BAC stands for "blood alcohol concentration". It refers to the percentage of alcohol (ethyl alcohol or ethanol) in a persons bloodstream. BAC is a crucial number in cases of operating a vehicle under the influence of alcohol or drugs, as it represents the results of testing to identify the percentage of alcohol in the bloodstream. BAC can be determined through a breath test or breathalyzer, like the Intoximeter EC/IR II, or through chemical testing. The legal maximum BAC limit allowed while driving a vehicle varies by country.