A quantitative tool, the isoperm method, is described; it quantifies the effect of environmental factors of temperature (T) and percent relative humidity (%RH) on the anticipated useful life expectancy of paper-based collections. The isoperm method provides answers to questions of the expected lifetime of the collection under various temperature and relative humidity conditions, as well as a way to demonstrate the effects in quantifiable terms. The method is based on paper-strength loss associated with the chemical reactions of cellulose hydrolysis and oxidation. Relative, rather than absolute, rates of deterioration and paper permanence are employed. The deterioration-rate ratio can be controlled through changes in temperature and percent relative humidity, but it is not possible to change nonenvironmental factors, such as fiber type and so forth. The isoperm method is a step toward using quantitative analysis techniques and models to help make preservation-management decisions. Seven figures illustrate the discussion. (Contains 8 references.) (SLD)