[tunap]

As to your 2nd point, go find your indoor blower(furnace closet for split system or outside in a package unit), pull the power supply, then open and take a look inside at the blower wheel. Unless the unit is a less than a year old you will will clearly observe why health risks do exist. If you need more proof see the evap coil & the primordial sludge in the condensate tray/drain. When finished, reassemble, plug power back in & change your air filter.

[LeifCarrotson]

I clean out my evap coils and condenser coils annually, and replace my filter twice a year (for efficiency, not for health). I'm well aware of the dust that my filter removes, which makes my indoor air cleaner than outdoor air. My allergen-sensitive wife proves experimentally that it has a strong positive effect for a couple weeks every spring and fall. And I'm aware of the crud that develops in the condensate tray. There's little crud in my central air system, but when I used a window AC in an apartment and the unit wasn't leveled properly it got pretty bad.

More generally, though, the presence of unattractive substances does not imply a health risk. My yard is, in the most basic sense, an enormous pile of dust, plus a few organics that grow and decay into dust. My home happens to be a few hundred meters from a lovely trout stream, and that stream and its floodplain contain more life from bacteria to algae to fish than any primordial sludge ever had. Neither are dangerous.