I live in an apartment complex in the southeastern United States. The apartment complex is made of brick so is probably fairly old, but my apartment was renovated just before I moved in. It has central electric AC and heat with the combination AC/heat unit inside a vented closet in a wall.

I recently began to notice small black pieces of crumbly material on the floor and furniture underneath the ceiling air ducts. Today, I removed the register from one of the ceiling vents and noticed that the ductwork inside is very dirty. Some observations:

  1. Most of the inside is covered in white material. I wonder if they left this vent uncovered whenever they were preparing/painting the ceilings.
  2. Some of these white dots appear to have become black.
  3. There is a gap between the metal vent and the ceiling, and there is more black material in this gap. This material has a fuzzy-looking surface.

The pieces I found on my floor/furniture look similar to the black pieces in (2) and (3) so I feel confident this is the source. So my question is this: is any of this material likely mold? My first thought is that it could be insulation from the attic space, but I couldn't imagine why it would be this color. Thanks for your help.

Air duct

Air duct close-up

Gap between duct and ceiling

Removed sample of material

  • 1
    Dirt that sticks to the surface when it is moist with condensation maybe a little mildew. And that piece of fluff is insulation
    – Kris
    Dec 7, 2020 at 23:50
  • Kris, thanks. Is there a reason the insulation is black? I imagine it wasn’t that color when it was new. Dec 8, 2020 at 0:02
  • 2
    Duct insulation usually yellow. It is dirty now because attics are dirty. May need a better sealing job done where duct attaches to the register.
    – Kris
    Dec 8, 2020 at 0:08
  • looks like the same stuff you find on a dirty air filter.
    – dandavis
    Dec 8, 2020 at 7:12

1 Answer 1


The ductwork was open when the Sheetrock was textured. The texture is just gypsum if it is damp it will absorb moisture and becomes a home for mold and mildew. It looks like even the fiberglass insulation has some gypsum spray on it that has started growing stuff.

I use a 3% or higher mix of hydrogen peroxide and water it will kill the mold. I would use a putty knife after spraying and scrape off the bumps of gypsum it comes off easily when wetted.

If you purchase consented hydrogen peroxide remember AAA always add acid to water to prevent an exothermic reaction.

  • Sorry to be pedantic, but acid and water always react exothermically regardless of which is added to which. The safest method is as you described (acid to water). A good explanation of why is here chemistry.stackexchange.com/a/897 That said, hydrogen peroxide is such a weak acid the order shouldn't matter.
    – tnknepp
    Dec 9, 2020 at 17:37
  • Ok to prevent an explosive reaction adding water to concentrated hydrogen peroxide will cause the water to instantly boil and can expel the mix. Adding the acid to water uses the water as a heat sink.
    – Ed Beal
    Dec 9, 2020 at 17:41

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