On Jan 1, 11:06*am, "bob" wrote:
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?

Possibly dirt that is already in the heating ducts from
before you put the MERV filter in the system?