Most people regularly wash their sheets, pillowcases, and blankets. The mattress underneath them, however, may go years without receiving more than an occasional vacuuming. That is understandable. A mattress is large, heavy, absorbent, and difficult to clean with ordinary household equipment. It also spends most of its life covered, which makes it easy to forget. Even beneath sheets and a mattress protector, a mattress can gradually accumulate dust, body oils, skin particles,