Living in bedding, carpet, and upholstery, as well as the forty pounds of surface dust the typical household accumulates each year, dust mites are so small that 1,000 of them can live in a single gram of dust. Unless you live in the desert or atop a mountain, your home likely shelters millions of these creatures who live off of the 50 million skin flakes each of us naturally loses each day. Worse still, we're not actually allergic to the mites themselves but to the waste particles and body parts they shed each day into the air we breathe. It's a primary cause for the development of these problems. It's a trigger.
Animal dander is a different story.
It consists of the skin flakes animals shed and particles of dried animal saliva from pet hair. These skin and saliva particles contain proteins that can cause allergic reactions. As with mite wastes, this dander can easily become airborne and inhaled. Rounding out our list of heavy indoor allergy hitters are molds. These fungus species can live almost anywhere and, as is the case with the other indoor allergens, it's not the molds themselves that cause allergies in most cases, but the microscopic spores they release into the air to reproduce.