Since purchasing my town home last year, I've noticed only in our guest bedroom, black dust and a yellow layer building up around the door jamb - right where the door meets the trim when closed.

The black dust is on the left trim and top trim (I just cleaned the top trim with some disinfecting wipes so it isn't pictured). The top trim had more significant build up compared to the picture I've attached showing the dust on the left trim. It was a black dusty layer all across the top from left to right, from end to end.

The hardened yellow film/layer is all along the top and right trim, from end to end. I don't know if this is something to do with when the paint dried? I believe the previous owner applied fresh paint prior to sale.

The black dust came off when I wiped across. The yellow film takes some firm wiping/scraping to remove.

The large dark spots pictured looks like protruding pieces of the yellow that have blackened (I think). I've pressed at these pretty hard and only a little bit has come off. It seems I'll have to get a scraper or knife to remove it.

I'm a bit worried this could be mold so I wanted to consult this forum.

This bedroom has very old windows and during the cold season you can feel the draft and if I remember correctly it's like that in the summer too. Very hard to keep this room warm or cold during seasons.

I'm not sure if it's the air that's passing through that door space that caused this build up. Today is the first time I'm actually cleaning it, and it doesn't look like it has gotten much worse than the first time I saw it last year.

Thanks for your help!