Nichibei Anime Club has a good page on the care 
and regular feeding of cels (with money, of 
course) at this location:

http://www.nichibeianime.com/care.html

To save time, here's the passage most relevant to 
your question:

"Most careful Japanese collectors ... place their 
cels in bags made of relatively inert 
polypropylene film.... This combination of cel 
books and bags are the tried and true method of 
preservation used by Japanese collectors over the 
years. However, there has been little research 
done on the long-term effects of keeping 
animation cels in polypropylene. ANSI (American 
National Standards Institute) allows it for the 
preservation of photographs, but there is some 
evidence that acetate may benefit from being in a 
more breathable environment. That is one reason 
why we clip a corner off of the cel bag. On the 
other hand, the polypropylene can create a micro 
climate inside the bag that buffers sudden 
changes in humidity. We change the bags on our 
personal cels every few years and after over 
eight years they appear quite well preserved."

In my personal experience, I changed all the cel 
bags in my collection once, about two years ago, 
not because I saw any damage to the cels but 
because the cel paint had clearly reacted with 
the plastic, severely puckering the back of the 
bag.  In one case, the paint had (very slightly) 
stuck to the plastic.  This didn't seem good for 
the cels, so I invested in some new bags and 
moved them over.  Some have puckered the new 
bags, but not as severely.

I do not know if it's true that there are "fumes" 
created.  In some cases, it looks as if certain 
cel paint colors (especially yellows and oranges) 
have reacted with the trace lines, softening them 
to brown.  But I assume this is due to 
direct contact between the pigments, 
not "fumes."  So I don't really know if it is in 
fact a good idea to clip a corner of cel bags to 
help your cels "breathe."  (It does make it more 
obvious in the middle of the night when one of 
your cels is snoring.) 

The only cels that I've seen visibly change are 
the ones that I displayed on the wall.  Despite 
UV resistent glass and protection from sunlight, 
all of them have very gradually had trace lines 
soften.  Those I've kept in the dark have not 
visibly changed since I acquired them, including 
the ones where the trace lines had softened to 
brown when I got them.  So I think that light is 
probably a more damaging factor than "fumes," and 
that it's likely that these items had previously 
been displayed by dealers in stores before I got 
them.