What is a rising BAC (Blood Alcohol Content) defense?
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It is unlawful to have an excessive blood-alcohol concentration (BAC) at the time of driving -- not at the time of being tested. Since it takes between 45 minutes and 3 hours for alcohol to be absorbed into the system, an individual's BAC may continue to rise for some time after he is stopped and arrested.
Commonly, it is an hour or more after the stop when the blood, breath or urine test is given to the suspect. Assume that the result is .12%. If the suspect has continued to absorb alcohol since he was stopped, his BAC at the time he was driving may have been only .07%. In other words, the test result shows a blood-alcohol concentration above the legal limit -- but his actual BAC at the time of driving was below. The other side of that is if a person is intoxicated while driving and is stopped, and is no longer absorbing alcohol, as time elapses his blood alcohol level would begin to fall. For that reason, and also to help rule out a rising BAC defense, police officers typically try to do testing as soon as possible.
In addition, this defense is why officers always ask exactly how many drinks were consumed, and most importantly, when. The prosecution is usually helped out by legislation that creates a presumption that the BAC level found at the time of testing was the same as when driving, as long as the test is done within a set time limit. This puts the burden of proof on the defendant, which means he has to hire a toxicology expert to prove his BAC at the time of driving.