Although the general rule is that relative humidity (RH) below 60 percent provides microbiological safety, in real museum situations micro-organisms do not always develop on sensitive materials, despite high RH. Conversely, they may develop on materials that are not particularly sensitive, and when the RH in the room is relatively low. Rooms with average conditions below those needed for fungal growth may contain microzones that do allow growth. These can arise in closed volumes as a result of differences in temperature, or near external wallsof buildings and overloaded storage rooms. Air circulation can eliminate such conditions. Low temperature slows down the growth of microorganisms at any humidity level. The freezing of wet objects controls growth (wet by accident, or wet at excavation). Limited winter heating of architectural monuments of stone, plinth and brick, in central and northern Russia, has been found to provide effective control of microorganisms on plaster, wall paintings and iconostasis carving.