The Ounce: Oh, The Humidity!

Welcome to The Ounce, an occasional missive for Harvard Library (HL) colleagues from Preventive Programs in HL Preservation Services.

During the summer season (June 21^st-September 21^st) indoor humidity levels rise, presenting risks for library and archive collections. Humidity at 65% can encourage mold to bloom, increase pest activity, and cause metal corrosion (eg. photographs and metal objects). High humidity exposure is cumulative from summer to summer, and collections that have already had mold can be susceptible to rebloom.

If humidity remains between 60-65% for 48-72 hours (2-3 days) consult with your building’s Facilities Manager and Preservation to consider:

• Storing and working with collections in a location with more moderate humidity
• Increasing airflow by using floor fans
• Reducing the amount of outside air that your Heating/Ventilation/Air-conditioning (HVAC) system takes in for human occupancy
• Increasing the HVAC’s unoccupied mode when staff are working offsite to lower the amount of air flowing in and out
• Raising the temperature, which lowers the humidity because of their inverse mathematical relationship (but generally, no higher than 74 F)

While summertime humidity can feel brutal, the human ability to detect changes in humidity can vary and is not recordable. So, we recommend using dataloggers to collect information about temperature and humidity in your collection storage areas. Preservation Services can assist with installation and analysis of humidity and other environmental conditions in your library and assess which collections are more vulnerable. Your building’s facilities manager can explain what the air-conditioning system is capable of doing to keep humidity moderate and stable.

PEM2 Datalogger                                                              Onset MX1101 Logger