How do airborne contaminants typically enter air ducts?

Airborne contaminants typically enter air ducts primarily through the return air system and leaks in ductwork. The return air system is designed to draw air from various areas within a building, and this air often contains dust, allergens, and other particulates that are present in the environment. If there are leaks in the ductwork, these can create pathways for outside air, which may carry contaminants into the duct system, exacerbating the buildup within the air handling units and ducts.

Understanding the mechanism of how these contaminants enter the system is crucial for effective air quality management and cleaning. While other options point to possible entry points, such as open windows, water leaks, or movements of personnel, they do not represent the primary and most significant means of airborne contaminant infiltration into air ducts. Focusing on the return air system and ductwork integrity highlights the importance of regular inspections and maintenance to prevent these contaminants from affecting indoor air quality.