The primary reason that electrostatic equipment does not function properly is low surface resistivity of components that should be insulators: belts, rollers, supports, and brushes. The primary reason for low surface resistivity is contamination with surface water and other contaminants.
The surface resistivity of most insulators is a linear function of the relative humidity. If you store the equipment in a warm dry environment and keep the equipment clean, less than 60% relative humidity is usually acceptable; however, a much lower relative humidity (RH < 40%) is helpful because a higher level of surface contamination can be tolerated.
Irrespective of the ambient relative humidity, the equipment should be stored at the same temperature or warmer than ambient to avoid condensation on the equipment. If this is not possible, heating the unit with a blow dryer for a few minutes before use should remove any condensed water.
If a charged Van de Graff belt is left exposed to the environment, it will tend to attract volatile organics compounds and dust. Equipment can be cleaned by wiping it down with clean cloths wet with alcohol.