Biological contaminants include bacteria, molds, mildew, viruses, animal dander and cat saliva, house dust, mites, cockroaches, and pollen
(see more about Asthma triggers at www.epa.gov/asthma).

There are many sources of these pollutants. Pollens originate from plants; viruses are transmitted by people and animals; bacteria are carried by people, animals, and soil and plant debris; and household pets are sources of saliva and animal dander. The protein in urine from rats and mice is a potent allergen. When it dries, it can become airborne.

Contaminated central air handling systems can become breeding grounds for mold, mildew, and other sources of biological contaminants and can then distribute these contaminants through the home.

Dust, dead skin cells, fibers, and other nutrient rich material accumulate in the ductwork over time as the air is recirculated. As this picture shows the material adheres to the interior walls of the ductwork, thus accelerating the build-up. As moist air returns through the system it combines with the contamination and microbial amplification occurs. Spores become airborne and are 'bumped' out into the living areas every time the fan is activated for heating or cooling. Many of the spores are small enough to bypass most filtration systems.