The article considers the effects of excessive acidity and inferior storage conditions on the useful life of books in libraries. The acidity of paper is the most significant cause of paper impermanence today. Certain atmospheric pollutants increase paper acidity and contribute to impermanence. The effect of temperature is believed to be the second most significant cause. The combined contribution of pH and storage temperature to paper impermanence was considered by specifying the folding endurance half-life of an ideal paper under certain conditions and then varying the pH and the temperature. The procedure demonstrated the theoretical increase in useful book life which can be obtained by paper deacidification and air-conditioning.