2002 Subaru Tribeca gas smell when heater is on

In the winter time when it is below freezing and we warm the car up and turn the heater on, the vents blow a terrible gas smell. It takes 30-45 minutes before the gas smell goes away. It is better if you turn the button on that circulates that air within the car (as opposed to the outside air), but it still has a very strong gas smell. Strong enough to have to open the windows. This only happens when it is below freezing. Any suggestions as to the cause of the problem? Thanks.