BAC stands for blood alcohol content, which is a measure of the amount of alcohol in a person’s system. Blood alcohol content means the grams of alcohol per 100 milliliters of blood, not an exact percentage, contrary to popular belief. The legal limit for BAC in Illinois is 0.08 for drivers aged 21 and over. The legal limit for BAC is 0.04 for commercial driver’s license holders operating a commercial vehicle. The legal limit for BAC is 0.00 for drivers under 21 years old.