Since mould grows on the surface of materials, surface conditions of susceptible materials should be examined in order to determine the likelihood of growth in any circumstance. In uncrontrolled museum environments moisture content in surface layers is the only factor that is likely to be influenced by atmospheric conditions and that is known to be critical to growth. Experimental data are presented which demonstrate the influence of variable atmospheric RH on thin sections of balsa wood. Variable RH is unlikely to be the only determinant of mould growth however, and other possibilities are discussed.