Criminal Law

What is “BAC” (Blood Alcohol Concentration)?

By John McCurley, Attorney
Learn about blood alcohol concentration as it relates to DUI law.

You might know that it’s illegal to drive with a BAC of .08% or more. But what exactly does “BAC” mean?

Measuring Blood Alcohol

“BAC” stands for “blood alcohol concentration,” sometimes called “blood alcohol content” or “blood alcohol level.” Basically, BAC is a measurement of the amount of alcohol in your blood.

All states have laws that prohibit driving with a BAC of .08% or more. The number of drinks it takes for a person to reach .08%, however, depends on a number of factors—these include sex, body weight, and how quickly the person consumes the drinks. Three beers over an hour might put one person at .07%, and another at .09%.

A driver's BAC is one of the main factors that determine the penalties the driver will face.

The two most common ways police measure the amount of alcohol in a suspect’s system are blood and breath tests. When blood tests are involved, police will normally take an arrested driver to a place—often a police station or medical facility—where a phlebotomist or other medical personnel can take a blood sample. The results aren’t instant: The blood sample has to go to a lab to be analyzed. It can take a month or more for the lab to report how much alcohol was in the blood. And prosecutors ordinarily won’t decide whether to charge you with a DUI until they receive the lab report.

How BAC Relates to Breath Alcohol Concentration

Though blood testing is still fairly common in DUI investigations, many officers prefer using breathalyzers to determine how much a suspect has been drinking. Breathalyzers give instantaneous results, and officers can administer breath tests without the assistance of medical personnel.

But breathalyzers don’t measure BAC—they measure breath alcohol concentration.

In the past, “per se” DUI laws prohibited driving with a BAC of .08% or greater but said nothing about breath alcohol. To be used in DUI cases, breath test results had to be multiplied by a “partition ratio.” In court, defense attorneys often challenged the accuracy of this breath-to-blood conversion. To avoid continuous legal battles over partition ratios, most states—including California and Florida—changed their per se DUI laws to prohibit driving not only with a blood alcohol concentration at or above .08% but also with a breath alcohol concentration at or above that level. (Whereas blood alcohol is normally measured in grams of alcohol per 100 milliliters of blood, breath alcohol is usually in grams of alcohol per 210 liters of breath.)

So, for purposes of assessing whether a motorist is guilty of a per se DUI, BAC and breath alcohol concentration are basically equivalent. Many DUI attorneys use the term “BAC” to refer to both.

Talk to an Attorney

The consequences of a DUI are serious. If you’ve been arrested for or charged with driving under the influence, make sure to consult with an attorney. An experienced DUI lawyer can tell you about the law in your state and help you understand what you’re up against.

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