Of these three, circulation is one of the most critical, and the most often neglected. The literature often mentions in passing the importance of good air circulation. Unfortunately, the significance of this factor, particularly in areas where the environment is not temperature and humidity controlled, has been largely overlooked. Air movement causes the evaporation of moisture, lowering the surface temperature. This is evident to anyone who has ever experienced the cooling effect of a sudden breeze on a hot still day. Good air circulation in the library results in the evaporation of moisture, lowers the surface temperature, and alters two of the environmental factors on which mold growth depends.
It is, in general, much less expensive to move existing air around, thereby modifying the temperature and humidity than it is to introduce an artificially created supply of air with characteristics radically different than that of the surrounding air. Good air circulation can do much to reduce the problems associated with lack of control of conditions three and four.