I have all
my yarn stored in a large clear plastic zippered bag (kingsize bed set came in it). I never thought about it not being safe.
The truth is, it is usually open at the corner, as I am constantly getting in and out of it, and don't usually bother zipping it all the way back shut.
I recently purchased some wool yarn off eBay. When I first got it, it looked great. I couldn't wait to make something out of it, as it was my first wool purchase. Wal-Mart is my only local yarn source, and they only carry acrylics. Anyway, when I went to unravel it to cast on, I noticed a bad place in the yarn. Further unravelling revealed similar damage throughout. I inspected the other two skeins and found the same thing. Apparently moths had lunched on them. :(
Incidentally, I keep cedar balls (like mothballs, but made of cedar) in my plastic bag.
I'd already left positive feedback for the seller, since they appeared fine when I first took them from the package, so I wasn't really sure if she'd be willing to make it right or not. I wrote to her yesterday, informing her of the yarn's condition, and she said she'd get me a check right out.