When Willis Carrier invented his “Apparatus for Treating Air” back in 1902, he did so with a single purpose: to control heat and humidity that’s adversely affecting the paper being manufactured in the paper mill he was working in—but how exactly does heat and humidity affect paper?
Air is a compound, which means it contains a lot of other gases which includes oxygen, nitrogen, and water vapor. Specific temperatures allow air to only hold a maximum amount of water vapor, which is measured by virtue of relative humidity or RH. As an organic material, paper can readily absorb water vapor when it is exposed to great amounts.
Relative humidity is closely related to the terms “dryness” and “wetness.” They both denote the proportion of water vapor present in the air. At a certain stage called the “dew point”, which occurs at low temperatures, the air is very saturated—it holds every ounce of water possible which manifests in forms of dew or mist. On the other hand, rising temperatures cause the dew to be suspended mid-air where it hovers until the temperature is lowered again.
The continuous rising and falling of temperatures spell doom to paper because of this. This is why paper manufacturers, as well as document archivists, must invest in efficient air conditioning so as to preserve the paper they store.