Attacking the Breath Tests

If you are stopped by a police officer for driving under the influence of alcohol, you may be asked to blow into a Preliminary Alcohol Screening (PAS) device. You may assume that the breath test results are generally unimpeachable. What you may not know is that the measure of your blood alcohol level resulting from a breath test may result in such an overestimated blood alcohol level that you are arrested based on an inaccurate test. The science of breath testing is far from infallible and often based on faulty assumptions.

The reason for this disparity is founded in the process of converting a sample of breath from a PAS device to one’s blood alcohol concentration (BAC) under California DUI law. California breathalyzer machines apply a nationally accepted scientific formula known as “Henry’s Law,” which is used to convert the amount of alcohol vapor expelled from a person’s lungs into a blood alcohol level. This conversion is done by way of a mathematical constant known as a “partition ratio.” The law mandates the partition ratio be calculated at 2,100-1 under California DUI law, which means the amount of alcohol in 2,100 milliliters of breath is presumably equivalent to the amount of alcohol in one milliliter of blood.

Partition ratios vary widely throughout the population and fluctuate for an individual from time to time. Although California DUI law establishes the partition ratio at 2100-1, actual partition ratios can range from 1500-1 to 3400-1. If a partition ratio falls below 2,100-1 at the time of the breath test, the test will overstate a person’s true BAC. The difference between the partition ratio presumed under California DUI law and a person’s actual partition ratio can mean the difference between a conviction and an acquittal on DUI charges.

A driver who is arrested under California DUI law is usually charged with two separate counts (1) Driving Under the Influence of Alcohol (“generic DUI”) and (2) Driving with an 0.08% BAC (“per se DUI”). Traditionally, a person charged with DUI could not challenge the breath test based on the submission of partition ratio evidence. In a decision on July 9, 2009, the California Supreme Court in People vs. McNeal indicated that partition ratio evidence is admissible to challenge a generic DUI charge. A number of factors that can affect partition ratios are listed below:

  • Temperature: The standard partition ratio of 2100-1 is based on a normal body temperature of 98 degrees Fahrenheit. If a person is ill or has an elevated body temperature for some other reason, this may cause their BAC breath test result to be artificially inflated. A one-degree increase in body temperature will result in a 7% higher level in the BAC result. This means that a person who is running a fairly low-grade temperature of 100.4 Fahrenheit with an actual blood alcohol level of 0.0935% will yield a 0.10 BAC. This example shows how a slight fever can mean the difference between being arrested and taken to jail and driving home.
  • Cellular Composition: Blood is only partially a liquid as it also contains suspended cells and proteins. The 2100-1 partition ratio assumes a hematocrit level (cell volume of blood) of 47%. Actual levels can range leading to a variability of up to 5%.
  • Absorptive Phase (Time Lapse): During the absorptive phase, the gastrointestinal tract is still absorbing alcohol. The partition ratio would be overestimated at this phase leading to an artificially inflated BAC.

Other factors that may affect the variability in a person’s partition ratio include medication, environmental factors such as lung temperature and air humidity and perhaps even Radio Frequency Interference (RFI).

Our experienced Los Angeles DUI Attorneys can challenge breath test results based on partition ratio variability. If you or someone you love has been arrested for DUI, you should consult the Reynoso Law Firm today for a FREE consultation.   Because scientific evidence must be generally accepted in the scientific community to be admissible, you may need experienced Los Angeles attorneys to make sure that any proposed partition ratio evidence or testing is admissible. Our Los Angeles County Attorneys have extensive experience and proven results in defending clients who have been arrested for DUI.

Los Angeles DUI Attorneys

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