Originally Posted by CivicSiRacer

If you run recirculate in your car all the time that's where you get that rotten smell. You have to use fresh air once in awhile to get all the moisture build up in your ventulation system. When you run recirculate all the time your body's moisture and moisture in general builds up and causes mold to grow.

You can also just blast some Lysol into the vents just below the windshield wipers.