How to clean your mattress

 The original link is broken, but I found this gross, disgusting detail at LiveScience about what's going on in our mattresses. "According to materials published by Ohio State University, a typical used mattress may have 100,000 to 10 million mites inside. Ten percent of the weight of a two-year-old pillow can be composed of dead mites and their droppings. Mites prefer warm, moist surroundings such as the inside of a mattress when someone is on it. One of their favorite foods is dead skin, and people shed about one fifth of an ounce of the stuff every week, some of which surely ends up flaking into your mattress. (Also gross: About 80 percent of the material seen floating in a sunbeam is, in fact, flakes of dead skin.)"

May be easier to buy a new one, or hire someone to do it. Consumer Reports suggests: https://www.consumerreports.org/mattresses/how-to-clean-a-mattress/