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Environmental Control

Temperature and Relative Humidity

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.

This graph illustrates recurring high humidity and frequent fluctuations in humidity, both of which accelerate chemical and physical deterioration of collections.

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.

Temperature and RH are interrelated

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.