Mould in air conditioning
Cooling coils and drip pans are permanently wet by design, so some growth is expected. Blocked condensate drains are the usual cause when it gets bad.
Start from the colour
Colour is unreliable on its own, but it is how people search — and each of these narrows to a short list.
What it is likely to be
Things that are not mould are listed first.
Follows airflow or the framing pattern behind the surface, in straight lines.
Powdery, granular surface that puffs when disturbed.
Velvety, suede-like texture that is dry rather than slimy.
Blue-green colour with a white growing edge.
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.
Narrow it down
Already filtered to air conditioning or ducts. Add the texture and surface.
1.Where is it?
Location narrows things faster than appearance, because each part of a house has its own moisture problem and its own usual suspects.
2.What does it feel and look like?
optionalTexture is far more diagnostic than colour. Slimy, powdery and crusty point in three completely different directions — and crusty usually means it is not mould at all.
3.What is it growing on?
optionalMaterial matters. Nothing grows on bare masonry that also grows on bread, and a white crust on concrete is almost certainly a mineral deposit.
4.What colour is it?
optionalLast, and deliberately so. Dozens of unrelated moulds are black, and colour changes as a colony matures — it is the least reliable thing about mould and the thing people lead with.
- Black Dust Around VentsNot mouldSoot, dust and ghosting
- AspergillusInvestigateAspergillus species
- CladosporiumRoutineCladosporium species
- PenicilliumRoutinePenicillium species