If you are under 21 in Illinois, you cannot drive with any amount of alcohol in your system. This law is called "zero tolerance." In other words, if you are 19 years old and get behind the wheel after just one drink, you could lose your driving privileges. Zero tolerance, as the name suggests, means that even a BAC of .01 can result in license suspension.
Less than .08 BAC
If you are caught driving with a BAC of less than .08, but more than .05, you can still be charged with driving under the influence. In these cases, it depends on your actions and the discretion of the police officer who pulled you over.
BAC of .08 and above
Again, the general rule is that driving with a BAC of .08 or higher results in DUI charges. Your license will be suspended, and you face losing your license for a year as well as jail time and fines. The penalties increase for subsequent offenses. The penalties are greater if you are under 21; you will lose your license for two years.
If you hold a commercial license, your BAC cannot be above .04. This applies to truck drivers, for example. If you are a school bus driver, there is zero tolerance while driving a bus.
If you are caught driving with an extremely high BAC, such as .20, you face enhanced penalties. Driving with a BAC above the legal limit when you have a child in the car also can result in enhanced penalties, such as longer jail time and higher fines.