A model study was conducted in which samples of eight calcareous biological materials were exposed for 12 months to acetic acid vapor at concentrations of 0, 5, 20, 35, 125, 217, and 795 ppm and 54% relative humidity in 1 liter glass jars. Each month, the samples were weighed and examined visually for gloss and crystal growth on the surface. The results support the theory that there is a concentration under which deterioration cannot be observed, but this concentration depends on the detection limit of the technique used to determine deterioration. The name <i>acceptable damage concentration</i> (ADC) is suggested for this concentration.