How BAC Is Measured and How It Impairs You

Blood alcohol concentration (BAC) is a measurement of the percent of alcohol is in a person's bloodstream. This, rather than the exact amount of alcohol the person has consumed, will determine the effects the alcohol will have on the person's brain, body and behavior. In all states, the legal limit is any number below .08.

There are many factors that affect BAC, including size, gender, physical condition; what has been eaten and when; how much sleep is logged; medications; and alcohol content of drinks. As BAC increases so does the level of alcohol-related impairment a person will experience.

How BAC Measurement Is Calculated

BAC can be assessed accurately with a breath test or breathalyzer. To calculate a BAC, the amount of alcohol in the bloodstream is measured in mg of alcohol per 100 ml of blood. It is usually expressed as a decimal, such as 0.08 or 0.15.

For example, a BAC of .10 percent means that an individual’s blood supply contains one part alcohol for every 1,000 parts blood.

How BAC Affects Impairment

The Centers for Disease Control and Prevention provides the following chart which conveys how alcohol consumption and BAC can impact behavior, judgment, physiology, and driving ability.

BAC*

Typical Effects

Predicted Effects on Driving

.02%
About 2 alcoholic drinks**

Some loss of judgment

Relaxation

Slight body warmth

Altered Mood

Decline in visual functions (rapid tracking of a moving target)

Decline in ability to perform two tasks at the same time (divided attention)

.05%

About 3 alcoholic drinks**

Exaggerated behavior

May have loss of small-muscle control (e.g., focusing your eyes)

Impaired judgment

Usually good feeling

Lowered alertness

Release of inhibition

Reduced coordination

Reduced ability to track moving objects

Difficulty steering

Reduced response to emergency driving situations

.08%

About 4 alcoholic drinks

Muscle coordination becomes poor (e.g., balance, speech, vision, reaction time, and hearing)

Harder to detect danger

Judgment, self-control, reasoning, and memory are impaired

Concentration

Short-term memory loss

Speed control

Reduced information processing capability (e.g., signal detection, visual search)

Impaired perception

.10%

About 5 alcoholic drinks**

Clear deterioration of reaction time and control

Slurred speech, poor coordination, and slowed thinking

Reduced ability to maintain lane position and brake appropriately

.15%

About 7 alcoholic drinks**

Far less muscle control than normal

Vomiting may occur (unless this level is reached slowly or a person has developed a tolerance for alcohol)

Major loss of balance

Substantial impairment in vehicle control, attention to driving task, and in necessary visual and auditory information processing

*Blood Alcohol Concentration Measurement

The number of drinks listed represents the approximate amount of alcohol that a 160-pound man would need to drink in one hour to reach the listed BAC in each category.

**A standard drink is equal to 14.0 grams (0.6 ounces) of pure alcohol. Generally, this amount of pure alcohol is found in:

12-ounces of beer (5% alcohol content)
8-ounces of malt liquor (7% alcohol content)
5-ounces of wine (12% alcohol content)
1.5-ounces or a “shot” of 80-proof (40% alcohol content) distilled spirits or liquor (e.g., gin, rum, vodka, whiskey)