The concentration of CO in a garage with a running car can quickly reach dangerous levels. In one study, the CO concentration in a garage with a running car reached 1,000 parts per million (ppm) within 10 minutes. The National Institute for Occupational Safety and Health (NIOSH) recommends that the CO concentration in workplaces not exceed 50 ppm.
If you fall asleep in a garage with a running car, you may not be able to wake up before CO poisoning occurs. It is important to never leave a car running in a closed garage, even for a short period of time.