A breathalyzer test measures the concentration of alcohol in your breath to estimate how much alcohol is in your blood. It works by analyzing a sample of air you exhale into a handheld or desktop device, then converting that measurement into a blood alcohol concentration (BAC) using a fixed mathematical ratio. Law enforcement uses breathalyzers during traffic stops, and employers use them for workplace safety screening.
How Alcohol Gets Into Your Breath
When you drink, alcohol absorbs into your bloodstream through your stomach and small intestine. As that blood circulates through your lungs, alcohol vapor transfers into the air inside your airways. The traditional explanation is that this exchange happens deep in the tiny air sacs where oxygen enters your blood. But research published in the Journal of Applied Physiology found that exhaled alcohol originates almost completely from a different source: the blood vessels embedded in the walls of your airways, not the air sacs themselves.
Here’s what happens in practice. As you inhale, the moist lining of your airways absorbs alcohol from the blood flowing through the airway walls. When you exhale, that alcohol releases back into the outgoing air. By the time the breath leaves your mouth, it carries a measurable concentration of alcohol vapor that reflects, with some variability, how much alcohol is in your blood.
The 2100:1 Conversion Ratio
A breathalyzer doesn’t directly measure your blood. It measures alcohol in your breath and then multiplies that number using a standard conversion factor. The standard ratio is 2100:1, meaning that on average, 1 milliliter of blood contains 2,100 times more alcohol than 1 milliliter of exhaled air. The device uses this ratio to calculate a BAC number, which is what appears on the screen or printout.
The catch is that this ratio varies from person to person. According to Duke University’s Alcohol Pharmacology Education Partnership, the actual blood-to-breath ratio can range anywhere from 1,500:1 to 3,000:1 depending on individual physiology. Someone whose natural ratio is lower than 2,100:1 could get a reading that overstates their true blood alcohol level. Someone with a higher ratio could get a reading that understates it. The 2,100:1 figure is a population average baked into the device, not a personalized measurement.
What Happens During the Test
You blow steadily into a mouthpiece attached to the device. The machine needs a deep breath sample, not just a quick puff, because the air from deeper in your lungs more accurately reflects your blood alcohol level. Requirements vary by jurisdiction, but Washington state’s standards are typical: you need to blow for at least 5 seconds, exhale a minimum of 1.5 liters of air, and maintain a steady airflow throughout. The device also checks that the alcohol concentration in your breath has leveled off toward the end of the exhale, confirming you’ve provided a deep enough sample.
Most law enforcement protocols require two separate breath samples taken a few minutes apart. If the two readings don’t agree within a set tolerance, the results may be invalid and the test is repeated.
Two Main Sensor Technologies
Breathalyzers fall into two broad categories based on how they detect alcohol.
- Fuel cell sensors use a chemical reaction to detect alcohol. Your breath passes over a platinum electrode, and the alcohol reacts to produce an electrical current. The stronger the current, the higher the alcohol concentration. Fuel cell sensors are specific to alcohol, which makes them less prone to interference from other chemicals. Most handheld devices used by police on the roadside rely on fuel cell technology.
- Infrared sensors shine a beam of infrared light through a chamber filled with your breath sample. Alcohol molecules absorb infrared light at specific wavelengths, and the device measures how much light is absorbed to calculate the concentration. These are typically larger desktop units found at police stations and are used for evidentiary testing, meaning the results carry more weight in court.
Factors That Skew Results
Breath temperature is one of the biggest variables. Breathalyzer machines are calibrated in the lab against a standard breath temperature of 34°C (about 93°F), but actual breath temperatures range from 32°C to 36°C across individuals. For every degree Celsius your breath runs above the calibration standard, your reading jumps by roughly 8.6%. One study found that when subjects sat in a hot tub and raised their core body temperature, their breath alcohol readings climbed to 23% higher than their actual blood alcohol level. Exercise, fever, or even sitting in a hot car could produce a similar effect.
Mouth alcohol is another common source of error. Normally, a breathalyzer is measuring alcohol vapor that came from your bloodstream through your airways. But if there’s residual alcohol in your mouth or throat from a recent drink, a burp, or vomiting, the device picks up that concentrated alcohol on top of the lung air and produces an inflated reading. This is why officers are generally required to observe you for 15 minutes before administering a test, making sure you don’t eat, drink, burp, or vomit during that window.
Medical conditions can undermine that observation period. Gastroesophageal reflux disease (GERD), acid reflux, and hiatal hernias all cause stomach contents to push back up into the esophagus. If there’s undigested alcohol in your stomach, these conditions can silently introduce mouth alcohol without any visible burp or belch. An officer watching you for 15 minutes may not notice anything, but the device reads the extra alcohol.
Certain metabolic states also cause problems. People on very low-carb or ketogenic diets produce acetone as a byproduct of burning fat for fuel. Some breathalyzer sensors, particularly older or less specific ones, can mistake acetone for alcohol and register a false positive. At least one case involved a flight attendant who reported that his keto diet triggered a positive breathalyzer result that cost him his job.
Legal BAC Limits
In every U.S. state, the legal BAC limit for drivers 21 and older is 0.08%. Commercial drivers holding a CDL are held to a stricter standard of 0.04%. Drivers under 21 face zero-tolerance laws in most states, where any detectable alcohol can result in penalties. These thresholds apply regardless of whether alcohol is measured by breath, blood, or urine.
A roadside breath test (sometimes called a preliminary breath test) is often used to establish probable cause for an arrest, but it may not be admissible as evidence in court depending on your state. The evidentiary breath test, typically administered at a police station on a more sophisticated machine, is the one that produces legally binding results.
Calibration and Maintenance
For a breathalyzer’s results to hold up in legal proceedings, the device must be properly maintained. The U.S. Department of Transportation requires that every approved evidential breath testing device come with a manufacturer’s quality assurance plan specifying how often calibration checks must be performed, what tolerances are acceptable, and who is qualified to do the work. Environmental factors like temperature, humidity, and altitude all influence how often recalibration is needed.
If a device fails a calibration check, it must be taken out of service immediately and cannot be used for testing until it’s repaired and passes a new check. Only certified maintenance representatives, whether from the manufacturer or a state health agency, can perform the calibration. Defense attorneys frequently challenge breathalyzer results by requesting calibration and maintenance records for the specific device used, since a machine that was overdue for service or out of tolerance could have produced unreliable numbers.