California Vehicle Code § 23610 specifies that in a prosecution for driving under the influence, it shall be presumed that the person was “under the influence” if their blood alcohol level was .08 percent or more at the time of driving. However, mandatory presumptions of this nature are unconstitutional in criminal cases, and thus jurors are instead instructed that they may infer the defendant was under the influence if they find he or she had a .08 percent or higher alcohol concentration at the time of driving.
Breath-alcohol testing is used in most California DUI prosecutions to try and prove a defendant’s blood-alcohol concentration. The percentage of alcohol in 100 milliliters of blood is equated to the percentage of alcohol in 210 liters of breath, so the government multiplies the amount of alcohol found in a breath sample by 2100 to arrive at what is supposedly the defendant’s blood-alcohol concentration. The problem with this nifty trick is that the 2100-1 ratio is just a guess—nobody has a fixed partition ratio, and everyone’s partition ratio is constantly changing. It is affected by, among other things, breathing patterns, body temperatures, hematocrit levels, and environmental conditions.
In People v. Bransford (1984) 8 Cal.4th 885, the California Supreme Court held that on the separate charge of driving with a .08 percent or higher alcohol concentration, evidence of partition-ratio variability is not relevant (and is thus inadmissible) because the Legislature has declared it to be unlawful to drive with a .08 percent or higher using the 2100-1 partition ratio (i.e., the Legislature supposedly does not care if your true blood-alcohol level is under .08 percent—you are guilty if your breath-alcohol level is .08 percent or higher using the 2100-1 ratio). It is highly doubtful that the Legislature actually intended this absurdity, since it is not alcohol in the breath that causes impairment (impairment is caused by alcohol entering the bloodstream and being carried to the central nervous system and brain). Nevertheless, that is the law as interpreted by the State Supreme Court.
What happens though, when a prosecutor wants to use a breath-alcohol test result to argue that the defendant was also under the influence? In People v. McNeil (2009) 46 Cal.4th 1183, the California Supreme Court held that partition ratio variability becomes relevant and admissible in that circumstance. To combat the McNeil ruling, many prosecutors will waive the presumption/inference jury instruction and argument as a means to try and persuade trial court judges that evidence of partition ratio variability should be excluded from evidence. Yet in Arizona v. Cooperman (No. 2 CA-CV 2011-0197; 2012 WL 3324227), an appellate court in Arizona rejected this very contention in a published decision issued on August 14, 2012. The Cooperman Court held that a statute containing a presumption similar California’s (Vehicle Code § 23610) applies even if not expressly invoked by the prosecution, and that introduction of breath-alcohol test result automatically triggers it. Since it’s there, the defense may introduce evidence of partition ratio variability to rebut the permissive inference that he or she was under the influence. Moreover, the Cooperman Court held that expert testimony concerning the accuracy of breath-alcohol testing may be challenged on either charge (driving under the influence and/or driving with a .08 percent or higher alcohol concentration) by introducing evidence that the factors referred to above can change the result.
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