Fall is a welcome time for so many of us, who look forward to turning off the air conditioning, opening up the windows, and enjoying the cool air before the heating season starts. However, if you are managing a collection storage space, it is a time to be thinking about MOLD.
In fall and winter when leaves fall to the ground, plants die, and extra rainfall produces moisture, the decaying plant matter often releases a greater amount of mold spores into the air. At the same time, the amount of moisture in the air tends to be high. Whatever is in the outside air comes into your building and is dealt with by the HVAC system.
Obviously there are regional differences affected by temperature, humidity, amount of precipitation, and the type and amount of vegetation in the area. However, for those institutions in regions that experience mold and high humidity in the fall, it is important to pay close attention to temperature control from August through October. If the air is cooled too much during this period, the relative humidity can rise to dangerous, mold-encouraging levels. It’s important to consider both temperature and relative humidity and not just assume that cooler temperatures will always be better for preservation. Very high relative humidity is potentially more harmful to collections than a period of moderately warm temperatures. Extended periods of humidity at 65% and above can lead to mold growth in vulnerable collections. It’s better to add a little heat and avoid conditions where mold can grow.
The map (from the Weather Channel at www.weather.com [2]) illustrates dew point temperatures around the US, measured at 10:10 AM on September 17th. Overnight levels are even higher. With a 55° dew point, a temperature setting of 72°F will result in 55% relative humidity. Drop the temperature to 65°F and the RH rises to 70%. Let the temperature go to 60°F and the RH will be 84% - and you have a serious mold risk. If your dew point is 60°, 72°F gives you 66% RH, and a drop to 65°F gives you 84% RH again. At this same dew point, a temperature of 60°F will mean 100% RH! Use IPI's Dew Point Calculator (www.DPCalc.org [3]) to run your own numbers and reduce the risk of mold.
As noted in The Massachusetts Experiment—MBLC article in this issue, during the months of August, September and October, institutions should be careful not to over cool the spaces with air conditioning or else to judiciously use heating to counteract high RH levels.