Mold forming on brand new painted timber window sills

[Smudge01]

I painted some external window sills at 1st floor level only about 5 months ago. Black mold has formed across some large areas, we can't work out what has caused it.

It's on a busy street so my best guess is, it's pollution.

Has anyone come across this before?

The blue thing is the shop sign below.

Thank you in advance.

 

[blup]

Are you above a takeaway or restaurant, with hot air rising from an external extractor fan or open windows. This warm air may be condensing on the sill.

Blup

 

[foxhole]

Pollution doesn’t cause mould , mould spores are in the air and settle on damp areas with poor ventilation.
Improve ventilation which will reduce condensation (causing the damp sill)and prevent spores settling .

 

[Smudge01]

Thank you for your replies.

It is not above a takeaway or restaurant, external extractor or anything like that.

And I'm not sure how ventilation can be improved on an external window sill, however, it is this line of thinking which has led me to the answer. The sill is about 12.0m long, at one end there is no mold on the sill, this is because the air flow around this area is good because it is on the corner of the building with a side street running alongside. The mold is much worse further along at the other end, I can imagine there would be very little air flow at the other end. They have the windows cleaned weekly, and the rainwater will sit on the sill, it will dry at the far end/corner, but not at the other end where the building continues as part of a run of terrace buildings.

Thank you very much for pointing me in the right direction.