What type of contaminants are typically found in air duct systems?

The correct answer focuses on the common contaminants that are typically found in air duct systems, which include dust, mold, pollen, and allergens. These particles can accumulate over time in ductwork, especially in environments where humidity levels fluctuate or where there is insufficient airflow.

Dust is prevalent in indoor environments and can originate from a variety of sources such as human activity, pets, and construction materials. Mold can develop in damp areas within duct systems, particularly if there's a moisture problem. Pollen is frequently introduced into indoor spaces from outside through open windows or doors and can contribute to allergy symptoms. Allergens from pets, dust mites, and other biological sources can also settle in ductwork, potentially affecting indoor air quality and health.

Other choices like chlorine and fluoride, heavy metals and pesticides, or carbon monoxide and radon represent contaminants that are less common in air duct systems specifically. While they may appear in various environmental settings, they do not typically accumulate within HVAC systems in the same way that the typical biological and particulate contaminants do. Thus, focusing on the typical sources of household dust and biological growth offers a more accurate reflection of what is usually found in air duct systems.