"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 the correlation between BAC and impairment, the "per se" DUI laws of all 50 states prohibit driving with a BAC of .08% or more (.05% or more in Utah). And the implied consent laws of all states require drivers lawfully arrested for driving under the influence to submit to testing.
Officers often use breathalyzers to determine how much a motorist has been drinking. But breathalyzers don't measure blood alcohol, they 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 attorneys 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 with 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.