also, from this page (tells how to build a cooler)

"Previously, most pads were made of aspen shavings, excelsior or plastic. Now on the market are cellulose pads impregnated with insoluble antirot salts. Another modern cooling pad system consists of a bonded plastic fiber pad coated with an absorbent cellular foam. Unaffected by continual exposure to moisture, disinfectants, chlorine or water acidity, these new pads are so efficient that you do not even see water trickling down the side.