Gas fume smell coming from air vents

So I have been having this problem for a while now. Whenever I have the air or heat on in the car, the smell of gas fumes comes through the vents, it stinks really bad. It’s only when the ac/heat is off or its on recirculation that the smell doesn’t come through. Also, when standing outside even a decent distance away from the car this strong smell is noticeable. One other symptom is a loud humming towards the front of the car. Since this is most likely an exhaust problem, could it mean I have been breathing in carbon monoxide or basically huffing gasoline fumes? How toxic to the health of the driver/passengers is this car problem?