“BAC,” or “blood alcohol concentration,” is a measurement of the amount of alcohol in a person’s blood. As you continue to drink, your BAC generally rises. So there’s a relationship between BAC and your level of impairment. Because of this correlation, the “per se” DUI laws of all 50 states prohibit driving with a BAC of .08% or more.
(For an estimate of how many drinks it takes to put a person over the legal limit, take a look at this BAC chart.)
But instead of testing blood, officers often use breathalyzers to determine how much a motorist has been drinking. How do breath-test devices measure BAC?
They don’t. Breathalyzers measure breath alcohol concentration.
Although breathalyzers have been around for a long time, for many years, per se DUI laws prohibited driving with a blood alcohol concentration at or above a certain limit, but said nothing about breath alcohol. When an officer used a breathalyzer to measure a suspect’s alcohol level, the reading had to be converted to BAC. The conversion involved multiplying breath alcohol concentration by a “partition ratio.” Breathalyzers used a preset partition ratio of 2,100.
However, partition ratios vary from person-to-person. And even the same person can have different partition ratios at different times. (Partition ratios are affected by factors such as sex, body weight, breathing patterns, and body temperature.) So, defense attorney often attacked the accuracy of this breath-to-blood conversion in court.
The lawmakers of most states—including those in California and Texas—responded by changing their state’s per se DUI laws. In almost every state, it’s now illegal not only to drive with a BAC of .08% or more, but also a breath alcohol concentration of .08% or more. (Whereas BAC is normally measured in grams of alcohol per 100 milliliters of blood, breath alcohol concentration is ordinarily in grams of alcohol per 210 liters of breath.)
In states that made this change, BAC and breath alcohol concentration are essentially equivalent for purposes of determining whether a driver is guilty of a per se DUI. Police and attorneys frequently use “BAC” to refer to both breath and blood alcohol concentration.
If you’ve been arrested for or charged with a DUI, you should talk to an attorney right away. The DUI laws of every state are different. A local DUI attorney can help you understand the laws in your state and what you’re up against.