What is blood alcohol content (bac)?

Quick Answer
Blood alcohol content is a measurement of the amount of alcohol in a person’s circulatory system at a given time. This measurement is commonly obtained by law enforcement officers in the field, who check drivers for operating a vehicle under the influence of alcohol. Blood alcohol content is less often used as a measure in medical treatment.
Expert Answers
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Blood alcohol content (BAC) is a measure of the amount of ethanol (or ethyl alcohol) in a person’s bloodstream. In the United States, BAC is measured in grams of alcohol per 100 milliliters (ml) of blood. If a person has 0.10 grams of alcohol in his or her bloodstream for every 100 ml of blood, the BAC for that person would be 0.10. Another way to think of BAC is that it is the percentage of a person’s blood that is composed of alcohol.

BAC is most often measured to determine if a person is impaired by alcohol while driving. Because drawing blood is an invasive procedure, law enforcement officers usually use a breath analyzer (or breathalyzer ) in the field to estimate a person’s BAC. Breathalyzer results accurately reflect blood-alcohol levels. In the United States (all states), the legal blood-alcohol limit for drivers who are old enough to legally drink alcohol is 0.08. Some states have stiffer penalties for drivers whose BAC is 0.17 or higher. The legal blood-alcohol limit for persons younger than age twenty-one years is 0.02 in most states, rather than 0.0 because some legal drugs or mouthwashes contain small amounts of ethyl alcohol, which could register as alcohol in one’s BAC.

Relativity of BAC Levels

Although blood alcohol levels are directly proportional to the amount of alcohol consumed, BAC levels vary significantly from person to person and from situation to situation for the same amount of alcohol. Factors that affect BAC are a person’s weight and gender, the length of time in which the alcohol was consumed, the presence or absence of food in the stomach at the time of alcohol consumption, and a person’s genetic makeup.

Women’s bodies generally contain less water than do men’s bodies, so alcohol has a greater relative impact on women. Also, the greater a person’s weight, the more that person can consume alcohol before feeling its effects. For instance, a 200-pound man who drank two 12-ounce beers in one hour would likely have a BAC of 0.04, whereas a 120-pound woman drinking the same two beers in the same amount time would likely have a BAC of 0.08.

Food in the stomach at the time of alcohol consumption can keep a person’s BAC lower because the alcohol makes its way into the bloodstream at a slower rate. One factor that does not affect BAC is caffeine, which can mask the depressant effects of alcohol but does not improve impaired judgment or increase a person’s reaction time while, for example, driving. Finally, a physician may be interested in a patient’s BAC if the physician suspects acute alcohol poisoning or when making a diagnosis of alcoholism.


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