The ABC’s of BAC: Understanding Blood Alcohol Concentration and Alcohol Impairment
Source: National Highway Transportation Safety Administration, StopImpairedDriving.org
Q: What is “BAC”?
A: The amount of alcohol in a person’s body is measured by the
weight of the alcohol in a certain volume of blood. This is called the
blood alcohol concentration, or “BAC.”
Alcohol is absorbed directly through the walls of the stomach and the
small intestine, goes into the bloodstream, and travels throughout the
body and to the brain.
Alcohol is quickly absorbed and can be measured within 30 to 70 minutes
after a person has had a drink.
Q: Does the type of alcohol I drink affect
A: No! A drink is a drink, is a drink.
A typical drink equals about half an ounce of alcohol (.54 ounces, to be
exact). This is the approximate amount of alcohol found in:
- one shot of distilled spirits, or
- one 5-ounce glass of wine, or
- one 12-ounce beer.
Q: What affects my BAC?
A: How fast a person’s BAC rises varies with a number of factors:
- The number of drinks. The more you drink, the higher the BAC.
- How fast you drink. When alcohol is consumed quickly, you will
reach a higher BAC than when it is consumed over a longer period of
- Your gender. Women generally have less water and more body fat
per pound of body weight than men. Alcohol does not go into fat
cells as easily as other cells, so more alcohol remains in the blood
- Your weight. The more you weigh, the more water is present in
your body. This water dilutes the alcohol and lowers the BAC.
- Food in your stomach. Absorption will be slowed if you’ve had
something to eat.
Q: What about other medications or drugs?
A: Medications or drugs will not change your BAC. However, if you drink alcohol while taking certain medications, you
may feel – and be – more impaired, which can affect your ability to
perform driving-related tasks.
Q: When am I impaired?
A: Because of the multitude of factors that affect BAC, it is very
difficult to assess your own BAC or impairment. Though small amounts
of alcohol affect one’s brain and the ability to drive, people often
swear they are “fine” after several drinks – but in fact, the failure to
recognize alcohol impairment is often a symptom of impairment.
While the lower stages of alcohol impairment are undetectable to others,
the drinker knows vaguely when the “buzz” begins. A person will likely
be too impaired to drive before looking – or maybe even feeling –
Q: How will I know I’m impaired, and why
should I care?
A: Alcohol steadily decreases a person’s ability to drive a motor
vehicle safely. The more you drink, the greater the effect. As with BAC,
the signs of impairment differ with the individual.
In single-vehicle crashes, the relative risk of a driver with BAC
between .08 and .10 is at least 11 times greater than for drivers with a
BAC of zero, and 52 times greater for young males. Further, many studies
have shown that even small amounts of alcohol can impair a person’s
ability to drive.
Every State has passed a law making it illegal to drive with a BAC of
.08 or higher. A driver also can be arrested with a BAC below .08 when a
law enforcement officer has probable cause, based on the driver’s
The following chart contains some of the more common symptoms people
exhibit at various BAC levels, and the probable effects on driving
Predictable Effects on Driving