PI values, expressed in units of years, show the combined effect of temperature and RH on the decay rate of vulnerable organic materials in collections and give a general idea of how long it would take for them to become noticeably deteriorated, assuming

PI values, expressed in units of years, show

the combined effect of temperature and RH on the decay rate of vulnerable organic materials in collections and give a general idea of how long it would take for them to become noticeably deteriorated, assuming that the temperature and RH did not change from the time of measurement onward. PI helps

to quantify how good or bad the environmental conditions are at that moment. If conditions change over time, the life expectancy of an object also changes.

PI is not meant as a predictor of the useful life of any particular object. It is simply a convenient measure of the effect of current environmental conditions on the overall life expectancy of a collection.