Paper is made from a natural fibre that reacts to humidity and is affected by temperature. Paper for printing is made to be in equilibrium with an atmosphere with Relative Humidity between 50 and 60%. Paper made for photocopying or laser printing is made dryer to improve image retention and flatness after toner fusion, so may have a measured Relative Humidity of 35%. Paper for preprinting then laser printing has a compromise relative humidity.
It is therefore strongly recommended that paper be left in its mill moisture proof wrappers until just before processing. Paper pallets should be allowed to stand in temperature conditions similar to the pressroom until approximately at equilibrium. Cold paper attracts moisture from the air similarly to a cold bottle in a warm room.

Stacks of paper should be covered between press passes and any other processing. A printed job may be extremely difficult to fold if it has wavy edges or is so dry as to be full of static.

Many printers now air condition their print room, however the normal conditions are too moist for laser papers, so these still need to be covered as much as possible. Atmospheric conditions in all pressrooms, including those with air-conditioning should be monitored. A dial hygrometer available for less than $50.00 will be adequate to raise the alarm.

Few finishing rooms are air conditioned so care should be exercised to keep external doors closed, especially in times of extreme humidity, (low is just as bad as high), and keep stacks covered until dispatch.