Silica Gel is basically a moisture absorbant. It absorbs moisture as trees absorb carbon-di-oxide.It is a porous, granular form of silica. a microporous structure of interlocking cavities gives a very high surface area (800 sq.mtrs) per gm. It is this structure that makes silica gel a high capacity desiccant. Water molecules adhere to the gels surface because it exihibits a lower vapour pressure than the surrounding air. When an eqilibrium of equal pressure is reached, no more absorbtion occurs. Thus the higher the humidity of the surrounding air, the greater the amount of water that is absorbed, before equilibrium is reached. It is in these higher humidity conditions (above 50% Relative Humiditiy) that stored or in-transit items are suspectible to damage. The beauty of silica gel is the physical absorption of water vapour into its internal pores. There is no chemical reaction or side effects. Even when saturated with water vapour, silica gel has the appearance of a dry product and its shape remains unchanged.