Blood alcohol content (BAC) is a measure of the amount of alcohol circulating in the bloodstream. It is expressed in terms of weight (milligrams) per unit of volume (milliliters); BAC levels are usually shown as a percentage. Blood alcohol content is used for legal and medical purposes to indicate a person's level of intoxication.
Synonyms for BAC include blood alcohol level, blood alcohol concentration, and blood ethanol concentration.
Blood alcohol content is the amount of alcohol present in 100 milliliters (mL) or its equivalent of 1 deciliter (dL) of blood. For example:
A blood-alcohol content of 0.1 (0.1% or one-tenth of 1%) 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.
In 49 of 50 states and the District of Columbia, the legal limit for driving under the influence of alcohol is 0.08. In Utah, the legal BAC limit is 0.05. Commercial drivers have a limit of 0.04 (this is a federal standard). Any detectable blood alcohol content is a violation for individuals under the age of 21.
When alcohol is consumed and absorbed into the bloodstream, it travels throughout the body and 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:
A 2018 report from the National Academies of Sciences, Engineering, and Medicine calls for the lowering the BAC limit for driving to 0.05% from the current legal limit (in most states) of 0.08%.
Many physical and situational factors determine blood alcohol content and the level of impairment you may experience. These can vary from person to person and also from situation to situation.
It is safest not to drink at all if you will be driving, since driving skill is impaired before reaching illegal BAC levels. If you do drink, it is important to limit your intake, space your drinks out over a longer period of time, and drink plenty of water. "Moderate" alcohol intake is defined as two or fewer drinks per day for men and one or fewer for women.
Research published in 2015 showed that people are not good at estimating their own BAC or level of impairment, leading to making poor decisions.
There are many simple weight/gender charts that estimate BAC levels. For example, a 120-pound woman can reach a 0.08 BAC level after only two drinks within one hour. 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 a typical can of beer, and the serving size is often a full pint (16 ounces), compared to 12 ounces in a can.
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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