RJW is right.
Plasticisers used with some jewel cases bring down the acidity of the micro climate inside a jewel case.
This can speed up corrosion significantly.
Also, the inlet paper (if any) can also be possibly hazardous.
If you want, you can also invest in copper impregnated corrosion resistant materials for inlet cards (see the link RJW gave you).
There is further research on humidity, temperature, noxious gases and EM radiation (mainly UV) that suggest you should protect your discs from exposure to these. Also in the case of temps/humidity, it's not just protecting from absolute numbers, but relative changes (i.e. temp/relative humidity delta).