[Cadaverous Pallor]

I thought of a science question that I'm having trouble googling. Perhaps you braniacs can help out.

Why would a razor (or other sharp metal implement) go dull from disuse?

I understand that certain metals chip away in tiny fractions when used and eventually lose their edge. I also understand that certain metals become sharper with use for the same reason, as the tiny chipping varies due to the properties of the metal.

But to have an object just sit and get dull? My muddy guess is that it has something to do with oxidation, something to do with the air...but to my mind, any dulling from air would take a very long time, unless the razor were in an extreme environment (humid, dry, salty sea air...whatever it is that causes the dulling). When people say "dull from disuse" are they referencing an effect that takes 50 years to manifest? Or, if you buy a very large pack of disposable razor blades, you'll find that the last razor you use is measurably duller than the first?

Thanks for any illumination. Other science questions are welcome here.