Breath alcohol concentration, or BrAC, is the amount of alcohol in the air you exhale, expressed as grams of alcohol per 210 liters of breath. It’s the number a breathalyzer displays. In 49 states, driving with a BrAC of 0.08 or higher is per se evidence of impairment; Utah uses 0.05. Commercial drivers face a 0.04 limit, drivers under 21 face zero-tolerance limits of 0.02 or lower, and federally regulated workplaces start taking action at 0.02.
The Legal BrAC Limits
For a standard driver’s license, the threshold is 0.08 in every state except Utah, which lowered its limit to 0.05 in 2019 and saw a measurable drop in fatal crashes afterward.1National Highway Traffic Safety Administration. Utah’s .05% Law Shows Promise to Save Lives, Improve Safety No other state has followed, though the Utah change fueled an ongoing debate about whether 0.05 should become the national baseline.
Commercial driver’s license holders operate under a stricter federal rule. A BrAC of 0.04 or higher is enough to establish impairment for anyone behind the wheel of a commercial motor vehicle, and that limit applies in every state.2eCFR. 49 CFR 383.5 – Definitions
Every state also enforces a zero-tolerance rule for drivers under 21, setting the ceiling at 0.02 or lower.3National Highway Traffic Safety Administration. BAC Test Refusal Penalties The small allowance above absolute zero exists because mouthwash, medications, or trace fermentation can register on a device even in someone who hasn’t been drinking.
Enhanced Penalty Thresholds
Most states layer heavier penalties on top of the standard DUI when a driver’s BrAC reaches a specified high level. The most common trigger is 0.15, used in roughly two dozen states including Colorado, Florida, Georgia, Texas, and Virginia. Other states set the bar at 0.16, 0.17, or 0.20. Crossing those lines typically means mandatory ignition interlock installation, longer license suspensions, higher fines, and required alcohol treatment.
How a Breathalyzer Produces the Number
Breath-testing devices detect ethanol in a fixed volume of exhaled air, but they use different sensors to do it. Fuel cell sensors run the alcohol through a chemical reaction that produces an electrical current, with a stronger current meaning more alcohol. These are common in both handheld roadside units and larger station-house machines. Infrared spectrometry passes infrared light through the breath sample and measures which wavelengths get absorbed; because ethanol absorbs at specific wavelengths, infrared devices can tell ethanol apart from other substances in the sample. Semiconductor oxide sensors, which detect electrical resistance changes when alcohol contacts a heated surface, show up in some lower-cost consumer devices but are less common in enforcement equipment.
For any of these to produce a legally defensible result, the device has to be maintained and calibrated on schedule. Federal DOT rules require each evidentiary breath-testing device to follow a manufacturer-submitted quality assurance plan setting the calibration intervals, tolerances, and maintenance procedures. A device that fails a calibration check has to be pulled from service until it passes again.
BrAC Is Not the Same as BAC
BrAC and blood alcohol concentration measure the same thing from different places. BAC comes from a blood draw; BrAC comes from exhaled lung air. The two are linked by a conversion factor called the partition ratio: on average, 2,100 milliliters of lung air contains the same amount of alcohol as one milliliter of blood. That 2,100-to-1 ratio is built into every breathalyzer’s math, so when a device shows “0.08” it’s reporting the equivalent blood level based on that assumption.4The Alcohol Pharmacology Education Partnership. The Breathalyzer Assumes a Specific Blood-to-Breath-Ratio to Calculate the BAC
The problem is that 2,100:1 is a population average. Real partition ratios range from about 1,500:1 to 3,000:1 and vary with age, sex, genetics, body temperature, and whether you’re still absorbing alcohol or already eliminating it. If your personal ratio is below 2,100:1, the device overstates your actual blood alcohol level; if it’s higher, the device understates it. That built-in variability is one of the primary grounds defense attorneys use to challenge breath test results.
Despite the imprecision, law enforcement sticks with breath testing because it’s fast and non-invasive. A blood draw needs trained medical staff, a sterile setting, and hours of lab work. A breath sample takes about a minute and produces a number immediately.
What Can Push a BrAC Reading Up or Down
Two people drinking the same amount can produce very different readings. Body weight is the biggest variable, because a smaller person has less water volume to dilute the alcohol. Biological sex also matters, since differences in body composition and enzyme activity affect how fast the liver processes ethanol. Drinking speed matters too: when drinks arrive faster than the liver can keep up, the excess accumulates in the bloodstream and lungs.
Mouth Alcohol
The biggest source of falsely high readings is mouth alcohol. Residual alcohol from a recent sip, a burp, or acid reflux can sit in the oral cavity, and if the device captures those concentrated vapors instead of deep lung air, the number comes back far higher than what’s actually in the blood. That’s why evidentiary testing protocols require a 15-minute observation period during which the operator confirms you don’t eat, drink, burp, or vomit, giving any mouth alcohol time to dissipate.5eCFR. 49 CFR Part 40 Subpart M – Alcohol Confirmation Tests Breath temperature can also nudge the result, since warmer breath carries slightly more alcohol vapor per liter.
Medical and Dietary False Positives
Certain medical conditions can produce a positive reading with no drinking involved. The clearest example is ketosis, a metabolic state tied to very low-carbohydrate diets or uncontrolled diabetes. Ketosis loads the bloodstream with acetone, which the liver can convert into isopropanol. Some breath-testing devices, especially fuel-cell-based ignition interlock units, detect isopropanol and read it as ethanol, producing a false positive. Infrared devices handle this better because they can tell the two alcohols apart by their absorption patterns.
Gastroesophageal reflux disease raises a related worry about stomach acid carrying alcohol vapor into the mouth, but controlled research has found the risk small when proper testing intervals are followed, and the 15-minute observation period further reduces it.
Roadside Versus Evidentiary Tests
Not all breath tests count the same in court. The handheld device an officer uses at the roadside is a Preliminary Breath Test, or PBT. Its purpose is to help the officer decide whether there’s enough evidence to arrest, not to prove guilt. PBT devices are smaller, less sophisticated, and more vulnerable to environmental interference than station-house machines, and in many jurisdictions their results are inadmissible at trial. Some courts admit PBT results if the prosecution can show the device was calibrated and the operator followed procedure, but that’s the exception.
An evidentiary breath test, or EBT, is run after arrest, usually on a larger stationary machine at the police station or booking facility. These tests follow strict protocols: a trained and certified operator, current calibration records on the machine, and the 15-minute observation period before the subject blows. Under DOT rules, calibration checks have to follow manufacturer-specified intervals, and any EBT that fails a check is pulled from service until repaired.6U.S. Department of Transportation. 49 CFR Part 40 Section 40.233 – What Are the Requirements for Proper Use and Care of EBTs? EBT results are routinely admitted as evidence of intoxication if those protocols were followed, which is why defense attorneys so often request the maintenance and calibration logs for the specific machine used.
Refusing the Test
Every state has an implied consent law: by driving on public roads, you’ve already agreed to submit to a breath test if an officer has reasonable grounds to suspect impairment.3National Highway Traffic Safety Administration. BAC Test Refusal Penalties Refusing doesn’t close out the case. In nearly every state, refusal triggers an automatic administrative license suspension lasting six months to a year for a first offense, and at least a dozen states treat refusal as a separate criminal charge on top of any DUI.
In Birchfield v. North Dakota (2016), the U.S. Supreme Court held that officers may require a breath test without a warrant as part of a lawful drunk-driving arrest, and that states may impose criminal penalties for refusing one. Blood tests sit on the other side of the line: because they’re more physically invasive, the Fourth Amendment requires a warrant before officers can compel a blood draw, and states cannot criminalize refusal of a warrantless blood test.7Justia. Birchfield v. North Dakota In practice, officers who want blood evidence after a refusal can apply for a warrant, and many departments do.
DOT Workplace Testing
Federal DOT alcohol testing covers safety-sensitive positions including commercial truck and bus drivers, airline crew, railroad workers, and pipeline operators. The protocols are tighter than the criminal DUI system, and the consequences start at a far lower BrAC.
A screening result of 0.02 or higher triggers a confirmation test on an EBT after a mandatory 15-minute wait.5eCFR. 49 CFR Part 40 Subpart M – Alcohol Confirmation Tests A confirmed result between 0.02 and 0.039 requires the employer to pull you from safety-sensitive duties for at least 24 hours.8U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.23 A confirmed result of 0.04 or higher is treated the same as a positive drug test: immediate removal from duty, referral to a DOT-qualified substance abuse professional, and a supervised return-to-duty process before you can work again.9Federal Motor Carrier Safety Administration. Return-to-Duty
Refusing a DOT alcohol test carries the same consequences as testing positive. You’re pulled from safety-sensitive work right away and can’t return without completing the full return-to-duty process, including an evaluation by a substance abuse professional and a return-to-duty test conducted under direct observation.10Federal Motor Carrier Safety Administration. What if I Fail or Refuse a Test? The refusal stays on your testing record for future DOT-regulated employers to see.