Hello Joan:
There are brush hairs, cat hairs, and a variety of dust, ashes, et.al in most old master paintings. I saw what I thought might be a cat hair in a Rembrandt - I am pretty sure he had cats. Clearly it is an age old problem. I now paint on a slightly textured board, so the dust is not quite as visible. For a couple of years I tried to get a glass smooth surface on my paintings and it drove me crazy because there was always that one little proud spot in the dark area that caught the light...
Nowadays I still follow a habit I got into from those paintings - I begin each session with a picking and smashing bit where I use my palette knife to pick off, or smash in as many of those annoying spots as I can. And, as I mentioned, I now paint on a textured surface so a lot fewer of them show or are noticable.
That said, I recommend an air filter in your room - helps to keep out the dust. Some even use drying boxes - too complex for my taste, so I just pick and smash.
