Mildew commonly occurs in carpeting, on walls, around doors and windows, and in bathroom and laundry areas. Cut an onion in half and place it on a plate on the basement floor. Onions work great at absorbing mold and mildew smells. Two slices of bread placed on a dish in the basement overnight will absorb smoke smells, mildew, stale air and animal odor. Place a dish of it on the floor or hang it from a bag year round to help keep your lower level odor-free.
Millions swear by the power of common household baking soda.
Simply open container and place out of sight in the basement for up to three months at a time. Baking soda can also be applied directly to stubborn smells and problem areas. Once odor has seeped into carpeting, it is difficult to remove. If carpets have been severely damaged , it's best replaced. For more minor carpet odors, regular cleaning and humidity control will go a long way in treating the problem and preventing its recurrence.