Relative humidity specifications for museums became very stringent during the 1960s and 1970s, without detailed explanation. The Canadian Conservation Institute has reviewed the relevant data in order to answer common cost/benefit questions, not just stipulate correct RH. Mechanical, biological, and chemical deterioration all rise sharply beyond 75per thousand RH, and increase significantly for every increment to 100per thousand RH. Fracture in rigid, constrained organic artifacts, given a relaxed state at some middle RH, becomes probable in one cycle only for a drop of -25per thousand to -50per thousand RH. Fatigue models imply that each reduction of fluctuations to one half of this critical value will reduce deterioration per cycle to 0.01per thousand- 0. 00001per thousand. Many flexible or sliding assemblies are immune since constraint is missing. -- CAL