How do I get rid of this

[lavalamp08]

Can anyone help .... I've got the following on the majority of my windowsills and no idea how to sort it out, I've scrubbed it but it still looks really grimy.
What can I use to get rid of it
Many thanks in advance

The cracks need filling and then the sils repainted

It looks like mould build up. You can get some good sprays for that. But ultimately you need to air the room a little and fix the cracks.

Yep, as the others have said - mould spray, air the room and then use caulk to fill the gaps and repaint.