Blood Alcohol Content and the Legal Drinking Limit

Impairment Begins With the First Drink

Vial of Blood
Blood and Urine Test Measure BAC.

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Blood alcohol content (BAC) is a measure of the amount of alcohol circulating in your bloodstream. It is expressed in terms of weight (milligrams) per unit of volume (milliliters) and is usually shown as a percentage. Blood alcohol content is used for legal and medical purposes to indicate a person's level of intoxication. In all 50 states and the District of Columbia, the legal limit for driving under the influence of alcohol is 0.08. Commercial drivers have a limit of 0.04. Any detectable blood alcohol content is a violation for persons under the age of 21.

Blood Alcohol Content

Blood alcohol content is the amount of alcohol present in a volume of blood equal to 100 milliliters (ml) or its equivalent of 1 deciliter (dl). For example:

  • 80 mg is 0.08 grams.
  • 0.08 grams of alcohol in 100 ml is 0.08 percent
  • This can also be expressed as 80 mg/dl or a BAC of 0.08

In the United States, a blood-alcohol content of 0.1 (0.1 percent or one-tenth of one percent) means that there are 0.10 grams of alcohol for every deciliter of blood in the person's body at the time of the test.

Synonyms for BAC include blood alcohol level, blood alcohol concentration, and blood ethanol concentration

Levels of BAC and Impairment

When alcohol is consumed and absorbed into the bloodstream, it travels directly to the brain, affecting many cognitive functions and the ability to perform physical tasks. Driving skills can be impaired long before someone reaches the legal limit, but at 0.08 the risk of having a vehicle crash increases dramatically.

The National Highway Traffic Safety Administration lists these effects on driving at different blood alcohol content levels:

  • 0.02 BAC: You are likely to feel relaxed and have some loss of judgment. You aren't able to quickly track the movements of other vehicles, pedestrians, or animals. You lose some of your abilities to do two things at once, so you are more likely to be distracted.
  • 0.05 BAC: You begin to exhibit loss of small-muscle control such as being able to focus your eyes, and you can have lowered alertness. You have even worse ability to track moving objects. Your ability to steer is degraded. If an emergency situation develops, such as needing to brake quickly or maneuver around an unexpected blockage, you are likely to have a poorer response. As a result, a 2018 report from the National Academies of Sciences, Engineering, and Medicine calls for the lowering the BAC limit for driving to 0.05 percent.
  • 0.08 BAC: At the legal level of 0.08, you will usually exhibit poor muscle coordination, loss of balance, slower reaction time, slurred speech, loss of acuity in vision and hearing, difficulty in detecting danger, and impaired judgment, self-control, reasoning, and memory. When driving, you have difficulties in speed control and in recognizing and reacting to signals and emergency situations. All of these impairments result in an increased risk of injuries in general, and particularly those related to the operation of a motor vehicle.
  • 0.10 BAC: At this level, you will have further deterioration of your abilities. It will be hard to maintain lane position and to brake when needed.
  • 0.15 BAC: You will have poor muscle control and ability to balance. You are likely to vomit. You will have significant problems in controlling your vehicle and paying attention to your driving and what is happening around you.

Reaching the BAC Limit

Many factors determine your blood alcohol content and the level of impairment you may be experiencing. On an individual level, there is a variance due to your weight, body fat percentage, hydration, digestion, and how alcohol affects your body chemistry.

The alcohol content of the drinks you have consumed, the spacing of the drinks, and the amount of time that has passed since consuming the drinks influence your BAC. Use of medications and other drugs can affect how much impairment alcohol produces. In addition, studies have shown people are not good at estimating their BAC or level of impairment, leading to making poor decisions.

There are many simple weight/gender charts that estimate the number of drinks and the blood alcohol content in one hour. For example, a 120-pound woman can reach a 0.08 BAC level after only two drinks and a 180-pound man can be at 0.08 after four drinks. A "drink" is either one shot of liquor, a five-ounce glass of wine, or one beer. These estimates should always be used with caution because of the variation in measures and alcoholic content of drinks within different classes of alcohol. For example, some craft beers have twice the alcohol as the typical can of beer, and you may be drinking a full pint (16 ounces) rather than the 12 ounces found in a can.

A Word From Verywell

Enforcing the legal blood alcohol content limit is important for public safety, but you must use caution anytime you consume alcohol and then drive. You will have some impairment from the first drink and it is always best to avoid driving after you have taken a drink.

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