What measurement unit is primarily used to determine Blood Alcohol Concentration (BAC)?

The primary measurement unit used to determine Blood Alcohol Concentration (BAC) is percentage by volume. BAC is expressed as the percentage of alcohol in a given volume of blood, which indicates the level of alcohol present in a person's bloodstream. For example, a BAC of 0.08% means that there are 0.08 grams of alcohol per deciliter of blood. This unit is widely recognized for assessing intoxication levels and is the standard in many legal contexts related to driving under the influence. This measurement is crucial for law enforcement and medical professionals when evaluating impairment and making decisions related to safe driving and potential legal ramifications. Using percentage by volume provides a clear, universal understanding of alcohol in the bloodstream across different jurisdictions, making it the appropriate answer in this context.