Breathalyzers are reasonably accurate but not perfect. The best devices used at police stations measure blood alcohol concentration (BAC) within about ±0.005 of your actual level, while portable roadside units typically fall within ±0.01. That means a reading of 0.08 (the legal limit in most U.S. states) could reflect a true BAC anywhere from 0.07 to 0.09. Several biological and environmental factors can push that margin even wider.
How Breathalyzers Estimate Blood Alcohol
Breathalyzers don’t measure alcohol in your blood directly. They measure alcohol vapor in your exhaled breath and then convert that number using a fixed ratio: for every unit of alcohol in your breath, the device assumes there are 2,100 units in your blood. This 2,100:1 ratio is baked into the math behind every breath test result in the United States, Canada, and Australia.
The problem is that ratio varies from person to person and even within the same person at different times. Your actual ratio depends on your body temperature, how long you’ve been exhaling into the device, your sex, and whether your body is still absorbing alcohol or already eliminating it. Different countries have acknowledged this uncertainty by adopting different ratios: some use 2,000:1, others 2,300:1 or 2,400:1. There is no international consensus on the correct number. Research published in Forensic Sciences Research found that in post-absorptive testing (after your body has fully absorbed the alcohol), none of 100 subjects had a ratio below 2,100:1, which means the standard ratio tends to produce readings at or below actual blood alcohol in that phase. But during the absorption phase, right after drinking, the ratio shifts lower, and breath readings can overestimate your true blood alcohol level.
Sensor Types and Their Accuracy
Not all breathalyzers use the same technology, and the sensor type determines how precise the reading is.
- Semiconductor sensors are found in cheap personal breathalyzers sold online. They react to a broad range of chemicals, not just alcohol, and degrade quickly. These are the least accurate option and are not used in law enforcement.
- Fuel cell sensors use a platinum electrode that reacts specifically with alcohol. Portable law enforcement devices like the Lifeloc FC20 achieve accuracy of ±0.005 at a BAC of 0.10. Less expensive fuel cell units, like the Alcolizer LE5, are accurate to ±0.01 at the same level.
- Infrared spectrometry is the gold standard, used in evidential breathalyzers at police stations. Devices like the Intoxilyzer 8000 identify alcohol by measuring how it absorbs infrared light at specific wavelengths. These machines are the most reliable and produce results that hold up as legal evidence, though they still carry an accepted margin of error of ±0.01 plus or minus 10% of the reading.
In practical terms, the device you blow into on the roadside is less precise than the one at the station. Roadside results are typically used to establish probable cause for arrest, not as courtroom evidence on their own.
Body Temperature Changes the Result
One of the most significant sources of error is body temperature. Breathalyzers are calibrated assuming a normal core temperature of about 37°C (98.6°F). When your body runs hotter, more alcohol evaporates from your blood into your lungs, and your breath reading climbs.
A study that raised subjects’ core temperature by 2.5°C using warm water immersion found breath alcohol readings jumped as much as 23% above their actual blood alcohol level. The researchers calculated that each 1°C increase in core body temperature inflated the breath reading by roughly 8.6%. A fever of 101°F (about 1.1°C above normal) could bump a true BAC of 0.07 to a reading near 0.076. A more significant fever, intense exercise, or sitting in a hot car for an extended period could push the distortion further. Most testing protocols do not include a temperature check before the breath sample.
How Breathing Patterns Affect Readings
The way you breathe in the moments before a test can shift the result. Air deep in your lungs has been in close contact with blood vessels and carries more alcohol vapor than air sitting in your throat and windpipe (called “dead space” air). Hyperventilating floods your lungs with fresh air that hasn’t had time to pick up alcohol, diluting the sample.
Researchers testing this found that hyperventilation immediately before a breath test dropped the average reading from 0.104 to 0.086, a reduction of about 17%. The effect faded quickly: after five minutes, readings had climbed back to 0.099. Still, the study noted that a drop of 0.02 or more from hyperventilation was “entirely plausible.” Conversely, holding your breath before blowing concentrates alcohol vapor and can push the reading higher. Longer exhalation times into the device also correlate with higher readings, because the last portion of a long exhale comes from deeper in the lungs where alcohol concentration is greatest.
Law enforcement officers are trained to observe your breathing before a test and may ask you to blow again if they suspect you tried to manipulate the result.
Medical Conditions and False Positives
Certain medical conditions can produce mouth alcohol or alcohol-like compounds that inflate a breath test.
People with acid reflux (GERD) have long worried that stomach contents burping into the mouth could spike a reading. However, a controlled study of 10 subjects with severe GERD found that even when gastric reflux occurred during testing, it did not produce significantly deviant readings compared to blood alcohol levels when samples were taken at five-minute intervals. The researchers concluded that reflux falsely inflating an evidential breath test is “highly improbable,” largely because standard testing protocols include a 15 to 20 minute observation period that allows any mouth alcohol to dissipate.
Ketosis is a different story. People on very low-carbohydrate diets or those with uncontrolled diabetes produce high levels of acetone in their blood and breath. Fuel cell sensors don’t react to acetone directly, but the body can convert acetone into isopropanol (a different type of alcohol), and fuel cell devices do detect isopropanol. A published case report documented a false positive breath alcohol result in a person following a ketogenic diet, traced to this exact metabolic pathway. Infrared devices are better at distinguishing ethanol from other alcohols, which is one reason evidential instruments at police stations are preferred over portable units.
Calibration and Maintenance Matter
Even the best breathalyzer drifts out of accuracy over time. The U.S. Department of Transportation requires that every evidential breath testing device follow a quality assurance plan specifying calibration check methods, acceptable tolerances, and testing intervals. Manufacturers set the schedule, and if a device fails an external calibration check, it must be pulled from service immediately and cannot be used again until it passes after repair.
In practice, calibration lapses do happen. Defense attorneys regularly challenge breath test results by requesting maintenance and calibration logs. A device that hasn’t been checked on schedule, or one that was used after a failed calibration, can have its results thrown out in court. If you’re facing a legal case involving a breath test, the calibration history of the specific device used is one of the most relevant pieces of information.
How Much Error to Expect in Real Life
Combining all sources of variability, a single breath test result is best understood as an estimate with a range around it, not a precise number. For a well-maintained evidential device used on a healthy person with normal body temperature, the reading will typically land within ±0.01 of your true BAC. That’s the mechanical margin alone. Layer on biological variability in the blood-to-breath ratio, breathing patterns, and body temperature, and the real-world margin can widen to ±0.02 or more in unusual circumstances.
This is why many jurisdictions require two breath samples taken several minutes apart and compare them for consistency. If the two readings differ by more than a set threshold (often 0.02), the test is considered unreliable and must be repeated or replaced with a blood draw. A blood test remains the most accurate method for determining BAC, which is why you generally have the right to request one instead of or in addition to a breath test.