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How Does a Breathalyzer Work?

Each time you breathe after consuming alcohol, a little bit of the alcohol in your blood vaporizes, passes through your lungs, and exits your body out of your nose or mouth. A breathalyzer device can check someone’s blood alcohol content by measuring the amount of ethanol, the chemical compound in alcohol, in your breath. The larger amount of alcohol consumed, the larger amount of alcohol a person breathes out.

There are several types of breathalyzers, but most work in a similar fashion. Sometimes a breathalyzer test can result in an arrest, yet fail as evidence in court, while other test are sophisticated enough to result in a DUI (Driving under the influence) conviction. A police officer administering a breathalyzer test tells the driver to blow into the machine for at least 3 seconds. This allows for deep lung air with the greatest concentration of alcohol to be tested. If there is still alcohol in the mouth, the test may read higher at first, but should stabilize and measure the deep lung air afterwards.


When you drink alcohol, it circulates in your blood and reaches the lungs. Similar to how carbon dioxide diffuses out of your blood, the alcohol diffuses as well. When you breathe, you exhale the alcohol. A breathalyzer can measure how much alcohol is present in your breath and provide an estimate of your BAC. The breath-to-blood alcohol ratio is 2,100:1, which assumes that there are 2,100 milliliters of alcohol in your blood for every 1 milliliter of alcohol in your breath.

In the United States, it’s illegal to drive with a BAC of .08 percent or higher, at which point it’s considered a DUI or DWI. However, the margin of error with portable breathalyzers can reach 15 percent, and up to 23 percent of the drivers charged for DUI/DWI may be victims of inaccurate breathalyzer results. The most accurate way to test BAC is by sampling blood. All other tests have a higher margin of error. This doesn’t mean that you can dispute your results if you’ve had a few, however.


Since breathalyzers do not physically measure BAC and only provide an estimate, several factors can lower the accuracy of results and make them lower or higher.

One factor that people cannot control is having a higher body temperature, which makes the breathalyzer result higher because of higher volatility. Differences in blood composition may cause higher readings as well. Lower readings could occur when the test is taken before the alcohol is fully absorbed into the blood, which takes one to two hours after drinking.

Electrical interference from police radios and smartphones is another possible factor that results in lower accuracy. Additionally, some breathalyzers are unable to tell the difference between alcohol and other simple chemical compounds that are present in the breath or vicinity of the test. Methyl compounds, for example, are present in vinegar, paint remover, cleaning fluids and the breath of people suffering from diabetes. More factors that can cause higher readings include acid reflux, blood or vomit in the mouth, breath freshener or mouthwash, or alcohol residue in the mouth.

Why Breathalyzer Tests Can Be Wrong?

We all know that drinking and driving is never a good idea.  There is no excuse for getting drunk and getting behind the wheel of a car.  However, what if you only had one drink or a sip of alcohol, even none at all, and you tested positive on a breathalyzer test?  This type of thing happens more than you think.  Recently in San Francisco over 1,000 false positive readings were thrown out, and prosecutions overturned because of such results.

How can Breathalyzers Yield False Positive Results?

 1. Improper Breathalyzer Calibration

Improper calibration was the main cause of the over 1,000 conviction turnovers in San Francisco. Every make and model is different, and law enforcement divisions must follow the equipment manufacturer’s guidelines for calibration exactly for the results to be accurate.  In the California incident, the 20 breathalyzer units were supposed to be calibrated every two weeks.  However, San Francisco law enforcement failed to maintain the equipment manufacturer’s guidelines and was forced to overturn hundreds of convictions based on the outcome of the breathalyzer test results.

  1.  Alcohol Residue in Mouth

This may sound silly, but a breathalyzer is designed to measure the amount of alcohol saturation from the air in a person’s lungs.  The human body releases alcohol through the lungs as a way of processing and expelling it.  If a person has just taken a sip of alcohol, they will have residual alcohol in their mouths that the breathalyzer assumes is from the lungs.  This can yield a much higher reading than the actual amount of alcohol in the person’s body. Therefore, each breathalyzer recommends that the testing officer wait a certain period of time (usually about 20 minutes) before administering the test for accurate results.  If you have just downed half a glass of wine before getting pulled over, the residual alcohol in your mouth will mislead the breathalyzer into thinking you just drank an entire bottle.

  1.  Breathalyzer Interference

Many things can cause false positive readings on breathalyzer tests, the most common being an elevated number of ketones in diabetics’ blood that causes an increase in acetone in their breath.  Some breathalyzer tests are sensitive to acetone and mistakenly register it as alcohol.  Other substances can cause false positive results as well such as paint fumes, chemical fumes, mouthwash, gum, cough syrup, and even herbal supplements.

  1.  Other Breathalyzer Reading Errors

A breathalyzer can also improperly measure BAC during the absorption phase of alcohol consumption. Absorption time varies depending on the person, but it can last anywhere from half an hour to two hours.  In the absorption period, alcohol is not evenly distributed through the blood stream and can yield erroneous breathalyzer results.

Did You Test Positive for Alcohol on a Breathalyzer Test?

If you tested positive on a breathalyzer test you need to contact a DWI lawyer immediately to start building your defense.  Only an experienced DWI lawyer will have the knowledge and expertise necessary to fight for you in court.  If you are facing DWI or DUI charges after testing positive on a breathalyzer test, hire a DWI  or DUI lawyer today.

Reference: https://sanfrancisco.cbslocal.com/2012/03/05/sfpd-breathalyzer-error-puts-hundreds-of-dui-convictions-in-doubt/


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