Leaks in air-handlers, such as worn seals around doors or holes in cabinets. We'll discuss the details in a two-part article. The top five are examined here. By far the number-one way to contaminate a duct system is to simply omit filters. This isn't a common occurrence, but when it happens, the duct cleaner has a real mess on his hands.
Dirt stacks up rapidly in the air handler and ductwork and is soon pouring into the building.
I recall one disaster where this occurred in a fiberglass-lined duct system. The lining looked like caked mud. A thorough cleaning was factually impossible. All HVAC systems must be filtered or they will be filthy within a year. Tears in the filters.