What is the brown mould on window sills?

There are 2 likely answers: Alternaria, Cladosporium. Velvety, slightly woolly texture with a raised profile.

Is brown mould on window sills dangerous?

Public health guidance is to remove indoor mould regardless of species, so the answer does not hinge on identifying it. Fix the moisture source or it returns.

Brown mould on window sills

Common on wood and in damp basements, and easily confused with staining from old water damage.

2 likely answers.

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.Alternaria

Routine

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.

How to know it is this one

Velvety, slightly woolly texture with a raised profile.

What it means: Standing water or a slow leak. Find the drip before cleaning the growth.

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