Comment by KaiserPro

It's normally stored in sealed boxes, but every time its taken out to be projected then the whole reel is unspooled and exposed to the environment. Most projectors are pretty good at not blowing unfiltered air on the film, but there is a surprising amount of dust that is just floating in the air.

The room that the scanners used to be in were temperature and dust controlled, everyone was supposed to wear dust jackets when you enter.