The room that we thought would be a good children's room has a strange dusty smell that is impossible to eradicate. And boy have I tried. The smell hits you when you walk into the bedroom as a tingle in your nose. It doesn't smell bad, but it does smell like there is something in the air. It sometimes makes me sneeze a little, and I am not allergic. So...

I washed everything.
I tried airing the room out. This makes the smell harder to detect, but only while the windows are open, and this is not practical in our climate.
I removed the drapes and threw them away, replacing them with those spring-loaded rolling shades that collect less dust.
I removed the carpeting, now it is hardwood floor.
I washed everything again.
Someone told me that the smell might indicate termites. I had a termite inspection. (No termites.)
I installed a new extre-large $350 sharp plasmacluster high-capacity HEPA air filter with an impurity sensor. It gives the room a "green" rating, but everyone agrees it smells dusty in there.
I've tried to control the humidity and the temperature in the room, keeping both down with a/c and a dehumidifier (humidity is now 45%).
I got rid of the bed in case it was harboring dust. I replaced it with a new mattress with an allergy cover.
There is one ceiling a/c vent. Just in case, I have scheduled a professional duct cleaning for the a/c system but honestly the vent looks pretty spotless now. If anything, we all think the dusty smell is BETTER when the a/c is running.
I moved my child's stuffed animals.
There is radiator heat. I washed the radiator very thoroughly.
You can see I'm getting desperate now.
I removed everything from the room.
I removed the fixtures and outlet covers and vacuumed them out and looked around for dust. (No dust, nothing unusual.)
I washed the walls and floors with trisodium phosphate.
I am planning to paint, but as far as I know this is unlikely to do the trick.

The smell is entirely localized to this room.

Does anyone have any ideas? This is REALLY driving me crazy. Obviously.

niftyc

Ceiling - is it plaster or a painted fiberboard? What is above the ceiling?

Old cellulose insulation or paper lining in the walls could be a source. Both the fiberboard and cellulose is basically paper and paper breaks down to dust. (I've taken some down where the insides of the walls were lined/stuffed with newspapers....)

When I've torn out old walls and ceilings I have been amazed at how much natural dust has collected in these cavities just through years of infiltration when the wind blows dust around outside. (100 years old....its lived through lots of dust storms. This room could have more than others just due to some small openings and direction of the wind.)

I'd certainly paint before I tore out walls and ceilings...Maybe use a sealer under the paint.

I don't know how the air filter has an 'impurity detector' - but air cleaners are fine.

I think you can be fairly certain you've narrowed it down to the room, cleaning your ducts is still a good idea, but the room smelling better when the a/c is on suggests it IS the room, because you're replacing the air in the room, hence it smells better.

Maybe put one of those UV air cleaner/steriliser things in your furnace air return (they go before the filter) - they're not that expensive....

Inspect your ceiling cavity for signs of mould - is there any? Is this room on the upper floor, under the roof? You might have bats or something, bat/rat messes can smell 'dusty' - make sure roof cavity is adequately vented - install a whirlybird or fan or similar, and make sure the roof itself is in good condition, a leaky roof might cause some of these issues too.

Hope you get to the bottom of it.

Sealing walls and sealing would be a good start.

As for the cuddly toys, my daughter had dust issues when she was small, still does but the allergy specialist wanted her to throw out all her fluffy toys which of course she didn't want to do, so the alternative is to vacuum them with a high efficiency HEPA type vacuum, then put them in the freezer overnight, in a plastic bag ( you can wash them too of course, but not before you put them in the freezer!) - the cold will kill dust mites.