The amount of moisture contained by a paper when its own relative humidity is equal to that of the surrounding atmosphere, and the point at which paper will neither gain nor lose moisture to the atmosphere. The primary constituents of paper, fibers of cellulose, have a strong affinity for water, and will gain (or lose) it readily, depending on the amount of moisture in the air. This hygroscopic characteristic of paper makes it dimensionally unstable, as the length and/or width of a paper can change depending on how much water the paper has gained or lost.
The equilibrium moisture content of a paper depends on its percentage of fillers—inorganic materials added to paper during papermaking—which neither gain nor lose moisture. Coatings also affect the moisture content of paper. Fillers and coatings increase a paper's dimensional stability, which improves printability in many printing processes.
The equilibrium moisture content of paper is also dependent on past exposure to moisture. (See Moisture Content and Relative Humidity.) The equilibrium moisture content will remain unchanged unless the relative humidity is changed; lowering the relative humidity of the environment will result in the paper losing moisture until a new equilibrium moisture content is reached. Similarly, increasing the relative humidity of the environment will result in the paper gaining moisture until a new equilibrium moisture content is reached. Generally speaking, if the relative humidity is 20:65%, a 10% increase or decrease in humidity will effect a 1% change in a paper's equilibrium moisture content. A related measurement is equilibrium relative humidity.