Several factors can cause a breathalyzer to produce a reading higher than your actual blood alcohol level, or even register alcohol when you haven’t had a drink. These range from everyday products like mouthwash and asthma inhalers to medical conditions, workplace chemical exposure, and even a mild fever. Understanding these causes matters whether you’re facing a questionable result or simply want to know how reliable these devices really are.
How Breathalyzers Work (and Where They Go Wrong)
There are two main types of breathalyzer sensors, and the type matters. Fuel cell breathalyzers use a platinum electrode that reacts specifically with alcohol to generate an electric current. The more alcohol in your breath, the stronger the current. These are the devices law enforcement typically uses for evidentiary testing, and they’re more resistant to interference from non-alcohol substances.
Semiconductor breathalyzers, commonly found in cheaper personal devices, use a metal oxide sensor that measures changes in electrical resistance when it detects alcohol. These sensors are significantly more prone to false positives because they can’t distinguish ethanol from other chemical compounds as effectively. They’re also more sensitive to temperature and humidity. If you’ve ever blown into a keychain breathalyzer and gotten a suspicious reading, the sensor type is likely part of the problem.
Mouthwash, Breath Spray, and Mouth Alcohol
Many mouthwash brands contain more than 20% alcohol, which is higher than most wines. If you use mouthwash and take a breath test within two minutes, the residual alcohol in your mouth can push the reading well past the legal limit, even if you haven’t had a single drink. After that two-minute window, breath alcohol levels from mouthwash decay rapidly. A reading taken six to eight minutes after rinsing may still detect some alcohol but typically won’t push you over the legal limit. By about 10 to 15 minutes, the effect largely disappears.
This is the same reason breath sprays, cough syrups, and other alcohol-containing oral products can temporarily spike a reading. The alcohol isn’t in your bloodstream or lungs. It’s sitting in your mouth, and the device can’t tell the difference.
Asthma Inhalers
This one surprises most people. A study from the University of Cádiz tested multiple common asthma inhalers on 60 volunteers and found that every single inhaler produced a false positive reading in the minutes after use, including inhalers that don’t contain ethanol as an ingredient. One minute after two puffs of a salbutamol inhaler (the active ingredient in many rescue inhalers), the average reading was 0.45 mg of alcohol per liter of breath. That’s a meaningful number on a breath test.
The readings dropped rapidly and linearly, reaching zero by about 10 minutes. Researchers believe the propellant gases in the aerosol, not the medication itself, are responsible for the interference. London’s Metropolitan Police have a standing policy of waiting at least 20 minutes before administering a breath test if someone has recently used an inhaler.
Ketosis and Diabetes
When your body burns fat instead of carbohydrates for fuel, it produces acetone as a byproduct. Acetone shows up on your breath, and some breathalyzers, particularly semiconductor models, can mistake it for ethanol.
The amount of breath acetone varies enormously depending on the situation. A healthy person eating normally produces about 1 part per million (ppm). Someone on a low-carb or ketogenic diet can produce up to 40 ppm. Extended fasting pushes levels to around 170 ppm. And people with poorly controlled diabetes experiencing ketoacidosis can reach 1,250 ppm. The higher the acetone level, the greater the chance it interferes with a breath test. This is why people with uncontrolled diabetes or those deep into a ketogenic diet are more vulnerable to false readings.
Workplace Chemical Exposure
If you work around volatile organic compounds, you may carry enough residual fumes in your lungs and airways to trigger a false positive. Breathalyzers, especially semiconductor models, cannot reliably distinguish ethanol from hundreds of other chemical compounds.
Specific substances documented to cause elevated readings include:
- Toluene: found in glues, varnishes, lacquers, and paints
- Methyl ethyl ketone: used in cement, adhesives, cleaning fluids, and paint removers
- Isopropanol (rubbing alcohol): fumes can be inhaled or absorbed through the skin
- Diethyl ether: found in some plastics and automotive products
Workers regularly exposed to these compounds were found to have higher BAC readings than unexposed people who had consumed the same amount of alcohol. The chemicals effectively stack on top of any actual alcohol present, inflating the result. People who work with gasoline, oil-based paints, lacquer, or industrial cleaning fluids are at the highest risk.
Body Temperature
Breathalyzers are calibrated assuming a normal body temperature of about 37°C (98.6°F). When your core temperature is higher than normal, the alcohol in your blood evaporates more readily into your lungs, producing a more concentrated breath sample. For every degree Celsius (1.8°F) above normal, your reading can be inflated by up to 7%. So a fever of 38.5°C could push a true BAC of 0.07% into the range of 0.08%, tipping a borderline result over the legal limit. Exercise, sitting in a hot car, or illness can all raise your body temperature enough to matter.
Acid Reflux (GERD)
Gastroesophageal reflux disease brings stomach contents, including any alcohol you’ve consumed, back up into your esophagus and mouth. In theory, this could introduce alcohol vapor from your stomach directly into a breath sample, bypassing the normal lung-based measurement the device is designed to capture. Defense attorneys frequently raise GERD as a factor in DUI cases.
However, the science is less supportive than you might expect. A study published in the Journal of Forensic Sciences concluded that the risk of stomach alcohol erupting into the mouth during a reflux episode and meaningfully increasing an evidentiary breath test result is “highly improbable.” That said, the standard 15-minute observation period before testing exists partly to catch exactly this kind of mouth alcohol contamination.
Auto-Brewery Syndrome
This is rare but real. Auto-brewery syndrome is a condition where fungi in your gut ferment carbohydrates into ethanol inside your body. People with this condition can become measurably intoxicated, and produce genuine positive breath tests, after eating foods like white bread, pasta, potatoes, fruit juice, or anything high in simple carbohydrates. The alcohol is real, but the person hasn’t consumed any alcoholic beverages. It’s most commonly associated with an overgrowth of yeast in the digestive tract, sometimes following antibiotic use.
The 15-Minute Observation Period
Most of the mouth-related causes of false readings (mouthwash, inhalers, belching, recent eating) share a common timeline: they fade within about 10 to 15 minutes. That’s why U.S. Department of Transportation regulations require a minimum 15-minute waiting period between a screening test and a confirmation test. During this window, the person being tested must not eat, drink, put anything in their mouth, or belch, and must be directly observed the entire time.
The purpose is straightforward: to let any mouth alcohol dissipate so the confirmation reading reflects actual lung air, not residual contamination. If this observation period is skipped, shortened, or improperly conducted, any of the factors above become much more likely to produce a false or inflated result.
Calibration and Device Maintenance
Like any measurement instrument, breathalyzers drift over time and need regular calibration to stay accurate. The National Highway Traffic Safety Administration requires that all breath alcohol devices operate under quality assurance plans specifying inspection, maintenance, and recalibration intervals. Calibration must be performed by the manufacturer, a certified maintenance representative, or an authorized state agency. A device that’s overdue for calibration, or one with a documented history of maintenance issues, may produce readings that don’t hold up to scrutiny. Calibration records are often a key piece of evidence in contested breath test cases.