Every three to six months
One study found that 16 species of fungi can live in an average pillow, and the stuffing actually attracts allergy-causing dust mites. So to ensure you're only laying your head down in a clean space, wash your pillows a few times throughout the year.
1. Use the gentle cycle on your machine, hot water and liquid detergent (powder can leave a residue). Wash two regular-size pillows together to balance the load (king-size ones should go one at a time).
2. Run the rinse cycle twice.
3. Then, to dry them, place pillows in the dryer with two clean tennis balls, and set on low. Polyester-or cotton-filled pillows take one to two hours to dry; down requires two hours or more (check for moistness or clumps by pinching several spots on the pillow.)