I’m not the first, second, or probably even the thirty second person to write about improving indoor air quality problems by using the four P’s approach. It’s a well known thought process in the building science community- not sure if that’s the case in HVAC circles.
The first P is pollutant. In this context a pollutant is something that contaminants the air. It can be one or more of many things. Mold, moisture, particles, animal feces, harsh chemicals, are some of the more common ones in an average home. If you remove the pollutant, there is no longer an IAQ problem.
The next P is pressure differential. If the pollutant can not be removed, creating a pressure differential will prevent it from entering or spreading throughout the occupied area... But you already know this, that’s why you turn on the fan when going to the bathroom, or the range hood while cooking.
Another P is pathway. If there is a pollutant, with an unfavorable pressure differential, there still needs to be a pathway to the occupants for that pollutant to become an issue. Eliminate the pathway and the pollutant can be contained.
The last P, but definitely not least is people. You’ve heard this before- If a tree falls in the middle of the woods does it make a sound? Well, If there is a pollutant with a pathway and a pressure differential but no people, is it really an IAQ problem?
In short, it’s always best to deal with the source of the IAQ problem, but if you remove any one of the four “P’s” there will be an air quality improvement.