A breathalyzer displays your blood alcohol concentration (BAC) as a decimal number, typically ranging from 0.00 to 0.40. The number represents the grams of alcohol per 210 liters of breath, converted to an equivalent blood alcohol percentage. A reading of 0.08, for example, means your body contains roughly 0.08 grams of alcohol per 100 milliliters of blood, which is the legal limit for standard drivers in all 50 U.S. states.
What the Numbers Mean
The decimal on the screen is small, but the differences between readings are significant. Here’s what happens in your body at each level, based on data from the National Highway Traffic Safety Administration:
At 0.02, you may feel slightly relaxed with a mild mood shift. Your ability to track moving objects and divide your attention between two tasks starts to decline, even though you likely feel fine.
At 0.05, behavior becomes more exaggerated and judgment is noticeably impaired. You may have trouble focusing your eyes, and your alertness drops. Behind the wheel, steering gets harder and your response to sudden situations slows down.
At 0.08, muscle coordination suffers across the board: balance, speech, vision, reaction time, and hearing. Short-term memory is affected, and your ability to process information, like noticing a traffic signal change, is measurably reduced. This is the legal threshold for a DUI in every state.
At 0.10, reaction time and motor control clearly deteriorate. Speech slurs, thinking slows, and maintaining a lane or braking at the right moment becomes difficult.
At 0.15, muscle control is far below normal and vomiting is common. Balance is severely compromised. At this level, a person has substantial impairment in nearly every function needed to operate a vehicle.
Readings above 0.20 represent a medical risk. Above 0.30 is life-threatening, and 0.40 can be fatal.
Legal Limits by Driver Type
The 0.08 threshold applies to standard passenger vehicle drivers. Two other groups face stricter limits. Commercial vehicle drivers (CDL holders) are disqualified at 0.04 while operating a commercial vehicle, regardless of whether they’re on or off duty. Drivers under 21 face zero-tolerance laws in most states, where any detectable alcohol (often 0.01 or 0.02) triggers a violation.
If your breathalyzer shows 0.00, no alcohol was detected. A reading between 0.01 and 0.03 suggests minimal alcohol, possibly from a single drink or even a non-alcohol source. Any number at or above 0.08 puts you over the legal limit for standard driving.
How the Device Measures Alcohol
Breathalyzers work by detecting alcohol molecules in the air you exhale from deep in your lungs. When blood passes through your lungs, some of the alcohol in it evaporates into the air in your lung sacs. That air carries a predictable ratio of alcohol compared to your blood, which is what the device measures.
Two main sensor technologies exist. Fuel cell sensors use a chemical reaction: alcohol in your breath is converted into acetic acid, then into water and carbon dioxide. This reaction generates a small electrical voltage proportional to the amount of alcohol present. The higher the voltage, the higher your reading. Fuel cells are found in both personal handheld devices and professional law enforcement equipment.
Infrared sensors take a different approach. They pass infrared light through your breath sample and measure how much light gets absorbed. Alcohol molecules absorb infrared energy in a specific way, much like sunglasses absorb visible light. The more alcohol present, the more light is absorbed, and the higher the reading. Infrared technology is typically reserved for evidential-grade machines used at police stations.
Personal breathalyzers almost always use fuel cell sensors, which are accurate but less precise than the infrared instruments used in formal legal testing. This means a home device gives you a useful estimate, but the number may differ slightly from what a police-grade machine would show.
Why Your Reading Might Be Wrong
The most common source of inaccurate readings is mouth alcohol. After you take a drink, residual alcohol lingers in your mouth for up to 15 minutes. If you blow into a breathalyzer during that window, the device picks up alcohol vapor from your mouth and throat on top of the alcohol from your lungs, inflating the result. This is why law enforcement officers are required to observe you for a full 15 minutes before administering a breath test, making sure you don’t eat, drink, smoke, burp, cough, or sneeze during that period.
On an evidential breathalyzer’s readout, mouth alcohol contamination sometimes shows up as an irregular pattern. In a normal breath sample, the alcohol concentration rises steadily as you exhale deeper air from your lungs. If the reading spikes erratically, drops mid-blow, or shows a wavy pattern, the machine flags it as a potential mouth alcohol issue and alerts the operator. Personal devices generally lack this detection feature, so waiting at least 15 to 20 minutes after your last sip is important for getting a number you can trust.
Several other factors can produce false positives or inflated readings:
- Mouthwash and breath sprays: Many contain significant amounts of alcohol that linger in your mouth and register on the sensor.
- Over-the-counter medications: Products like Nyquil and certain cough drops contain alcohol as an ingredient.
- Asthma inhalers: Some inhaler medications can trigger false positives on breath tests.
- Acid reflux: Gastroesophageal reflux can push alcohol vapor from your stomach back into your mouth, creating the same problem as residual mouth alcohol.
- Diabetes: When blood sugar runs high, the body produces ketones that are chemically similar to alcohol. Some breathalyzer sensors, particularly cheaper ones, can mistake ketones for ethanol.
Getting an Accurate Reading at Home
If you’re using a personal breathalyzer, the number on the screen is only as reliable as the conditions under which you took the test. Wait at least 15 to 20 minutes after eating, drinking anything (including water), smoking, or using mouthwash. Take a deep breath and blow steadily into the device for the full duration it requires, usually four to six seconds. Shallow or short breaths produce readings based on mouth and throat air rather than deep lung air, which skews the result.
Take two readings a few minutes apart. If the numbers differ by more than 0.02, something is interfering with the test. Wait a few more minutes and try again.
Calibration matters more than most people realize. Personal breathalyzers drift over time as the fuel cell sensor degrades with use and exposure to environmental contaminants. Most manufacturers recommend recalibrating every 6 to 12 months, or after a certain number of uses (often 200 to 300 tests). Professional law enforcement devices are calibrated at least annually. Without regular calibration, your personal device may read consistently high or low, giving you a false sense of either alarm or safety.
What a 0.00 Reading Does and Doesn’t Tell You
A reading of 0.00 means no detectable alcohol was found in your breath at that moment. It does not mean you’re unimpaired. Breathalyzers measure ethanol only. They won’t detect impairment from marijuana, prescription medications, sleep deprivation, or any other substance. A 0.00 also doesn’t guarantee that alcohol has fully left your system. If you recently stopped drinking, your BAC may still be falling, and a reading taken 20 minutes later could differ from one taken now as your liver continues processing what’s in your blood.
The body metabolizes alcohol at a relatively steady rate, roughly 0.015 per hour for most people. So if your last reading was 0.06, it would take approximately four hours to reach 0.00. A breathalyzer snapshot tells you where you are right now, not where you’ll be in an hour or where you were an hour ago.