Scenario 1 - Passive system
You are in an air-conditioned office, classroom or other workplace and the AC return air grille is at one end of the room. You are sitting near that grille. The passive filter systems are contained inside the AC system. Someone down the other end of the room sneezes. All the pathogens in that sneeze, travel through the room on their way to the return air grille, passing over everyone in the path, meaning everyone in that air stream is now exposed to the bugs. Once in the duct, the contaminated air makes its way to the HEPA filter or UV-C light system where it is treated. It is then sent out into the room again. Once the air leaves the filter, it can be instantly recontaminated.
In a passive system, the only time the air is really purified is the short time it is passing through the filtration system or UV-C light. That is only milliseconds. The rest of the time it can be, or is, contaminated, until it returns to the filter again.
For this type of system to be effective everyone in the space would need to sit right near to the supply air grille, where there is less chance of the air being dirty. Furthermore, passive systems only clean the air and do not clean surfaces. One must ask, how are the surfaces kept clean of pathogens when using a passive system? The answer is obvious…they aren’t. The only way to keep surfaces clean is with manual labour, an expensive and impractical task.
Additionally, the amount of energy needed to force air through a dense HEPA type filter is high and many AC systems are not equipped for that task, resulting in significant cost increases to make them functional.
UV-C lights, as passive systems also have many constraints and limited efficacy unless a select range of criteria are complied with. Those constraints are not practical in the real world.
Contact ‘The Clean Air Company’ if you require further clarification on the efficacy of static UV-C lights in A/C systems.
Active systems however use much newer, more practical and relevant technology.
