This is my experience. If a bag is completely 
air tight, that is bad for just about anything, 
including comic books and other collectibles. 
However, just like a balloon deflates over time, 
even a sealed bag isn't really air tight, unless 
its a ziplock. So, open ends or not, air is 
getting in and out of there. However, an open-
ended bag, laying flat in a cel book, isn't 
getting a whole lot of air to it in the first 
place, so I am of the opinion that it doesn't 
matter if its even in a bag at all. My cels are 
in special sleeves, except for the oversized 
ones that are in bags. Technically they are open-
ended, but since I keep them flat (vertical, 
since I use books with hard bindings) and inside 
cel books, they aren't 'exposed'. I think that 
paint, more than air, has to do with line 
fading. Red and browns tend to be evil on the 
lines, along with sunlight and flourescent 
light, of course. I'd say, if you just take good 
care of your cels, you won't see their condition 
fall, unless it is a reaction with the paint, 
which you can't help in the first place.