Hmmmm.... I've actually noticed the exact 
opposite: the cels that I've kept sealed have 
less line fading than the cels that were kept in 
open-ended bags. 

I suppose though that are so many variables 
involved in line fading (the color and type of 
paint touching the line, the quality of the 
original line and the kind of copier machine used 
to make it, incidental exposure to light, 
humidity, migrant acidity, etc) that it's 
difficult to pinpoint a cause of fading simply 
from our personal observations. 

It would be interesting to do an experiment on a 
set of cheap, nearly identical cels from the same 
sequence to see how quickly colors and lines 
degrade under different conditions.