A breathalyzer is a handheld or benchtop device that estimates your blood alcohol concentration (BAC) by measuring the alcohol in a sample of your exhaled breath. Police use it during traffic stops and at the station, employers use it for workplace testing, and courts rely on its readings as the central evidence in most impaired-driving cases. For standard drivers, the legal threshold in nearly every state is a BAC of 0.08%, and a breath reading at or above that number is enough, on its own, to prove impairment under the law.
How a Breathalyzer Measures Alcohol
Alcohol you drink absorbs through your stomach and small intestine into the bloodstream. When blood passes through the lungs, some of the dissolved alcohol evaporates into the air inside the small sacs where gases exchange, and you breathe it out. The concentration of alcohol in that exhaled air tracks the concentration in your blood at a reasonably stable ratio: on average, 2,100 milliliters of deep lung air contain the same amount of ethanol as 1 milliliter of blood.1National Center for Biotechnology Information. Deconvolving an Estimate of Breath Measured Blood Alcohol Concentration
The device measures the alcohol in your breath sample and applies that 2,100-to-1 conversion to report a BAC. The problem is that 2,100-to-1 is only an average. Real partition ratios range from about 1,700:1 to 2,400:1, depending on body temperature, breathing patterns, and other biological variables. A person whose ratio runs lower than the assumed average can get a reading that overstates their actual blood alcohol level. That built-in variability is one of the standard angles defense attorneys use to challenge breath results.
Different machines use different sensors to detect the alcohol. Police handhelds generally use a fuel cell sensor that reacts specifically to ethanol. Larger evidential machines at the station usually use infrared spectroscopy, which measures how much infrared light the breath sample absorbs at wavelengths specific to ethanol.2ScienceDirect. Breath Alcohol Analyzer Consumer-grade personal breathalyzers often use cheaper semiconductor sensors that can react to acetone, methane, and other compounds, so their readings should be treated as rough estimates.
The Roadside Test and the Station Test Are Not the Same
If you get pulled over on suspicion of drunk driving, you may be asked to blow into a breathalyzer twice during the encounter, and the two tests have very different legal weight.
The first is a preliminary breath test, or PBT. It’s the small handheld device the officer carries in the cruiser. Its purpose is to help the officer decide whether there’s probable cause to arrest you. In most states, PBT results are not admissible as evidence of your BAC at trial. They are an investigative tool, not courtroom proof.
The second is an evidentiary breath test, almost always administered after arrest on a larger machine at the station. These devices must appear on a federal conforming products list maintained by NHTSA and meet strict performance standards, including the ability to distinguish alcohol from acetone at concentrations as low as 0.02%.3National Highway Traffic Safety Administration. Alcohol Measurement Devices They must print results, assign a unique test number, and pass external calibration checks on a set schedule. A machine that fails a calibration check has to be pulled from service until it is repaired.4eCFR. 49 CFR Part 40 – Procedures for Transportation Workplace Drug and Alcohol Testing This is the reading prosecutors will put in front of a jury.
Practically speaking: a roadside PBT of 0.09% probably won’t be shown to a jury in most jurisdictions, but it will almost certainly lead to an arrest and the more rigorous test at the station.
The BAC Numbers That Matter
BAC is expressed as grams of alcohol per 100 milliliters of blood. A BAC of 0.08% means 0.08 grams of alcohol in every 100 milliliters of blood. On a breath test, the equivalent unit is grams per 210 liters of breath, reflecting that partition ratio.1National Center for Biotechnology Information. Deconvolving an Estimate of Breath Measured Blood Alcohol Concentration The legal threshold depends on who’s driving.
- Standard drivers: every state sets a per se limit of 0.08%, with one state using the lower threshold of 0.05%. At or above that number, you are legally impaired no matter how you feel or perform.
- Commercial drivers: FMCSA sets the limit at 0.04% for anyone operating a commercial motor vehicle, and prohibits operating one within four hours of consuming alcohol.5eCFR. 49 CFR 392.5 – Alcohol Prohibition
- Drivers under 21: zero-tolerance laws in every state set the limit below 0.02%, with license suspension or revocation on violation.6National Highway Traffic Safety Administration. Zero-Tolerance Law Enforcement
Cross any of these thresholds and the consequences stack quickly: license suspension, mandatory fines, possible jail time, and a record that follows you into insurance quotes, background checks, and professional licensing.
Can You Refuse a Breathalyzer?
Physically, yes. No one will force a tube into your mouth. Legally, refusal carries its own penalties, because every state has an implied consent law. By accepting a driver’s license and driving on public roads, you have already agreed to submit to a chemical test if an officer has reasonable grounds to suspect impairment.7National Highway Traffic Safety Administration. BAC Test Refusal Penalties
Refusal triggers automatic administrative penalties in nearly every state. A first refusal typically brings a license suspension that can last a year or longer, with longer suspensions for repeat refusals. In many states the refusal itself is admissible at trial, which lets a prosecutor argue you refused because you knew you’d fail. Some states treat a second or subsequent refusal as its own criminal offense with fines and potential jail time.
There is no obviously right answer at the roadside. Refusing may deprive prosecutors of a BAC number, but it substitutes a separate penalty structure and an inference of guilt. For most drivers, the consequences of refusal are roughly comparable to the consequences of a failed test.
Why a Reading Might Be Wrong
Breath testing is reasonably reliable, but several things can push a reading higher than your actual blood alcohol level. These are the usual pressure points when a lawyer challenges a result.
Medical Conditions
Uncontrolled diabetes can produce ketoacidosis, raising acetone levels in the breath; semiconductor sensors in particular can mistake acetone for ethanol. GERD can push stomach contents, including residual alcohol, back into the esophagus and mouth, so the device picks up concentrated mouth alcohol rather than deep lung air. Some asthma inhalers contain trace alcohol as a propellant and can temporarily inflate a reading if used shortly before the test.
The Observation Period
Testing protocols generally require the officer to watch you continuously for at least 15 minutes before taking the sample. During that window you can’t eat, drink, vomit, or belch, because any of those can introduce mouth alcohol that doesn’t reflect what’s in your blood. A skipped or sloppy observation period is a legitimate reason to question the result.
Rising BAC
Alcohol doesn’t peak the moment you swallow it. BAC can keep climbing for roughly 30 to 45 minutes after your last drink as absorption continues. If you were tested during that window, your BAC at the station may have been higher than your BAC while you were behind the wheel. That gap is the basis of the “rising BAC” defense.
Calibration and Maintenance
Every instrument drifts. Evidential devices require periodic external calibration checks, and a device that fails a check has to be taken out of service until it passes again.4eCFR. 49 CFR Part 40 – Procedures for Transportation Workplace Drug and Alcohol Testing Defense attorneys routinely subpoena calibration and maintenance logs. Overdue calibration or a history of failed checks gives a lawyer something concrete to argue at trial.
Ignition Interlock Devices
After a DUI conviction, many states require a breath alcohol ignition interlock device (BAIID) in your vehicle. It’s essentially a small breathalyzer wired to the ignition. You blow into the mouthpiece before the car will start, and if your breath alcohol meets or exceeds the set point, the engine won’t turn over. Federal model specifications set the threshold at 0.02 grams per deciliter, well below the driving limit, to leave a safety margin.8Federal Register. Model Specifications for Breath Alcohol Ignition Interlock Devices
Interlocks also require random “rolling retests” while you drive. The device prompts you for another sample at unpredictable intervals; it won’t shut the engine off mid-drive, but it will log a failure or skipped test and may trigger your horn or lights. Those logs go to the court or supervising agency and can extend the interlock period or add penalties.
Installation, monthly monitoring, and calibration fees for interlocks typically run between $70 and $150 per month, plus an upfront installation charge. The full cost over a one- or two-year interlock period adds up, and it falls entirely on the driver. Courts rarely waive the requirement because of cost.