Blood Alcohol Content (BAC)

Blood Alcohol Content (BAC) and Blood Alcohol Concentration (BAC) are terms commonly used to refer to the amount of alcohol found in an individual's blood. These levels are measured in either percentage by mass or mass per volume. A Blood Alcohol Content (BAC) of 0.20% means that there is 1 gram of alcohol per every 500 grams of blood.

There is a difference between Blood Alcohol Content (BAC) and Breath Alcohol Concentration (BrAC). Breath Alcohol Concentration (BrAC) is utilized most often by law enforcement to extrapolate or estimate an individual's Blood Alcohol Content (BAC). In order to Ftruly determine Blood Alcohol Content (BAC), a blood sample must be obtained. Since it is very difficult to provide medical personnel to obtain this sample in field situations (while the officer is determining if an individual is Driving While Intoxicated (DWI) or Driving Under the Influence (DUI)), breath testing devices are utilized to determine Breath Alcohol Concentration (BrAC) which typically can be used to equivalate Blood Alcohol Content (BAC). Breath testing devices rely on an a ratio that equates the parts of alcohol found in breath to the correlating amount of alcohol found in blood.

Legal Limits

Law enforcement utilizes Blood Alcohol Content (BAC) to define intoxication and provide a rough measure of impairment. Due to the relative simplicity in measurement, Blood Alcohol Content (BAC) is utilized versus attempts to determine individual impairment. This places the burden of proof upon the individual arrested for Driving While Intoxicated (DWI) or Driving Under the Influence (DUI) to show that either their Blood Alcohol Content (BAC) was lower than the legal limit or that they were not impaired despite the Blood Alcohol Content (BAC).

All states in the US have adopted .08 as the legal limit for intoxication. Some US states will arrest individuals driving with Blood Alcohol Content (BAC) of less than .08. Additionally, some states have zero tolerance laws that apply to individuals under the age of 21 years old and are designed to allow law enforcement to arrest these individuals at substantially lower levels of Blood Alcohol Content (BAC).

Test Assumptions

Blood alcohol tests make several assumptions about the individual being tested. While these assumptions are averages that typically apply to most individuals, they most certainly will not apply to all individuals. One critical assumption is the ratio of Breath Alcohol Concentration (BrAC) to Blood Alcohol Content (BAC). This ratio (called the "partition ratio") is 2100:1. However, the actual ratio can vary substantially by individual from 1300:1 to 3100:1. These actual ratios can have a substantial influence on the accuracy of field tests and an individuals actual level of intoxication. For example, an individaul tested with a breath testing device who had a actual Blood Alcohol Content (BAC) of .08 and a partition ratio of 1700:1 would test at a .10 level on the breath testing device. Only a qualified and competent DWI or DUI defense attorney can provide the legal assistance and defense to address whether or not the estimated Blood Alcohol Content (BAC) determined by the breath testing device was correct.