The best means to prevent or control the spread of microorganism growth is to deny the spores the moisture necessary for germination. Therefore, regulating the environment, especially the RH, is essential for preventing the deterioration of a museum collection from microorganism growth.
RH levels should be routinely monitored. Spore germination is less likely to occur if RH is controlled between 45% and 55%, but RH should be kept below 65%. When RH levels rise above 65%, the use of portable dehumidifiers will be necessary to reduce the moisture content of the air. A temperature between 18 degrees C and 20 degrees C (64 degrees F to 68 degrees F) should be targeted. These levels only decrease the potential of germination and growth; they do not eliminate it. Therefore, other factors, such as adequate aircirculation should be maintained; a fan will help to increase circulation.
Problem environmental conditions that may contribute to higher humidity levels need to be corrected. Repair leaking pipes, gutters and downspouts, cracked windows, a problem roof, deteriorated brick, masonry pointing, or cracked walls.
It is also important to keep any area that houses museum collections clean and free of dust and dirt and organic debris that can nourish spores.
Silica gel and other buffers can help adjust RH conditions within a sealed space, such as in a storage cabinet or exhibit case. These buffers will absorb or release moisture into the surrounding atmosphere. The quantity of buffering material to place within the space must be customized for each situation and a conservator should be consulted for assistance in determining this need. It takes time, experience, and careful monitoring to ensure that the buffers are performing as intended. (See NPS "Museum Handbook," Part I (Rev 9/90), Appendix I, for additional guidance on the use of silica gel.)