I used to walk past a cotton mill on the odd occasion on which I walked between the homes of a couple of my relations. One thing that struck me at the time - and I've never solved the conundrum - was that the building was large and had a huge number of windows, albeit those little six inch panes set into a larger frame. Presumably this was to allow natural light to access the interior; but the windows were never cleaned - or at least did not appear to be so. Still, I'm talking of a town called Leigh which also had two or three active coal mines and hundreds of coal burning fireplaces so maybe it was just 'natural' deposits. The air that came out of those windows - many were hopper type, hinged at the bottom - was hot. I sympathise with anyone who had to work in that environment.