The California Supreme Court last Thursday entered a ruling allowing motorists accused of driving under the influence of alcohol (DUI) to question the reliability of the breathalyzer machinery used to secure convictions. The decision, however, leaves room for the conviction of drivers even when the machine is proved unreliable.

The high court recognized that a breath testing machine does not directly measure the alcohol content in a person's bloodstream. Rather, the device estimates from a sample of breath how much alcohol might be present in the blood using a conversion factor called the "partition ratio." California's breathalyzer machines assume that the amount of alcohol in 2100 milliliters of breath is equal to the amount of alcohol in 1 milliliter of blood.

"Simply put, the machines all automatically convert the amount of alcohol tested in the tiny amount of breath taken from the suspect," California DUI attorney Lawrence Taylor explained. "The internal computer multiplies the amount by 2100 -- using the average ratio of alcohol in blood to alcohol in breath -- to estimate the amount of alcohol in the suspect's blood. Problem: We are not all average. And ratios vary from 1300:1 to 3500:1."

Timmie Lance McNeal appealed his conviction for driving after his breath test showed a reading of 0.10, but the lower court refused to consider scientific evidence that at least one-third of the population has a partition ratio different from the one presumed by the breathalyzer machine. For that reason, he argued, he should not have been automatically presumed to be under the influence based solely on the machine's readout. McNeal was convicted.

In 1989, the state legislature codified the partition ratio of 2100:1 and made it a "per se DUI" offense to have a breath test reading of .08, regardless of whether the motorist was actually intoxicated or not. This crime would be treated separately from "general DUI" which requires a motorist actually to be intoxicated.

"The legislature passed section 23152(b) to facilitate the prosecution of drunk drivers," Justice Carol A. Corrigan explained in the unanimous ruling. "The creation of a per se DUI offense did away with the need to prove the defendant was actually impaired."

The court admitted that the legislature's presumption of a certain ratio could, in some cases, result in incorrect measurements.

"If, however, the defendant's own partition ratio at the time of testing is lower than the standard ratio, conversion of the breath result using the statutory formula produces an artificially high measure of his blood alcohol," Corrigan wrote. "2100-to-1 ratio overstates the actual amount of alcohol in his blood. For someone with an extremely low ratio of 1100 to 1, for example, use of the 2100-to-1 partition ratio would overstate blood-alcohol content by almost 50 percent."

With this in mind, the supreme court held that partition ratio evidence may now be raised as a defense to a general DUI charge. The court, however, in previous rulings made it clear that motorists could be convicted of per se DUI regardless of any scientific evidence regarding actual intoxication. The high court cited Lawrence Taylor as an authority on the subject three times in its decision, but Taylor blasted the decision as irrational.

"It takes a supreme court ruling to allow a citizen accused of DUI to defend himself with established scientific truth," Taylor wrote. "But in a typical retreat from logic, the court limited the admissibility of partition ratio evidence to defending against the charge of driving under the influence -- not to the accompanying charge of driving with .08 percent blood-alcohol concentration (BAC). So you can use scientific facts that the BAC reading is faulty to defend yourself against the BAC-based presumption of being under the influence -- but not against the charge that your BAC was .08 percent or higher."

A copy of the decision is available in a PDF file at the source link below.