A BAC test measures the amount of alcohol in your body, expressed as a percentage of alcohol in your blood. BAC stands for blood alcohol concentration (sometimes called blood alcohol content), and a reading of 0.08 means your blood is 0.08% alcohol by volume. These tests are used in traffic stops, emergency rooms, and workplace screening to determine how much alcohol someone has consumed and how impaired they may be.
How BAC Is Measured
There are three main ways to test BAC, each with different levels of accuracy and different situations where they’re used.
Breath tests are the most common. A breathalyzer is a handheld device that analyzes your breath to estimate the alcohol in your blood. Police officers use them during traffic stops because they give quick results on the spot. They’re convenient but less precise than other methods.
Blood tests involve drawing a blood sample and analyzing it in a lab. This is the most accurate way to measure BAC and can detect alcohol for up to 12 hours after drinking. If a legal case requires precise evidence, or if you arrive at an emergency room after an accident, a blood test is typically what’s used.
Urine tests check for recent alcohol use and can sometimes detect it over a longer time frame than blood or breath tests. They’re less common for measuring exact BAC levels but are used in some workplace and clinical settings.
What the Numbers Mean
BAC doesn’t just tell you whether someone has been drinking. It corresponds to specific, measurable changes in how the brain and body function. The National Highway Traffic Safety Administration breaks it down by level:
- 0.02: Mild relaxation, slight warmth, subtle shifts in mood and judgment. You may already have trouble tracking moving objects or dividing your attention between two tasks.
- 0.05: Lowered alertness, reduced coordination, and impaired judgment. Behavior may become exaggerated. Steering a vehicle becomes noticeably harder, and your ability to respond to sudden driving situations drops.
- 0.08: Poor muscle coordination affecting balance, speech, vision, and reaction time. Reasoning, self-control, and short-term memory are all impaired. This is the legal limit in 49 U.S. states.
- 0.10: Clear deterioration in reaction time and motor control. Speech becomes slurred, thinking slows, and maintaining a lane while driving becomes very difficult.
- 0.15: Significant loss of balance and muscle control. Vomiting is common. At this level, a person has substantial impairment in nearly every function needed to operate a vehicle or make sound decisions.
Levels above 0.15 move into dangerous territory. BAC in the range of 0.30 to 0.40 can cause loss of consciousness, and levels above 0.40 can be fatal.
What Affects Your BAC
Two people can drink the same amount and end up with very different BAC levels. The most significant factors are body weight and biological sex. BAC is calculated using what’s known as the Widmark equation, which accounts for the fact that alcohol distributes differently depending on body composition. Men typically have a higher water-to-fat ratio than women, so the same number of drinks produces a lower BAC in most men compared to most women of the same weight.
How fast you drink matters too. Your liver processes roughly one standard drink per hour. A standard drink is 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of liquor. Drink faster than that pace and alcohol accumulates in your blood faster than your body can clear it. Your BAC drops at an average rate of about 0.016 per hour once you stop drinking, and nothing speeds that up: not coffee, not food, not a cold shower.
Legal BAC Limits
In the United States, the legal BAC limit for driving is 0.08 in every state except Utah, which lowered its limit to 0.05 in 2018. Commercial drivers and people under 21 face stricter thresholds, often 0.04 and near-zero respectively, depending on the state. On federal lands, the limit can vary based on which agency manages the property.
It’s worth noting that impairment begins well below the legal limit. At just 0.02, your ability to track a moving object and divide your attention is already reduced. The legal limit is not a safe threshold; it’s the point at which driving becomes a criminal offense.
What Can Cause a False Reading
Breathalyzers estimate BAC indirectly, which makes them vulnerable to interference. Several common substances and conditions can produce a falsely elevated reading.
Mouthwash and breath sprays often contain significant amounts of alcohol. If you use one shortly before a breath test, the residual alcohol in your mouth can register on the device even if your actual blood alcohol level is zero. Over-the-counter cold medicines like NyQuil also contain alcohol and can trigger a positive result. Asthma inhalers can leave compounds in the mouth that a breathalyzer mistakes for alcohol.
Certain medical conditions create problems too. Acid reflux can push alcohol vapors from the stomach into the mouth, inflating the reading. People with diabetes sometimes produce ketones when their blood sugar is high, and ketones are chemically similar enough to alcohol that a breathalyzer may detect them as such. Even fermented foods like kimchi, sauerkraut, or foods made with yeast can leave trace amounts of alcohol in the mouth that show up on a test.
Environmental exposure to paint fumes, adhesives, or chemical solvents in poorly ventilated spaces has also been documented as a source of false readings. This is one reason blood tests are considered more reliable when accuracy is critical.
When BAC Tests Are Used in Medical Settings
BAC tests aren’t just for law enforcement. Emergency departments routinely test blood alcohol levels when someone arrives after a car accident, a fall, or with altered consciousness. Knowing whether alcohol is in someone’s system helps doctors figure out what’s causing symptoms and how to treat them safely, since alcohol interacts with anesthesia and many medications.
Hospitals also use BAC testing when monitoring people going through alcohol withdrawal, where knowing the starting level helps guide the intensity of care needed. In these medical contexts, a blood draw is the standard method because precision matters more than speed.