Grey mould on walls and ceilings
Usually a maturing colony or a mixed community rather than a distinct species.
2 of the 5 likely answers here are not mould at all.
The species matters less than you think
- Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
- “Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
- Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.
Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.
The 5 likely answers
1.Black Dust Around Vents
Not mouldGrey-black staining fanning out from air vents, or in fine straight lines on ceilings and walls that trace the framing behind them. Uniform, flat, and following an obvious airflow or structural pattern.
How to know it is this one
Follows airflow or the framing pattern behind the surface, in straight lines.
What it means: Poor filtration, candle or cooking soot, or thermal bridging where insulation is missing. Worth fixing for the staining and the cold spots, not for health.
More on black dust around vents →2.Efflorescence
Not mouldA white or off-white crystalline crust on brick, concrete or masonry, sometimes fluffy and needle-like. It dissolves in water and often reappears elsewhere as the wall dries.
How to know it is this one
Dissolves in water. Mould does not — this is the definitive test.
What it means: Water is migrating through the masonry. The deposit is harmless but the moisture behind it will eventually support real mould on adjacent materials.
More on efflorescence →3.Dry Rot
InvestigateWhite or grey cotton-wool sheets over timber, with grey-white strands that can run across masonry, and sometimes a flat rusty-orange fruiting body with a wrinkled surface. Affected wood darkens, shrinks and cracks into cubes.
How to know it is this one
Wood cracks into cube-shaped blocks across and along the grain.
What it means: Structural timber decay. This is a building surveyor and a builder, not a cleaning product. It needs the moisture source fixed and affected timber cut out well beyond the visible damage.
More on dry rot →4.Alternaria
RoutineDark grey-brown to black with a distinctly velvety or downy texture and slightly raised, woolly growth. Common in showers, under sinks and around window frames.
How to know it is this one
Velvety, slightly woolly texture with a raised profile.
What it means: Standing water or a slow leak. Find the drip before cleaning the growth.
More on alternaria →5.Cladosporium
RoutineOlive-green, brown or near black with a suede-like velvety texture, growing in discrete rounded patches that merge. Dry to the touch and powdery when disturbed.
How to know it is this one
Velvety, suede-like texture that is dry rather than slimy.
What it means: Relative humidity persistently above roughly 60%, or surfaces cold enough to condense. Ventilation and a dehumidifier address it.
More on cladosporium →Still not sure?
The key adds texture and surface, which separate these far better than colour does.
Identify it step by step