I have a bedroom window that looks out over the east (a backyard with mostly tiles and little plants) with a roller shutter which over the past couple of years has gotten a serious buildup of some sort of black soot, which I can't really figure out where it comes from.

Below are a couple of pictures. Apologies for the reflection of my bedroom, I can't open 2 of the windows and the 3rd has a mosquito screen which I'd rather not remove now that those critters are appearing again.

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This soot covers pretty much the entire side of the shutters and I'm worried it might be more than that based on a question on here about black mold that looks suspiciously like what I have. Only thing is my shutters are plastic, not wood.

If this is black mold, would it be safest to just have the entire shutter plus the casing side of my bedroom replaced?


Looks like fungus of some sort. It grows on many surfaces. Try household cleaners and a sponge.

There are many types of mold, and the hysteria surrounding them is often unwarranted. Mold spores are everywhere, and only a very specific type is potentially dangerous, and only in significant quantities.


Basically, if it can be cleaned up it's not a serious issue. You wouldn't throw away an item that can be wiped clean.

  • +1 If these shutters are outside (which I think they are), then just take them off and clean them. Wear a mask if you are really paranoid about it. – JimmyJames Apr 17 '18 at 14:26

Clean it up with bleach and a sponge (chlorine is the main ingredient of mold-cleaners). If your home is humid (RH >55%) consider installing a dehumidfier so with less moisture you get less mold.


This is indeed black mold. We had the same problem at home because:

  • A vent is built between the shutter box and our bedroom.
  • The shutter box wasn't insulated at all: the lath was cold and had a large surface in a small volume. It was basically a perfect condenser.
  • Our HVAC wasn't properly set up and the mechanical ventilation wasn't strong enough to stop the warm, humid indoor air flowing through the shutter box.
  • Condensed water and mold were trapped inside the box.
  • Whenever the outside air would come into the bedroom through the vent, it was already humid and filled with mold spores. It was really hard to keep the relative humidity below 60% and I developped a mold allergy.

It might be worth it to ask an HVAC expert to see if your problem isn't a structural one.

The box got insulated, we cleaned the laths and we installed better mechanical ventilation; we heated a bit more and we ventilate the whole flat more often. We don't have any problems any more.


A microscope examination of scrapings by a qualified analyst would settle the question of whether this is mold or not. The only other thing that comes to my mind is graphite powder. Someone might have used graphite powder to lubricate the mechanism of the shutter.

If you want to do some testing yourself, you could scrape some of the black material off the shutter and into a glass dish. Then put a drop of chlorine bleach on it. The black color of mold (or any organic dye) will be bleached by the chlorine bleach whereas mineral pigments will not be bleached out.

Another way to test the black substance is to wet a Q-tip and repeatedly wipe the black substance onto the Q-tip, then dip the Q-tip in a small amount of concentrated bleach in a little dish. Wait and see if the black color is bleached out.

  • 1
    Or you simply spray it with chlorine disinfection *vent properly while doing so!) and see what happens :-) – yo' Apr 17 '18 at 13:54
  • 1
    Hydrogen peroxide is another way to check, if you'd rather not mess with chlorine unnecessarily. It will foam up spectacularly on contact with mold, as opposed to inorganic matter. – junkyardsparkle Apr 17 '18 at 19:38

Do you burn a lot of candles? Candle soot is a real nuisance, especially those heavily scented ones in jars. I quit burning candles when I begam experiencing soot stains around the house. I'm afraid I don't have any suggestions how to clean your shutters, but once you do get them clean (or replace them with new ones), please read this: Candle soot

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