Temperature can be defined as a measurement of how quickly molecules are moving within a material. As molecules move faster, they collide more frequently, making chemical reactions more likely. Molecules move more rapidly at higher temperatures, so heat accelerates the chemical reactions that cause deterioration. As a general rule of thumb, the rate of many chemical reactions is doubled with an increase of 18°F. Thus, a lower temperature means a slower rate of deterioration.
Relative humidity refers specifically to the amount of water vapor contained in the air (e.g., 30%, 40%) at a given temperature (e.g., 65°F, 80°F), relative to the total amount of water vapor the air is capable of holding at that temperature (which would be 100%).
The amount of water vapor in the air is important for two reasons: moisture provides fuel for the chemical reactions that cause deterioration (e.g., acid hydrolysis), and it causes physical damage such as swelling and shrinking. Organic materials such as paper naturally try to come to equilibrium with the surrounding air, so they absorb moisture as the relative humidity rises, and release moisture as it falls. Thus, higher relative humidity results in a quicker deterioration rate. Very low relative humidity can result in dessication and cracking of some materials. Frequent fluctuations in relative humidity (and temperature) are even more damaging.
When managing climate within collections storage spaces, it is crucial to understand that air is capable of holding more water vapor at higher temperatures. Thus, given that the absolute amount of water vapor in the air remains the same (unless moisture is added or taken away through humidification or dehumidification), the relative humidity will go down if the temperature is raised, and it will go up if the temperature is lowered.
Take a few moments to reflect on the scenario below. Why do you think this problem occurred? Click on Show Answer to see.
Scenario: The River's End Historical Society, concerned that their collections suffer from extremely low humidity in winter, has recently added a humidification system to their building. When the curator comes to work one cold morning in December, she notices that there is water running down the inside of the windows in the library room.
Question: What is causing this and what should she do?
The water running down the window glass is condensation. When the relative humidity is 100%, the air is saturated with moisture, and excess moisture condenses out of the air—this is known as the dewpoint. As the temperature dropped outside, the window glass became cold. The heated air inside the building (which contained extra moisture added by the humidification system) cooled at the window, causing the relative humidity of that air to rise higher than desired. Eventually the dewpoint temperature was reached and moisture condensed out of the air.
The curator should immediately reevaluate the use of the humidification system. Condensation may also be occurring within walls even though it cannot be seen, and it could lead to mold or other problems. If careful monitoring to ensure the relative humidity does not get too high is not possible, the humidification system should not be used. A safer way to increase the humidity in winter would be to lower the heat in storage areas to about 60°F.