Alternaria
RoutineAlternaria species
Alternaria is one of the most significant airborne allergens there is and a well-documented asthma trigger. Indoors it turns up wherever water sits — shower corners, around leaking taps, on damp window frames. It is also abundant outdoors, so spores enter constantly and colonise any surface that stays wet.
Quick facts
- Appearance
- Dark 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.
- Moisture
- Needs surfaces that stay genuinely wet
- Smell
- Musty.
- Colours
- black, grey, brown, green
- Texture
- velvety, fuzzy
- Surfaces
- grout, drywall, fabric, wood
How to recognise it
- Velvety, slightly woolly texture with a raised profile.
- Dark grey-brown to black.
- Found where water actually sits rather than merely where air is humid.
- A recognised asthma trigger, so worth removing promptly in a household with asthma.
What it tells you about the building
Standing water or a slow leak. Find the drip before cleaning the growth.
What it gets confused with
One observation settles each of these.
Cladosporium is flatter and drier; Alternaria is woollier and needs genuinely wet conditions.
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.