It is illegal to have a blood-alcohol concentration (BAC) of .08% or greater while driving a vehicle. It is not illegal to have a BAC of .08% or greater while blowing into a breathalyzer in a police station. In other words, just because a breath test shows a level of, say, .09%, it does not mean that the BAC when the suspect was driving an hour earlier was .09%.
So what was the breath alcohol level when driving?
Well, we’ll never know: There is no evidence of the BAC at the time of actual driving. However, we can be fairly sure that it wasn’t .09%, since the body is constantly either absorbing or eliminating alcohol and the BAC is therefore constantly rising or falling. If it was falling, then we can expect the BAC when driving was higher — .10% or more. But if it was rising…..
Let’s take a typical example. The subject — let’s call her "Janet" — finishes dinner by throwing down "one for the road", a 12-ounce can of beer containing .05% alcohol. She is stopped by an officer soon after leaving the restaurant, alcohol is smelled on her breath and she is given field sobriety tests. She does marginally well but, to be sure, the officer takes her into the police station for breath testing. About 45 minutes after drinking the alcohol, Janet breathes into the breathalyzer. The result: .09%. She is booked and his driver’s license confiscated.
It will take, on average, about one hour for the alcohol to be absorbed and reach peak levels of concentration in the blood, thereafter to be eliminated from the body. This is only an average; it can vary from 15 minutes to 2 hours; some invidividuals can reach peak concentration ten times faster than others. Dubowski, ""Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects", Journal on Studies of Alcohol, Supp. 10 (July 1985). This makes trying to estimate earlier BAC levels no better than a rough guess, and scientists have unifromly condemned the practice. See, for example, "Breath Alcohol Analysis: Uses, Methods and Some Forensic Problems", 21 Journal of Forensic Sciences 9.
Applying averages to Janet, though, we can expect the last drink to have had little if any effect on her blood-alcohol concentration while she was driving. By the time she is being tested at the station 45 minutes later, however, she is reaching peak concentration. In other words, Janet’s BAC has been rising. At about 120 pounds, we can estimate (read "guess") that the can of beer has increased her BAC by about .031%.
Translation: the breathalyzer reading of .09% at the station indicates a BAC while driving of only .06%. She is not guilty. But the "evidence" will convict her.
Just to make things worse….As I indicated, attempts to guess BACs when driving earlier than when tested have been condemned by scientists. This makes things tough for prosecutors. Solution? As I discussed in an earlier post, "Whatever Happened to the Presumption of Innocence?", most states today have passed laws — contrary to scientific truth — which presume that the BAC at the time of being tested is the same as at the time of driving!
In other words, unless the defendant can prove that his BAC was different than when tested, the jury will be instructed that they must find that it is the same. In effect, the defendant is presumed guilty. And since there is no evidence of the BAC when driving, there is no way for the defendant to rebut the presumption. These laws do, however, make getting convictions much easier.