Mattresses are a prime habitat ( we generally spend one third of our lives there if were lucky ). A typical used mattress may have anywhere from 100,000 to 10 million mites inside ( 10% of the weight of a 2 year old pillow can be composed of dead mites & their droppings ).

They don’t carry diseases, but they can cause allergic reactions in asthmatics and others who are allergic to their feces. People sometimes confuse dustmites with bed bugs. See this page about bed bugs and if you are going to a hotel or motel, check this page for tips specific to staying in hotels and motels.

Skin cells and scales, commonly called dander, are often concentrated in mattresses, often harbor large numbers of these microscopic mites. Since the average human sloughs off 1/3 ounce (10 grams) of dead skin a week. That gives dust mites a lot to eat. Cats and dogs create far more dander for dust mites to eat.”

( reference – https://www.ehso.com/dustmites.php )

The average double bed can contain up to 10 million dust mites, although they may not all be alive; a 5-year-old pillow may be 50% heavier as a result of dead dust mites and excrement.

Dust mites are dispersed into the air when a person makes their bed or shakes out their sheets due to their small size and lightweight nature.

( reference – https://study.com/academy/lesson/dust-mites-size-life-cycle-facts.html )