Short answer
Gray/black mold is often a condensation phenomenon caused by insufficient air renewal or air treatment, sometimes worsened by another source of moisture.
Where to spot it
Typical areas are wall/ceiling corners, upper walls, cupboards, and behind furniture.
These locations often point to a combination of humid air and cold or poorly ventilated areas.
Condensation or another cause?
Condensation may be the main cause, but it may also be worsened by rising damp or water damage.
You should therefore avoid drawing conclusions based only on the color of the marks.
Do the marks keep coming back despite cleaning?
That is often a sign that the cause needs analysing, not just the surface.
Appropriate response
Air treatment aims to control moisture contained in the air.
Depending on the case, it can complement what already exists and improve conditions in sensitive spaces such as basements.
Frequently asked questions
Is a dehumidifier enough against mold?
That depends on the cause and the volume to treat. You need to distinguish air humidity from moisture in the building.
Is black mold always caused by condensation?
It is a common cause, but it may be worsened by other sources of moisture. Diagnosis helps separate the hypotheses.