Efflorescence
Not mouldMineral salt deposit · also called salt bloom, white powder on concrete
This is not mould. It is included because it is mistaken for mould constantly, and recognising it saves an unnecessary remediation.
Efflorescence is not mould and is not alive. It is mineral salt carried out of masonry by water moving through it, left behind when the water evaporates. It is the single most common false mould report in basements. What it does tell you is important: water is moving through that wall.
Quick facts
- Appearance
- A 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.
- Moisture
- Needs surfaces that stay genuinely wet
- Smell
- None.
- Colours
- white, grey
- Texture
- crusty, powdery
- Surfaces
- concrete, drywall
How to recognise it
- Dissolves in water. Mould does not — this is the definitive test.
- Crystalline and gritty between the fingers, sometimes needle-like.
- Only on masonry, concrete and brick, never on wood or fabric.
- No musty smell at all.
What it tells you about the building
Water is migrating through the masonry. The deposit is harmless but the moisture behind it will eventually support real mould on adjacent materials.
What it gets confused with
One observation settles each of these.
Drop water on it. Efflorescence dissolves; mould does not and often beads water.
Where it turns up
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.