The vehicle code, section 23152(b) makes it unlawful to drive with “0.08 percent or more, by weight, of alcohol in [a driver’s] blood.” You might immediately note that most DUIs are charged based on a breathalyzer measurement, not a blood measurement. I previously discussed how breathalyzers measure the alcohol content in a person’s blood but to complicate matters, the alcohol weight in a person’s breath must be translated into an equivalent blood weight measurement . If you continue reading section 23152(b) it states, “percent, by weight, of alcohol in a person’s blood is based upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.”
These different measurements of alcohol by weight depending on whether the test was of blood or breath is known as “partition ratio variability.” The more accurate measurement of BAC is by measurement of alcohol content in the blood. Breath analysis loses some accuracy and though rigorous scientific testing, test established that the average amount of alcohol in 230 liters of breath is equivalent to what would be found in the 100 milliliters of the same subject’s blood. This may vary from person to person on either side of the scale. To allow for possible variations among individuals and to give the benefit of the doubt to the driver, the California legislature, set the conversion at 210 liters of breath to 100 milliliters of blood, or a ratio of 2,100 to one.
Before it was amended in 1990, section 23152(b) defined blood alcohol concentration (BAC) only in terms of “grams of alcohol per 100 milliliters of blood.” Because, then as now, many DUI charges were based on breathalyzer results, the breath results had to be converted using the standard 2,100 to one ratio. But because the partition ratio was not written into the statute, a common DUI defense was based on the effect of partition ratio variability factors. Following amendment of the law, the California Supreme Court held that the amended language was meant to criminalize driving with the specified blood alcohol level or the specified breath-alcohol level. In other words, the amendment did not simply provide an alternate method for calculating BAC but was meant to make it unlawful to drive with an .08 percent or more BAC as measured per 210 liters of breath.