A breathalyzer can read high for reasons that have nothing to do with how much you drank. The most common causes of false positive breathalyzer readings are alcohol lingering in the mouth from mouthwash or dental products, medical conditions like uncontrolled diabetes, certain medications including asthma inhalers, exposure to industrial chemicals at work, and devices that have not been calibrated on schedule. Each of these can push a reading above the 0.08 legal limit while your actual blood alcohol level sits well below it.
The Built-In Assumption That Can Work Against You
Before looking at outside interferents, it helps to understand that every breath test starts with a fixed assumption about your body. Breath testing devices measure alcohol in air exhaled from deep in your lungs and convert it to a blood alcohol number using a ratio set by law at 2,100:1. The device assumes 2,100 milliliters of your breath contains the same amount of alcohol as 1 milliliter of your blood.
That ratio is not the same for everyone. Research puts the actual average closer to 2,380:1, with individual values ranging from roughly 2,125:1 to 2,765:1 depending on body temperature, breathing pattern, and physiology.1National Library of Medicine. Reflections on Variability in the Blood-Breath Ratio of Ethanol and Its Implications For most people, the 2,100:1 figure underestimates their true blood alcohol. For someone at the low end of the range, the device overestimates it. The instrument has no way to know which end of the range you sit on.
Two sensor technologies are in use. Fuel cell (electrochemical) devices generate a small electrical current when alcohol is oxidized on a platinum electrode. Infrared devices measure how a breath sample absorbs light at wavelengths associated with alcohol. Each has different weaknesses, and which one produced your reading matters when you challenge it.
Residual Alcohol in the Mouth
Mouth alcohol is the most common and best-documented cause of inflated readings. When alcohol is sitting in your mouth rather than coming up from your lungs, the device captures those concentrated vapors directly and reports them as blood alcohol. The number can be dramatically higher than your actual BAC.
Mouthwash is a frequent source. Listerine Antiseptic contains 26.9% alcohol, higher than wine.2Regulations.gov. Warner-Lambert Company Presentation to FDA Regarding Alcohol Content of Listerine A study measuring breath alcohol after rinsing with Listerine (26.9% alcohol), Scope (18.9%), and Lavoris (6.0%) found detectable alcohol on the breath for minutes afterward.3PubMed. Breath Alcohol Values Following Mouthwash Use Oral pain treatments containing benzocaine, such as Anbesol, and alcohol-based cough syrups create the same problem.
Burping or vomiting shortly before a test pushes raw alcohol vapors from the stomach into the mouth. To catch this, testing protocols in most jurisdictions require a continuous observation period, typically 15 to 20 minutes, during which the subject cannot eat, drink, smoke, vomit, or belch before providing a sample. If the officer skips or shortens that waiting period, or if a reflux episode goes unnoticed, the reading may reflect mouth contamination rather than blood alcohol. Failure to complete the observation period is one of the first things defense attorneys look for.
Medical Conditions
Some health conditions put chemicals into your breath that a device can misread as ethanol, though the science is more nuanced than the popular version suggests.
Diabetes and Ketosis
People with uncontrolled diabetes, and anyone on a strict low-carbohydrate diet, can produce elevated acetone and trace isopropyl alcohol when the body burns fat instead of glucose. Whether that matters depends entirely on the device.
Fuel cell breathalyzers oxidize alcohol at a specific voltage that effectively excludes acetone. A government-funded study confirmed that fuel cell devices are, in practice, specific to ethanol because acetone does not oxidize at the voltage the electrode applies. Infrared breathalyzers measure light absorption in a range where acetone overlaps with ethanol. Most modern infrared devices now include acetone-detection circuitry that flags or rejects contaminated samples, but older models, roughly 1,000 of which were still in police use when the study was conducted, lack this feature.4Bureau of Transportation Statistics. The Likelihood of Acetone Interference in Breath Alcohol Measurement If you have diabetes or follow a ketogenic diet and think your result is wrong, find out which device was used.
Gastroesophageal Reflux Disease
GERD is one of the most commonly raised medical defenses in DUI cases. The theory is that stomach acids and undigested alcohol vapors travel back up the esophagus and contaminate the sample. It is plausible, and the research is less supportive than many people assume.
A controlled study of subjects with confirmed gastric reflux found that reflux episodes during testing did not produce widely deviant breath alcohol readings compared to actual blood alcohol concentrations when samples were taken at five-minute intervals. The researchers concluded that the risk of stomach alcohol erupting into the mouth and falsely increasing an evidential breath test result is “highly improbable.”5PubMed. Reliability of Breath-Alcohol Analysis in Individuals With Gastroesophageal Reflux Disease GERD alone, without corroborating evidence like a blood test showing a lower BAC, is a difficult defense to win on.
Medications
Several common medications can produce false positives, and this is the category drivers tend not to think about until they are already facing charges.
Asthma inhalers are a documented source. A study testing salbutamol (albuterol), salmeterol, and budesonide found that using these inhalers produced false positive results on breath alcohol tests.6National Library of Medicine. Using Asthma Inhalers Can Give False Positive Results in Breath Alcohol Tests Because inhalers deliver medication straight into the airways, the contamination enters the breath sample path. If you use an inhaler and are asked to take a breath test, tell the officer and note the time of your last dose.
Over-the-counter cough syrups and cold medications can contain 10% or more alcohol by volume. Oral pain relievers and topical gum treatments like Anbesol leave alcohol residue in the mouth that registers on the device for minutes after use. These effects typically fade within the observation window, which is why the waiting period exists. When it is cut short, the contamination goes undetected.
Foods and Non-Alcoholic Beverages
Certain foods and drinks put trace ethanol into your breath without causing any impairment. Sourdough bread and overripe fruit undergo natural fermentation that produces small amounts of alcohol. The amounts are tiny and will not push a healthy adult above 0.08, but they can produce low-level readings that matter in specific situations.
Non-alcoholic beers are legally permitted to contain up to 0.5% alcohol by volume under federal labeling regulations.7eCFR. 27 CFR 7.65 – Alcohol Content Drinking several in a short period can leave enough residual alcohol to register on a sensitive device, especially if tested soon after. For drivers under 21, this is more than a curiosity. Every state sets a zero-tolerance threshold for underage drivers, often 0.02% or lower, and a trace reading that would be meaningless for an adult can bring fines, license suspension, and mandatory alcohol education.
Workplace Chemical Exposure
People who work around volatile organic compounds face a risk most drivers never think about. Painters, auto body workers, floor refinishers, and anyone routinely exposed to lacquer, toluene, or paint thinner can absorb these chemicals through the skin or lungs. The compounds enter the bloodstream and are eventually exhaled, and breathalyzer sensors, especially infrared ones, sometimes lack the specificity to tell them apart from ethanol.
Gasoline vapors, cleaning solvents, and adhesive fumes work the same way. A mechanic finishing a shift in a poorly ventilated shop may still carry enough residual compounds in their breath to register. The observation period does not help here, because these chemicals are not in the mouth. They are circulating in the blood and coming out through the lungs exactly the way alcohol would. For workers in these fields, the strongest protection against a wrongful charge is requesting an independent blood test, which measures only ethanol and is not thrown off by industrial chemicals.
Device Calibration and Technology Problems
Even when the person being tested has no interferents in their system, the device itself can produce bad numbers.
Calibration Drift
Breathalyzers need regular calibration to stay accurate. With time and repeated use, the sensors drift from their reference point and start reporting values that are too high or too low. States set their own schedules for how often devices must be recalibrated, some by number of uses and some by fixed time intervals. Once a device is used past its required calibration date, every reading it produces is suspect. Missing or incomplete calibration records are one of the most effective grounds for challenging a breath test in court.
Older Device Standards
NHTSA maintains a Conforming Products List of breath testing devices that meet its model specifications, including testing for acetone interference and the ability to detect other low-molecular-weight alcohols such as methanol and isopropyl alcohol.8Federal Register. Highway Safety Programs – Conforming Products List of Evidential Breath Alcohol Measurement Devices Devices that conform only to the older 1984 specifications may lack acetone-detection capability. If your case involves an older device, checking which specification it meets is worth doing.
Radio Frequency Interference
Electronic components inside breath testing devices can be disrupted by external radio frequency signals from police radios, cell phones, or nearby transmitters. Modern evidentiary devices use metal shielding and circuitry that terminates a test if interference above a certain threshold is detected. Older or poorly maintained devices may not have adequate shielding. Proving RFI caused a specific false reading is difficult, but it remains a recognized vulnerability and a viable line of cross-examination.
What To Do If You Think Your Reading Was Wrong
If you believe a breathalyzer produced a false positive, several avenues exist to challenge the evidence, and all of them work better with documentation gathered early.
The single most valuable step is requesting an independent blood test as soon as possible after the breath test. Blood tests measure actual ethanol and are not affected by mouth alcohol, acetone, or industrial chemicals. Many states give you the right to request an independent test at your own expense, and some require the arresting officer to inform you of that right. The wider the gap between the breath result and the blood result, the stronger the argument that the breath test was unreliable.
Defense attorneys challenging a breath test typically pursue several lines through pretrial discovery:
- Calibration records showing when the device was last calibrated, by whom, and whether it was done on schedule.
- Maintenance and repair history, particularly any error codes or repairs in the months before your test.
- Documentation that the officer who ran the test was properly trained and currently certified on that specific model.
- Proof that the officer observed you continuously for the full required waiting period. Any gap opens the door to a mouth-alcohol argument.
A motion to suppress can be filed if the defense can show the state failed to follow its own testing procedures. Once the defense raises these issues, the burden shifts to the prosecution to show the protocol was followed. If the state cannot produce complete records or an adequately trained operator, the breath test results may be excluded entirely. Cross-examination of the device operator at a suppression hearing is often where these cases turn, because an operator who cannot explain calibration gaps or procedural shortcuts can be more persuasive than any expert witness.