I’ve Been Charged with DWI… What Does BAC Mean?


BAC, three letters that don’t mean a whole lot… unless you’re talking about driving while impaired. The concept of BAC is central to proving whether a driver is impaired. This article will discuss what BAC is, the science behind it, and why it is so important in driving while impaired cases.

What is BAC, and How is it Calculated?

BAC stands for Blood Alcohol Content. BAC refers to the percentage of alcohol in a person’s bloodstream. It is presented as a decimal fraction of the number 1. For instance, a BAC of .10 would mean that a person’s blood supply contains 1 part alcohol for every 1,000 parts of blood. There are many factors that affect BAC, including:

  • Number of drinks consumed: This is the primary source of alcohol in the bloodstream.
  • Amount of time in which drinks are consumed: If a person drinks faster than one drink per hour, the alcohol simply stays in the body, waiting for its turn to be metabolized. The result is increasing levels of intoxication.
  • Body weight: The weight of a person greatly affects the distribution of alcohol throughout the body. The smaller the person, the less room for alcohol to distribute itself.
  • How hydrated a person is: Diluting alcohol with water or juices reduces the volume of alcohol in your bloodstream. Drinking straight alcohol or alcohol mixed with carbonated beverages speeds up absorption.
  • How a person produces certain enzymes: The most common of these are two enzymes—alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). These enzymes help break apart the alcohol molecule, making it possible to eliminate it from the body.
  • Hormone levels: Generally, men can handle more alcohol than women. A woman's ability to metabolize alcohol can be affected by her menstrual cycle due to higher levels of estrogen. All of this contributes to higher concentrations of alcohol in a woman's system even if she is drinking the same amount as a man.
  • Medications taken: Alcohol is a depressant. Any illegal, prescription or over-the-counter drug is likely to react with alcohol and may increase intoxication or have negative effects.
  • Food: Your stomach lining absorbs alcohol directly into your bloodstream. Food slows down the absorption of alcohol.

Some other points to remember are that a person's BAC can continue to rise even while passed out. Even after a person stops drinking, alcohol in the stomach and intestine continues to enter the bloodstream and circulate throughout the body. So simply taking a rest and trying to drive afterward may not guarantee that the BAC has decreased. Only time can lower your BAC, not coffee or cold showers. Once a person reaches .30 BAC, there is a risk of coma or death.

How is BAC tested?

BAC is measured by testing bodily fluids, either breath or blood. Breath testing is the most common form of testing for BAC. When law enforcement wants to test the BAC in someone’s breath accurately, they will use a device known as an intoxilyzer. The intoxilyzer uses infrared spectroscopy to determine the amount of alcohol in the breath sample, and its use is regulated by strict guidelines for maintenance and testing. The device itself consists of several components that work together to measure the concentration of alcohol, including a sample chamber, an infrared source, a detector, and a microprocessor. The process occurs nearly instantaneously. First the machine is tested using a sample cartridge to make sure that there are no false readings. Then the test subject is ordered to blow into the tube forcefully enough to ensure a reading. Two readings are taken and the lesser of the two readings is used to calculate BAC. BAC may also be determined from blood analysis, where blood is drawn from the body and tested in the laboratory.

Why is BAC Important in Court?

BAC is the scientific principle by which driving while impaired charges are proven in court. Therefore, the amount of BAC in a DWI charge is crucial. If a person tests below .08 BAC in North Carolina, that means that the prosecutor will have to prove their impairment through other means. This proof can come in the form of poor driving, bad performance on field sobriety tests, or proof of other substance abuse while driving. However, if the BAC is .08 or above, a judge or jury can find that as prima facie evidence of impairment. That means the judge or jury can presume that the person in question was impaired without considering additional evidence. While there are many issues to argue in a DWI case, arguing a lack of impairment becomes difficult once this threshold is crossed.

Have you been given a BAC result after you have been pulled over, and now need help with your DWI? Call one of our experienced attorneys at Jetton and Meredith today at (704) 931-5535!