Many people worry that drinking coffee before driving might cause a false positive on a breathalyzer test. However, the science of breath testing shows that standard brewed coffee is not a source of error. Understanding how breathalyzers work and what substances truly affect them provides a clear answer. The primary concern is not the coffee itself, but the temporary presence of alcohol vapor in the mouth, known as “mouth alcohol.”
How Breathalyzers Measure Alcohol
Breathalyzers measure Breath Alcohol Concentration (BrAC) to estimate the Blood Alcohol Concentration (BAC) in the bloodstream. This relies on the principle that alcohol in the blood evaporates into the deep lung tissue, called alveolar air. The device analyzes this air, assuming a consistent ratio between breath and blood alcohol concentration, typically 2100:1.
Modern breathalyzers are calibrated to detect ethanol, the alcohol found in alcoholic beverages. The technology differentiates between a deep-lung sample and one contaminated by residual mouth alcohol. Mouth alcohol is highly concentrated and evaporates quickly, creating a sharp, high peak in the breath sample.
To ensure the reading reflects deep-lung alcohol, law enforcement follows a specific protocol. This requires continuously observing the person for at least 15 minutes before administering the test. During this time, the person is prohibited from eating, drinking, smoking, or regurgitating, allowing any residual mouth alcohol to dissipate completely.
Analyzing Coffee’s Alcohol Content
Standard brewed coffee contains no ethyl alcohol (ethanol) and is a non-alcoholic beverage. Since the breathalyzer is specifically looking for ethanol vapor, a regular cup of coffee cannot register a positive reading. The brewing process itself does not create alcohol, eliminating it as a direct source of breathalyzer error.
Some specialty coffees may introduce trace amounts of alcohol through common additions like vanilla extract or certain flavor syrups. Vanilla extract is often made with ethanol as a carrier, sometimes having an alcohol by volume (ABV) content similar to liquor. However, the small volume consumed and the high volatility of ethanol ensure it would quickly evaporate from the mouth.
Unless the coffee is spiked with liquor, such as in an Irish coffee, or is a specialized alcoholic product, it poses no risk of registering a BAC.
Identifying Sources of False Readings
Mouth alcohol contamination is a valid concern that can lead to false readings when alcohol vapor from the mouth, rather than the lungs, is measured. The most common culprits for this temporary elevation are products with high concentrations of alcohol used daily.
Mouthwash is a primary source of false positives, as some popular brands contain alcohol concentrations as high as 20% to over 40%. Many over-the-counter liquid medications, such as cough syrups and cold remedies, also use ethanol as a solvent or carrier and can temporarily spike a reading. Even using hand sanitizer near the mouth can introduce enough alcohol vapor to register on the sensitive device.
Physiological Causes
Certain physiological conditions can also cause a false positive by mimicking alcohol on the breath. Gastroesophageal Reflux Disease (GERD), or acid reflux, can bring stomach contents back into the mouth, causing an elevated reading. For individuals with uncontrolled diabetes, the body may produce high levels of acetone during ketoacidosis, which some breathalyzers can chemically mistake for ethanol.