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Mouth Alcohol as a California DUI Defense

Just because you have been charged with a California DUI based on your DUI breath test... don't assume the prosecution has an open-and-shut case.

Residual mouth alcohol can lead to false DUI breath test results...

California DUI charges almost always rely on the results of DUI blood or DUI breath tests.

But such tests have inherent flaws.  Breath tests are particularly vulnerable to challenge in California DUI cases.  And if the results can be excluded from evidence, it could mean the prosecutor will reduce the DUI charges – or even dismiss the case entirely.

1. DUI Breath Tests and Residual Mouth Alcohol

When you consume something containing alcohol, small amounts of the alcohol remain in the mucosal linings of the mouth for approximately 15-20 minutes. This is known as “residual mouth alcohol.”

Residual mouth alcohol can be caused by many things, including:

  • recently consumed alcohol, even when --     
  • cough syrup,
  • homeopathic medicines, and
  • mouthwashes or breath sprays containing alcohol.

California regulations require that you be observed continuously for at least 15 minutes before beginning a DUI breath test.  This is to give mouth alcohol time to dissipate, and ensure that during that time you don't put anything in your mouth.

2. DUI Breath Tests Only Approximate Blood Alcohol Content

Unlike California DUI blood testing, DUI breath testing does not directly measure blood alcohol content (“BAC”).

Breath test machines approximate BAC by mathematically converting the amount of alcohol present in a breath sample to a comparable BAC. To do this, the machine needs to measure the alcohol present in the deepest part of the lungs – the part closest to the blood supply.

“Deep lung air” comes from your alveoli, the balloon-like sacs that fill and empty when you inhale and exhale. California regulations require that a DUI breath sample be “essentially alveolar” in composition. This is why you are asked to blow hard when you take a DUI breath test.

3. How Residual Mouth Alcohol “Tricks” DUI Breath Machines

When you exhale, any residual mouth alcohol mixes with deep lung air as it passes through your airway and out of your mouth. This mouth alcohol then fools the DUI breath testing device into believing you have consumed more alcohol than you actually have.

The manufacturers of breath testing machines are aware of this problem. To get around it, many have incorporated something known as "slope detection" into their machines.

4. How “Slope Detection” Tries to Get Around Mouth Alcohol

The concept behind slope detection is simple.  As air leaves your mouth, DUI breath machines measure the amount of alcohol over time. This is graphically represented as a curve.

A DUI breath sample that is free of mouth alcohol produces a generally smooth and consistent curve. But when mouth alcohol is present, there is a steep spike in the curve -- a “slope” -- at the onset of the sample.  This is followed by a similarly steep drop off as the sample levels out and the influence of mouth alcohol dissipates.

Breath machines are designed to abort a test as soon as a slope is detected. But it doesn't always work.

5. The Problem With Slope Detection

Slope detection is simply not sensitive enough to detect the presence of small amounts of mouth alcohol.  Such small amounts don't generate a spike large enough to stop the test – even though there is enough mouth alcohol to generate a falsely high BAC.

Our Los Angeles DUI lawyers understand that there are many possible defenses to DUI.  Some of these involve the use of lesser-known strategies.  Residual mouth alcohol is just one way we might challenge the prosecution's case.

We have successfully challenged chemical test results for clients throughout Los Angeles, Ventura, San Bernardino, Orange and Riverside counties. Contact us today for a free consultation to find out how we can help you.

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The police arrested you for DUI. You gave a blood or a breath sample. The DUI officer confiscated you drivers license. The police released you with a notice to appear in court in a few weeks. Where do you go from here? Our lawyers are here to guide you step by step.