J3 Now Offering National Air Duct Cleaner's Association (NADCA) Vacuum Test Analysis for Evaluating Particle Levels of Non-Porus HVAC Component Surfaces. Normal building occupation generates a great deal of contaminants and air pollutants, such as dander, dust, and chemicals. These contaminants are pulled into the HVAC system and re-circulated 5 to 7 times per day, on average. Over time, this re-circulation causes a build-up of contaminants in the ductwork. While dirty ducts don’t necessarily mean unhealthy air in the home, school or workplace, they may be contributing to larger health issues or harboring contaminants that could cause serious problems for people with respiratory health conditions, autoimmune disorders or some environmental allergies.