Fresh Air vs Clean Air
Unknowingly, there is a difference between fresh air and clean air but both these terms are being used interchangeably to mean the same effect on indoor air quality for the majority. Basically, the main difference would be the level of Oxygen / Carbon Dioxide and fine particles / impurities in the air.
Fresh air is normally felt outdoors. If the surroundings are filled with trees and plants, this is where you feel that the air you breathe in feels so refreshing. Reason being is that the amount of oxygen is far more than carbon dioxide. Green plants use carbon dioxide to produce oxygen through photosynthesis, and that is how you feel fresh air when surrounded by plants. While staying indoors, humans produce carbon dioxide through their normal respiratory process and at the same time reduce the level of oxygen. If there is no air ventilation for the room, this will increase the level of carbon dioxide and reduce oxygen level.
Unlike fresh air, clean air is felt when the air you breathe in does not contain impurities like fine particles, dust, mold and bad smell (Volatile Organic Compound, VOC). It would be impossible for a regular household to achieve a completely clean environment indoors as air flows freely. The best that one can do is to minimize the amount of impurities with the use of an air purifier. Some might feel that by closing the doors and windows from the outdoors will stop impurities from coming into the house but how many of us have no answer to that when there are still dust build-ups even with all the doors and windows closed.