In laser printing systems, paper is not just the surface the image is put onto, it is also part of the process to make a printout. Laser printing systems use static electricity to convey the toner from image components onto the paper. The paper is actually charged with static electricity to attract the toner to it.
As we all know, controlling static electricity can be unpredictable because of all the factors that affect it. All matter has different characteristics that affect the conductivity of electricity. For example, distilled water is actually a poor conductor of electricity, whereas sea water conducts electricity quite effectively. Paper can have subtle differences that affect its ability to accept a static charge. Moisture in the paper is a major factor.
A printing system has the ability to change the amount of static electricity applied to the paper, but it needs to know factors about the paper to do so. Damp paper requires more static electricity charge be applied to it to attain the proper charge for attracting the toner. Commercial laser-based printers have humidity sensors in them to help in making adjustments to the amount of static electricity it will apply to the paper. These work only if the paper has acclimated to the surrounding air.
For instance, if you just received a shipment of paper that was stored in a damp warehouse and put that paper immediately into the printer, the machine can only assume that the moisture content of the paper is close to that of the surrounding air. The amount of charge applied to the paper would be less than what is really required to charge the damp paper to effectively attract the toner. This can result in light images.
For more details about factors that can affect paper performance in your printing system, refer to the Operators Guide for your equipment. You will find a section about paper in the appendix.