In California, the legal definition of drunk driving is operating a vehicle with .08% BAC (blood alcohol content) or higher. Typically, law enforcement will use a breath test to determine your BAC. However, if you have used a product like mouth wash, you may have traces of alcohol in your mouth which throws off the test. You could test as more than .08% even if you are stone sober. This is known as the “mouth alcohol” defense, and it works in many cases.
How do breath tests work any why does mouth alcohol affect them?
Breath tests are not designed to measure the breath from your mouth. They are supposed to test air from deep within your lungs, known as “alveolar” air. This is why you are required to blow a long, continuous breath into the device—so that you get a good sample of deep-lung breath. In theory, this allows the breath test device to measure the percentage of alcohol vapor that was released by the blood flowing into and out of the lungs. This is supposed to be a direct measure of your blood alcohol concentration, or how drunk you are.
The problem is that other sources of alcohol vapor can interfere with the test, even if you are not intoxicated. For example, imagine that you rinsed your mouth with an alcohol-based mouthwash like Listerine shortly before taking the test. Your mouth would still taste and smell like mouthwash, because there is still some alcohol in your mouth. This alcohol releases its own vapors which can and will interfere with the test. In some cases, they could affect the results so seriously that you appear to “fail” the test (score higher than .08%) even if you were not drinking.
I’ve heard that breath mints, dentures, tobacco, etc. can all cause “mouth alcohol.” Is that true?
Yes. There are many possible sources for mouth alcohol other than mouthwash. These can include:
- Breath mints and breath sprays which sometimes include small amounts of alcohol
- Chewing tobacco which often contains pure ethyl alcohol
- Herbal medicines such as tinctures which are mixed with a small amount of alcohol
- Some cough syrups if they are alcohol based
- Dentures, dental bridges and dental caps which can trap alcohol consumed earlier
- Burping or vomiting after drinking alcohol
- GERD or acid reflux (see our detailed GERD page here)
Some other factors can mimic the effects of mouth alcohol, like eating a low carb or ketogenic diet.
What if I was drinking? Can I still use the mouth alcohol defense?
This is actually quite common. It’s important to remember that it’s not necessarily illegal to have a few drinks and drive. Under California Vehicle Code 23152(b), it is only illegal if your BAC is .08% or more. Under other California DUI laws, it’s only illegal if the alcohol impaired your driving—such as if you were weaving or driving dangerously. But simply having some alcohol in your system is not enough to convict you of DUI.
That means that, hypothetically, you could have had two beers and gotten in the car, with a BAC of .05% and no impairment to your driving. If you were then pulled over and given a breath test, mouth alcohol could cause you to falsely read as .08% or above. In other words, the two beers you drank didn’t put you over the limit; the mouth alcohol did.
Often, mouth alcohol is not from breath spray but from burping after having one or more alcoholic drinks. You can still use this defense even if you were drinking.
Don’t police check for mouth alcohol before administering the test?
In theory, yes. California law enforcement officers are supposed to observe a 15 minute “observation period” immediately prior to any DUI breath test. This observation period is supposed to give them time to make sure that you do not:
- Drink anything
- Use any breath products
This is supposed to reduce the risk of mouth alcohol affecting your results.
Unfortunately, it doesn’t always work. First of all, police are busy and are often distracted with paperwork and other activities during the “observation period,” meaning they may not be observing you closely at all. Secondly, 15 minutes may not be enough time for mouth alcohol to subside, depending on the source of the mouth alcohol.
You can potentially use the mouth alcohol defense whether or not police observed you.
Do “slope detectors” prevent mouth alcohol from affecting my breath test?
Not always. A slope detector is a device added to some breath test machines, which is supposed to screen out the impact of mouth alcohol and give a “pure” reading from deep lung air. Unfortunately, two factors make the slope detector unreliable:
- Operator error
- The device may not be functioning during your test, meaning you get a false result that the officers believe is “pure”
For this reason, a DUI lawyer may still dispute the results of a breath test even if a slope detector was used.
How can I prove that mouth alcohol affected my breath test? Can I win my DUI case?
There is lots of evidence you can use depending on the source of the mouth alcohol. A good DUI lawyer will sit down with you and discuss the possible factors and help you construct a case for what happened. Possible evidence you can use may include:
- Receipts for mouthwash/breath spray products, or the presence of such products in the “inventory search” police conducted of your vehicle
- Restaurant or bar receipts showing exactly what you ate or drank before driving
- Eyewitness accounts that you used a product that may cause mouth alcohol
- Eyewitness accounts that you were experiencing heartburn, indigestion or burping
- Medical documents showing that you have braces, dentures or dental fixtures or, alternately, that you have GERD/acid reflux
Additionally, your DUI lawyer will subpoena data on the breath test device that was used. If the device does not have a slope detector, the detector wasn’t working, or there is evidence of operator error, the test itself may be called into question.
Mouth alcohol is a valid defense and has saved many drivers from DUI charges.
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