What is BAC?

You may have seen or read about “BAC” when doing your research on Driving Under the Influence, drunk driving, blood alcohol level, and chemical test results, but maybe you have some further questions about BAC.

BAC stands for Blood-Alcohol Concentration. The alcohol concentration in your blood can be shown in different ways. The most common way is weight per volume, or grams/volume of blood. In California, this is shown as grams per 100 milliliters of blood, gram % or percent (W/V), and g/210 L breath.

In the late 1990s, Congress established a grant program, requiring states to set the legal limit of .10 percent BAC for the first three years in order to be eligible. To continue to qualify after the first three years, the states had to lower the limit to 0.08 percent BAC.

In 2000, Congress passed a law that said the states must pass their own 0.08 BAC laws by 2004 or lose federal highway construction funding.

The movement of alcohol in the body, its absorption into the bloodstream, its metabolism, and its elimination are all dynamic processes. However, in California, there is a presumption in Vehicle Code Section 23152b that the driver has the same BAC level at the time of driving as at the time of the blood or breath test if the test is done within three hours after driving. Although there is little science to justify this presumption, it still applies in court and at the DMV. It is just as likely that your BAC may be rising, falling, or staying the same over the same time period.

BAC Laws in California

When the prosecutor charges you with a DUI, the BAC is significant evidence against you. Vehicle Code Section 23152 says it is illegal to drive with a BAC of 0.08 percent. Vehicle Code Section 23140 makes it illegal for someone under 21 to drive with a BAC of 0.05 percent or more. VC 23136 says it is unlawful for a person under 21 to drive with a BAC of 0.01 percent or more. VC 23153 makes it illegal to drive with a blood-alcohol concentration of 0.08 percent, or higher and cause injury to someone else. VC 23152 (d) and (e) make it illegal for “passenger for hire” drivers to drive with a BAC of 0.04 percent or more.

DUI Attorneys often consult with forensic toxicologists, who are experts on BAC. These discussions cover estimates about what a drinker’s possible BAC might be when the person was driving and/or taking the chemical tests (breath or blood tests). There are formulas used for these estimates, and there are also factors and assumptions that can have an effect on the calculations.

Many experts warn of the dangers associated with what is called “extrapolating” to past BAC levels. If all that is known is the time you were pulled over by the police and the chemical test result, Dr. Kurt Dubowksi, the University of Oklahoma Distinguished Professor of Medicine and Director of Toxicology Laboratories at the University of Oklahoma, says that “no forensically valid forward or backward extrapolation… is ordinarily possible.” Distribution and Elimination of Alcohol: Highway Safety Aspects; Journal of Studies on Alcohol, 1985, p. 106.

It is sometimes useful to estimate how many drinks someone would have to drink to be at a certain BAC. Wine is generally 12% alcohol, beer is 4%, you can say that 4 ounces of wine (glass of wine) has the equivalent alcohol content to a 12-ounce beer. Furthermore, 1.25 ounces of 80-proof liquor (40% alcohol) is equal to 1 ounce of 100-proof liquor (50% alcohol).

A BAC of 0.15 percent and 0.20 percent will cause the prosecutor to file sentence enhancements for high BAC asking the judge to increase the penalties under California law. Vehicle Code 23578. VC 23538(b)(2).

There are potential challenges to BAC evidence:

Attacks on Breath Tests

If you took a breath test while in the absorptive phase, your breath alcohol concentration (BrAC) would be higher than your BAC. Simpson, G. Accuracy And Precision of Breath Alcohol Measurements in the Absorptive State, Clinical Chemistry, 1987.

There can also be alcohol contamination in your mouth by recent drinking, regurgitation, burping, or vomiting. If the officer failed to properly administer the breath test to you, the BAC results are unreliable.

The breath sample shall be collected only after fifteen continuous minutes during which time the subject must not have ingested alcoholic beverages or other fluids, regurgitated, vomited, eaten, or smoked. 17 CCR 1221.1. A DUI Defense attorney should also review the calibration records of the breath machine to see if it was properly maintained.

Attacks on Blood Tests

If you took a blood test, DUI Defense Attorneys attack the collection, transportation, and storage of your blood sample.

Was the person who took your blood qualified to draw your blood? Was your blood taken in a reasonable manner according to accepted medical practices? Was your sample properly preserved? Alcohol in the blood vial can be produced by fermentation. The device analyzing your blood cannot tell where the alcohol came from. Therefore, the lab report may not reflect your actual BAC.


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