The black dust on the outside of your window sill could have multiple causes. Without further information, it's challenging to determine the exact cause. However, here are a few possible explanations:
Mold or Mildew: If the black dust has a fuzzy or powdery texture and appears in damp areas, it could be mold or mildew. These fungal growths thrive in humid environments and can develop on surfaces that are exposed to moisture. Ensure proper ventilation and address any sources of moisture to prevent mold or mildew growth.
Dirt and Pollution: If your window sill is exposed to outdoor elements, the black dust could be a combination of dirt, dust, and pollutants from the surrounding environment. Traffic, construction, and industrial areas can contribute to the accumulation of airborne particles on surfaces.
Soot or Smoke Residue: If your window is near a source of combustion, such as a fireplace, wood-burning stove, or heavy traffic, the black dust could be soot or smoke residue. These particles can settle on surfaces and create a black, powdery substance.
Insect or Pest Activity: Certain insects, like carpenter ants or termites, can leave behind frass, which is a mixture of their waste and wood particles. Frass can appear as a black, granular substance and may accumulate near window sills if there is insect activity in the surrounding area.
To determine the exact cause and appropriate action, it's recommended to inspect the affected area closely and consider the environmental factors and activities nearby. If you're concerned about potential health hazards or the presence of mold, it may be advisable to consult a professional, such as a mold remediation specialist or a pest control expert, who can assess the situation and provide appropriate recommendations.