My 10+ year old 2000 sqft house is always dusty.
We have no carpets or drapery. The doors and windows are always closed. We have leather and wood furniture. We don't have children. We do have one dog that does shed and she mostly sleeps. I don't use the central AC because I suspect there's dust in the ductwork so I put in several window AC's (happily, my electrical bill plummeted). We don't live in a dusty area (e.g. a desert).
There are dust bunnies under the couch several days after I vacuum and the leather chair that isn't used is covered with a layer of dust within days of being damp cleaned.
So, I bought six( 6) Sharper Image Quadra units. After running them for a week or two, they all picked up near zero particles. When I returned them, the metal blades were still nice and shiny.
I brought back all of the Quadra units and bought four (4) of the multi-fan driven Sharper Image Hybrids. Nearly $2,000. After a week, they didn't collect any dust either and I brought them all back.
I bought five(5) of the severely maligned Sharper Image Ionic Breezes off of eBay. These units actually do collect tons of dust and I have to clean them weekly but the house is still very dusty. However, my chronic sneezing and dripping nose came to an immediate end with the Sharper Image air purifiers.
Then I bought four (4) IQAir HealthPro Plus Air Purifiers ((Amazon.com product link shortened)).
These are the size of small refrigerators. Each one is rated to clean 900 sqft at max fan speed. I leave them running at max speed for over 20 hours/day but the house is still dusty. I have two (2) IQAir running on opposite ends of the open area downstairs plus a Sharper Image unit.
The IQAir units are quite powerful. If you put a small piece of tissue paper on the floor about 6" away from the bottom of the unit, the tissue paper travels across the floor and gets sucked up.
I went on a multi-week cleaning rampage. I tore the house apart and washed everything from ceiling to floor and then did it all over again and again. Call me compulsive or extremely thorough.
I water vacuum wash the floors once/week to ensure that the floors are truly spotless.
Even with the 5 Sharper Image cleaners and the 4 IQAir units, the house is still substantially dusty (small particles and long fibers). Even the filters to the air conditioners have to be cleaned every few weeks.
My latest idea was to turn on all the ceiling fans in each room to prevent dust from settling and let the air purifiers do their job.
Where is all the dust coming from? What more can I do?