Relative humidity

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Organic materials expand and contract as ambient moisture levels rise and fall.

A low RH (less than 40%) reduces chemical change but can cause materials to shrink, stiffen, crack, and become brittle.
A sustained RH above 65% can cause adhesives in both modern (magnetic and optical media), and traditional library material to soften and lose their strength.
In areas of poor air circulation RH should not exceed 60%; and even when air circulation is good RH should not exceed 65% in order to avoid mould growth.
Above 70% RH and there is serious probability of mould growth.