This little fellow is a dust mite. Most of the dust that enters your home stays in. And of the possible 40 pounds that get in each year, up to 15 pounds of it can be trapped in your duct system. And it's much more than common dirt. One ounce of dust contains nearly 42,000 living dust mites.
Dust contains pollen.
One million tons of pollen sail over the U.S. every year from every corner of the world. Dust contains fungi and bacteria that can grow, reproduce and spread throughout your home. It may be germ producing or spore producing fungi, but it does grow. Dirty air ducts provide the ideal environment for these creatures because they are damp and contain bits of dead human and animal skin. They also contain dog, cat and human hair.