Why do old polyethylene bottles smell?

[Carlisle Landel]

Hi!

I unearthed an old carboy yesterday, and was considering using it for
RO water storage (we've been having problems with our system, and
I wanted to stockpile some).  I noticed that it smelled funny inside.
Only water has ever been stored in it, as far as I know.  It is the
same smell that your polyethylene water bottle that you'd use for
hiking (the nalgene bottles we steal from the lab and other people
buy from the outfitter stores) gets with age.  The question is, what
is that smell, and does storing water in it cause degredation of the
water quality?  Just curious.

Of course, I'll not store water in it, because anything I can smell 
probably goes into the water.

Carlisle Landel