White mould on walls and ceilings
Often the early stage of something that will darken later, and often not mould at all — efflorescence on masonry is white and crystalline.
1 of the 4 likely answers here is 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 4 likely answers
1.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 →2.Aspergillus
InvestigatePowdery colonies with a granular surface, in an unusually wide colour range — green, yellow-green, white, brown or black — often with a paler edge and darker centre as the colony matures.
How to know it is this one
Powdery, granular surface that puffs when disturbed.
What it means: Persistent damp. If anyone in the household is immunocompromised, has asthma or has a lung condition, this is the case where professional remediation is proportionate rather than excessive.
More on aspergillus →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.Penicillium
RoutineBlue-green or grey-green powdery colonies with a characteristic white fringe at the growing edge, and a texture like fine felt. On food it is the familiar blue-green fuzz on bread and citrus.
How to know it is this one
Blue-green colour with a white growing edge.
What it means: Recent or ongoing water damage, or humidity high enough to keep soft materials damp. Materials that have been wet for more than 48 hours generally need replacing rather than cleaning.
More on penicillium →Still not sure?
The key adds texture and surface, which separate these far better than colour does.
Identify it step by step