What is the grey mould in air conditioning?

There are 2 likely answers: Black Dust Around Vents, Cladosporium. Follows airflow or the framing pattern behind the surface, in straight lines.

Is grey mould in air conditioning dangerous?

Black Dust Around Vents is not mould at all. For anything that is, public health guidance is to remove it regardless of species and fix the moisture behind it.

Grey mould in air conditioning

Usually a maturing colony or a mixed community rather than a distinct species.

1 of the 2 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 2 likely answers

1.Black Dust Around Vents

Not mould

Grey-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.

2.Cladosporium

Routine

Olive-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.

Still not sure?

The key adds texture and surface, which separate these far better than colour does.

Identify it step by step