The air particles that are visible to you will mostly be caught in your nose and cilia, and either swallowed or spat out. They can still cause plenty of allergies. They are also too heavy to float long enough in the air to pass through the filter. That's why you need to kick it up from surfaces to see. You can remove it from surfaces with vacuum and wiping.
Smaller invisible particles, for example pollution or pollen, hang in the air and move with the circulation, so they will eventually be caught as air passes through the filter.
The amount of these particles could be reduced by cleaning the air at the "intake" of the house. If there is intake air passing through pipes, these pipes need to be kept clean. If the house is set up such that air moves in both direction through passages, the whole thing is fucked and need to be fixed. Even detecting where intake air is entering the house can be really difficult.
Because of indoor temperature and outdoor wind, a house functions like a pump that sucks in outdoor air, heats it, and ejects it. Intake air may be sucked from any crack in the walls, floors and foundation, where it picks up dust and mould. Equipping the house with an intake fan (and filter) will pressurize it and prevent this "sucking".
Smaller invisible particles, for example pollution or pollen, hang in the air and move with the circulation, so they will eventually be caught as air passes through the filter.
The amount of these particles could be reduced by cleaning the air at the "intake" of the house. If there is intake air passing through pipes, these pipes need to be kept clean. If the house is set up such that air moves in both direction through passages, the whole thing is fucked and need to be fixed. Even detecting where intake air is entering the house can be really difficult.
Because of indoor temperature and outdoor wind, a house functions like a pump that sucks in outdoor air, heats it, and ejects it. Intake air may be sucked from any crack in the walls, floors and foundation, where it picks up dust and mould. Equipping the house with an intake fan (and filter) will pressurize it and prevent this "sucking".