If choosing natural fiber pads for a swamp cooler, aspen is a good choice. Units with metal pads use corrugated metal to distribute the water for evaporation. Metal pads require less frequent changes but are often subject to mineral buildup.
Swamp coolers use the natural cooling of evaporation to reduce the temperatures inside a building. Pads increase the surface area of the water so that as the blower pulls air through the moisture the rate of evaporation increases for more effective cooling. A new set of aspen pads generally lasts the entire cooling season. Inverting the pad so the water flows in the opposite direction helps to increase their useful life.