzer device works. Understanding this test is essential if you have been charged with a DUI, especially if you believe that the device gave a false reading. The best thing to do in this situation is to contact an experienced DUI attorney, but you can start by familiarizing yourself with the breath test and some of the factors that may confound it.


If you are pulled over by a police officer for a moving violation or stopped at a DUI checkpoint, the officer needs to have probable cause to charge you with a DUI. Because field sobriety tests have proven to be unreliable, a breath test is more commonly administered in order to measure the level of alcohol in the driver’s blood. In all fifty states, the BAC limit is .08, which means you can be charged with a DUI if the breath test shows that your blood has at least eight one-hundredths of a percent of alcohol.


A breathalyzer is designed to measure BAC based on the alcohol in a deep-lung exhalation. The test assumes that there is a consistent ratio between BAC and alcohol exhaled in a deep lung breath, but in reality, the blood/breath partition ratio may vary depending on the individual. We’ll talk more about confounding variables in a second, but first let’s look at how a breathalyzer measures the level of alcohol based on your exhalation.


After a suspect breathes into the breathalyzer mouthpiece, any alcohol they have exhaled undergoes a chemical reaction with sulfuric acid, potassium dichromate, and water in a vial. The mixture that results is compared to an unreacted mixture in another vial, producing an electric current that moves the needle on the breathalyzer’s meter and allows the police officer to read the BAC level.


Factors That Affect the Breath Test


Factors That Affect the Breath Test in Colorado

Although the results of a breathalyzer test are often used as grounds for charging someone with a DUI, there are many different factors that may cause the test to give a high reading, even if the person being tested hasn’t been drinking. These factors include:


Body temperature. One of the assumptions of the breath test is that the body is at a temperature of 98.6 degrees Fahrenheit, but a healthy person may have a body temperature that is up to two degrees above or below that mark. Every degree above 98.6 will cause the breathalyzer to read up to 3.9% higher.


Diabetes. People with diabetes produce acetones and ketones that a breathalyzer misreads as ethyl alcohol, resulting in a higher reading. If you have diabetes and you have been charged with a DUI, be sure to tell your attorney immediately.


Interfering substances. In addition to the acetones and ketones produced by people with diabetes, there are a number of substances that may falsely register as ethyl alcohol. These include camphor (an ingredient in chewing tobacco), substances found in asthma inhalers, and substances found in aerosols.


Alcohol absorption. A breath test is designed to measure BAC after alcohol has been absorbed into the bloodstream, and if the test is performed during the absorption process, the reading may be anywhere from 100% to 230% higher than it should be. This means that someone who had one or two drinks at a bar and didn’t feel impaired might still end up being charged with a DUI.


Do not refuse if a police officer asks you to take a breath test, as this will result in the suspension of your license. However, keep in mind that you can challenge the results of the breath test if you think they are inaccurate. Make sure you contact a defense attorney as soon as possible to learn more.


About the Author:

Kimberly Diego is a criminal defense attorney in Denver practicing at The Law Office of Kimberly Diego. She obtained her undergraduate degree from Georgetown University and her law degree at the University of Colorado. She was named one of Super Lawyers’ “Rising Stars of 2012” and “Top 100 Trial Lawyers in Colorado” for 2012 and 2013 by The National Trial Lawyers. Both honors are limited to a small percentage of practicing attorneys in each state.  She has also been recognized for her work in domestic violence cases.



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