Challenging Breathalyzer Results in California DUI Cases

Understanding Breathalyzer Tests and Their Flaws

Breathalyzer tests check for alcohol in your breath sample. When you blow into the device, it measures alcohol from your deep lung air. The machine then estimates your blood alcohol based on this breath sample.

Bay Area law enforcement agencies use breath testing devices that need to be properly calibrated and checked often. If a breathalyzer isn’t properly calibrated, it can’t give accurate results. Many police departments don’t follow proper protocol for keeping these tools in good shape.

Breath testing has big problems even when done right. These machines can have a margin of error of up to 15% in either direction. This error range could mean the difference between being under or over the legal limit in many DUI cases.

The Two Types of Breath Tests in a California DUI Investigation

There are actually two different breath tests that may be administered during a California DUI investigation, and understanding the difference between them matters for your defense.

The first is the preliminary alcohol screening (PAS) test, typically administered at the scene of the stop. Officers use this test to help determine whether there is sufficient probable cause to arrest the driver for DUI. The PAS test is generally considered less reliable and, for most adult drivers, is optional before arrest.

The second is the evidentiary breath test, typically given at the police station or jail after someone has been arrested on suspicion of DUI. This is the test whose results the prosecution primarily relies on in court. Both tests use machines designed to analyze the alcohol content in a breath sample and convert that into an estimated blood alcohol concentration — but as discussed below, that conversion process has significant scientific problems that a skilled defense attorney can exploit.

The Partition Ratio: Why Breath Tests Don’t Actually Measure Your Blood Alcohol

Breathalyzer devices do not directly measure the alcohol content in your blood. They analyze a sample of your breath and then perform calculations to convert the breath alcohol reading into an estimated blood alcohol concentration. This conversion is where a significant scientific problem arises.

California law makes it illegal to drive with a BAC of 0.08% or higher — not a breath alcohol concentration of 0.08%. This distinction matters because the conversion from breath to blood alcohol is based on an assumption that is not accurate for every individual.

Breath test devices are programmed to assume that for every one part of alcohol in a person’s breath, there are 2,100 parts of alcohol in that person’s blood. This 2,100:1 ratio is called the partition ratio — and it represents only a population average. In reality, individuals display a very wide range of partition ratios, from as low as 900:1 to as high as 3,400:1.

The machine does not calculate your actual partition ratio. It applies the 2,100:1 average to everyone. If your individual ratio is lower than 2,100:1, the device will artificially inflate your blood alcohol reading — potentially pushing a result that is actually under the legal limit to one that appears to be above it. This is a powerful argument for reasonable doubt in many DUI cases.

Henry’s Law: How Temperature Affects Breath Test Accuracy

Breathalyzer devices make a second critical assumption that can produce inaccurate results: they assume a standard body temperature for everyone tested. A scientific principle known as Henry’s Law states that in a closed system — like the human body — the amount of a volatile substance in a gas in contact with liquid is directly related to temperature. As temperature increases, the measured result also increases.

Breath test devices are programmed to use an average body temperature of 34 degrees Celsius. But body temperature varies from person to person and can be affected by illness, fever, or physical exertion. A study published in the Journal of Forensic Science found a breath alcohol increase of 8.6% for every one degree Celsius increase in core body temperature. This means a person with a slightly elevated body temperature could produce a falsely high reading even if their actual blood alcohol concentration was under the legal limit.

Temperature also affects calibration. Breath machines are calibrated using a simulator solution containing a specific percentage of alcohol. If the temperature of that calibration solution is not exactly correct during the calibration process, the machine will produce unreliable results — not just for one test, but for every test administered using that machine until it is properly recalibrated. Many machines do not record the individual’s breath temperature at the time of testing; they simply apply the standard average.

How to Challenge Breathalyzer Test Results in Court

The legal process for fighting a breath test starts with looking at your whole DUI investigation. We check everything from why the traffic stop happened to how the officer conducted field sobriety tests and breath testing.

Filing motions to suppress evidence is often our best defense. We argue that test results shouldn’t be used because:

• The traffic stop lacked reasonable suspicion
• The person performing the test didn’t follow the rules
• The breathalyzer device had faulty sensors

Cross-examination of police officers helps us find weaknesses in their cases. We ask whether they observed the required observation period, were properly trained to use the device, and if they followed all proper procedures. Even small mistakes by the police can help us win your case.

Expert witnesses strengthen our defense strategy. We bring in specialists who explain why breathalyzer readings can be wrong. They help judges and juries understand that your test results might not reflect your actual blood alcohol content at the time you were driving

Can Breathalyzer Test Results Be Inaccurate?

Yes, breathalyzer test results are often wrong. From weather to medical issues to poor care of the machines, these devices make mistakes. An experienced DUI attorney knows how to spot and use these problems when fighting for you.

When building your defense, we look for all the things that might have made your breath test inaccurate. Let’s look at the common problems that lead to false readings.

Factors That Can Skew Breathalyzer Results

Weather and temperature can throw off breath tests. Cold weather can make readings too high if the device isn’t set correctly. Radio equipment in police cars creates electromagnetic waves that can mess with the machine’s sensors.

Mouth Alcohol and the Observation Period Requirement

The term “mouth alcohol” refers to any alcohol remaining in someone’s mouth due to recent ingestion, alcohol trapped in dental cavities, or other sources. If this alcohol is blown into the breath test device, the machine includes it in its measurement — producing an artificially high BAC reading that does not reflect the alcohol actually in the person’s bloodstream.

Officers are required to take specific steps to account for this, but they do not always follow through:

Slope detector — Some breath test devices contain a “slope detector” designed to identify mouth alcohol by checking for a rapid fall-off in breath alcohol readings — a pattern associated with mouth alcohol rather than deep lung air. These slope detectors do not always work reliably and may not have been functioning properly at the time your test was administered. Defense counsel can present evidence of the slope detector’s unreliability to cast doubt on the prosecution’s test results..

Observation period — Officers must observe the subject for at least 15 minutes before administering the test to ensure the subject has not ingested alcohol, and has not belched, vomited, or regurgitated any alcohol from the stomach into the mouth. A skilled defense attorney can attack whether the officer actually observed this period or simply wrote down a time without genuinely watching.

Agreement of tests — Officers must collect at least two separate breath samples, administered a few minutes apart. If the two results disagree by more than 0.02%, the tests must be run again, because disagreement at that level suggests the presence of mouth alcohol. Officers do not always follow this procedure, and the defense can use any deviation to challenge the reliability of the results.

Many things can trigger false positives on breath tests. Mouthwash, certain foods, and some medicines can all make it seem like you drank alcohol. Even some health issues can cause breath tests to give wrong results when you haven’t had any alcoholic beverages.

Medical Conditions Like Acid Reflux and Their Impact

Gastroesophageal reflux disease (GERD) can greatly affect breathalyzer readings. This condition pushes stomach contents into your throat and mouth. Any alcohol in your stomach can create “mouth alcohol” that makes test results much higher than your actual blood alcohol level.

Diabetes and similar health issues can cause false readings. When your body makes ketones, some breathalyzers mistake them for alcohol. Someone with diabetes might fail a breath test without having a single drink.

Other health problems that can affect test results include liver disease, dental work, and breathing issues like asthma. People who have trouble breathing might not give a good breath sample, leading to wrong readings. We check your medical history to find any conditions that might have affected your test.