The answer to 'why is my air duct stained' is that air ducts can become stained for several common reasons, including dust buildup, moisture condensation, and mold growth. Stains in air ducts are often a sign that the ducts need cleaning or maintenance.
Dust and dirt can accumulate in air ducts over time, especially if the ducts haven't been cleaned in a while. This can lead to visible stains and discoloration. Moisture condensation can also cause staining, as water droplets can pick up and deposit dirt and debris. In some cases, mold growth inside the ducts can cause significant staining and discoloration.