Expressed as a percentage, relative humidity is the ratio of the amount of water vapor present in the air to the greatest amount that could be present at a given temperature. Ideally, the relative humidity in the storage area should be below 75%. Very moist or very dry conditions must be avoided. Where ventilation is necessary, it should be kept to a minimum. Condensation cannot be allowed to occur. Some materials, such as polyester-based polyurethanes, are hygroscopic; they absorb moisture from the air. This moisture attacks the polymer’s chemical backbone, resulting in chain scission (division of the polymer chain into smaller, weaker segments). Over time, the material becomes soft and cheesy. In humid environments, this can happen in a matter of mere weeks unless precautions are taken.

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