Blood alcohol concentration (BAC)

The amount of alcohol in a person’s body is measured by blood alcohol concentration (BAC). Many factors determine the level of alcohol in an individual’s blood at the time of an alcohol test.

Scope

A commercial driver’s BAC determines whether the individual is in violation of state traffic code and the Federal Motor Carrier Safety Regulations. Enforcement often performs an alcohol test (if the officer suspects alcohol use) following a commercial motor vehicle crash or during a roadside inspection.

Regulatory citations

None

Key definitions

Blood alcohol concentration (BAC): The amount of alcohol in a person’s body, measured by the weight of the alcohol in a certain volume of blood.

Summary of requirements

Alcohol is absorbed through the walls of the stomach and the small intestines. It goes directly into the blood stream and throughout the body and to the brain. Its effects can be measured within 30 to 70 minutes after a person has a drink.

The type of alcohol does not affect an individual’s BAC. On average, a drink equals about half an ounce of alcohol (.54 ounces). This amount of alcohol is found in one shot of distilled spirits, one 5-ounce glass of wine, or one 12-ounce beer. How fast this alcohol enters the blood stream to raise the BAC varies on few factors, including:

The number of drinks. Obviously, the more you drink, the higher your BAC.
Time frame. If a person consumes alcohol quickly, the BAC will rise. If the person drinks the same amount of alcohol slowly over a longer time period, the BAC will not rise to the same level.
Gender. Women have less water and more body fat per pound of body weight than men. More alcohol remains in a female’s blood because alcohol does not go into fat cells as easily as other cells.
Weight. The alcohol is diluted by more body weight because there is more water present. It lowers the BAC.
Food. Individuals who eat before they drink have slowed the absorption of the alcohol because of the food in the stomach.

Medication or drugs will not change the BAC but will further impair a person’s driving ability.

BAC readings and effects. The effects of alcohol on the ability to operate a motor vehicle can be seen in the following BAC readings:

BAC	Symptoms	Effects on driving
.02	Some loss of judgment Relaxation Slight body warmth Altered mood	Decline in visual function (rapid tracking of moving target) Decline in ability to perform two tasks at the same time (divided attention)
.05	Exaggerated behavior May have loss of small-muscle control (e.g., focusing eyes) Impaired judgment Lowered alertness Release of inhibition	Reduced coordination Reduced ability to track moving objects Difficulty steering Reduced response to emergency driving situations
.08	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	Cleared deterioration of reaction time and control Slurred speech, poor coordination, and slowed thinking	Reduced ability to maintain lane position and brake appropriately
.15	Far less muscle control than normal Vomiting may occur (unless this level is reached slowly or a person has a 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

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Blood alcohol concentration (BAC)

Scope

Regulatory citations

Key definitions

Summary of requirements