Massachusetts has two methods for a prosecutor to prove a DUI case in court. First, the prosecutor can proceed under the per se prong of the Massachusetts DUI statute, Chapter 90 Section 24 and submit evidence that the breathalyzer result was over the legal limit of .08. This is known as the per se law. A second method of proof is to present evidence that the defendant’s operation was impaired by the affects of alcohol to the extent that the defendant could not operate a motor vehicle safely. Under the impairment prong, the evidence consists of the officer’s observations of the motorist’s speech, appearance, balance and coordination as measured by the field sobriety tests and mental alertness as indicated by responses to the officers questions and ability to follow instructions.

The Massachusetts OUI law, Chapter 90 Section 24 provides that in any prosecution evidence of percentage of weight of alcohol in the defendant’s blood at the time of the alleged offense, as shown by chemical test or analysis of breath, shall be admissible and deemed relevant to the determination of the question of whether the defendant was operating under the influence of alcohol.

A major DUI decision was issued by the California Supreme Court in People v. McNeil on July 9, 2009. The California Supreme Court addressed whether a defendant could present evidence on the partition ratio in a case where the Government charged a defendant with both violating the per se prong and impairment prong of California’s DUI law.

The Court explained the science behind breathalyzer testing relying on a treatise from Attorneys Taylor and Taynac from California. The court explained that when a person blows into a breathalyzer machine, the breathalyzer is measuring deep lung air from the alveolar. From this measurement of breath alcohol, a blood alcohol percentage is obtained through a mathematical constant using a theory of chemistry known as “Henry’s Law”. The breathalyzer machines in Massachusetts and throughout the country use a conversion factor of 2100 to 1, which means that the amount of alcohol in 2100 milliliters of alveolar breath is equivalent to the amount of alcohol in one milliliter of blood.

One defense that DUI lawyers pursued in McNeil and that is also available in defending a Massachusetts OUI charge is whether that assumption regarding the conversion ratio of breath to blood alcohol percentage is accurate for the person being tested. The conversion ratio for individuals will vary depending on body temperature, medical condition and sex, as well as a number of other factors. The ratio used by the breathalyzer machine was considered a rough estimate.

DUI lawyers attacked the accuracy of the breathalyzer test that is was unfair to group everyone together on a machine that would make unfair assumptions regarding an individual’s partition ratio. Accordingly, the legislature amended the California DUI law to remove this defense by defining the DUI offense as failing the breathalyzer test despite the assumption of the machine. The law defined the DUI offense as being based on grams of alcohol per 210 liters of breath. Accordingly, the California courts precluded defendants from presenting evidence of the partition ratio on the grounds that the new law made the difference irrelevant.

The McNeil case hold that when the Government is pursuing a conviction both based on the per se portion of the law and the traditional impairment approach, relying on observations, field tests and opinion testimony, then the defense can introduce evidence challenging the assumptions behind the breathalyzer and evidence regarding the partition ratio. Significantly, the court indicated that it would allow evidence of the general impact of the partition ratio not related specifically to the defendant into evidence.

Partition ratio evidence has never been ruled inadmissible in Massachusetts and given the language of the Massachusetts OUI statute it would be difficult to foresee Massachusetts courts excluding the evidence from the jury. A partition ratio defense could be presented by retaining an expert to offer an opinion as to what a particular individuals breath to blood alcohol conversion ratio is or it can be used to attack generally the theoretical foundation behind breathalyzer testing and the fact that the machine is making generalized assumptions to obtain a specific reading. The theory behind a partition ratio defense is to convince the jury that the standard formula assumed by the breathalyzer machine overstates an individuals blood alcohol content. This will occur when an individual has a lower partition ratio than assumed by the breathalyzer machine.

If you were arrested for DUI in Massachusetts and took a breathalyzer, call Attorney DelSignore for a free Massachusetts OUI consultation. Attorney DelSignore will explain the defenses to the breathalyzer and review your cases to discuss your defenses and ways you can win your DUI case.