"bob" wrote in message
...
Due to allergy I installed a filter cabinet and a merv 12 4-inch filter
in my furnace. I thought I'd be getting very clean air.

To verify, I cover two floor registers with furnace filters. After a few
weeks these filters turned dark grey and can be seen from either side of
the filter. I flipped it over and there is no dust accumulation.

Is this normal? Where do the dark fine particles come from?

When did you last have your ducts cleaned out ?

You might want to have a closer look at those dust particles under
magnification to have a better idea of their source.