Anything that touches film can potentially scratch it.
I have seen a jet of air scratch film. Some idiot decided that he was going to use an air compressor to clean stuff in the projection booth but he didn't put a dirt trap in line with the hose and he blew millions of microscopic dust particles directly into the film, from close range at high speed. He basically sandblasted the film.
When projected, the film looked like a cat had gone crazy and clawed the screen... millions of tiny, little black lines all over.
The manager called me up and said, "The picture looks funny." (They always say "funny" when they know they did something wrong.)
I took one look and said, "You used an air compressor. Didn't you?"
The guy lied and denied it but I could tell from the way his face went white when I called him on it that he had used a compressor.
The best way to keep film clean is to not let it get dirty in the first place.
Easier said than done... I know...