Car fumes are dangerous when inhaled because the fumes take the place of oxygen in your blood. A few breaths of car fumes will not kill you but prolonged exposure can cause hypoxia, a dangerous lack of oxygen in the system.
Yes, car fumes can turn lethal very quickly. Cars also put out other fumes that can make you sick such as anti freeze fumes. Carbon monoxide is released when a car sits and idles and in an enclosed place can overcome you before you know it.