Moisture can be absorbed directly from the air. This causes a chemical reaction called binder hydrolysis. Molecules in the recording and backing layers react with the moisture and break down, leaving a sticky residue. Eventually the tape becomes too sticky to use. Binder hydrolysis is the factor most experts use in predicting the life expectancy of magnetic tape.
Heat aggravates binder hydrolysis.
It also causes tape to expand and contract producing creases, stretching and structural damage. Dirt and debris contaminate the surface of the tape and block playback machinery from reading the signal. Dirt also gets inside tape wraps, causing deformation and abrasion of the tape surface. Storage environments must be cool, dry, clean and relatively stable to maintain tape in good condition. Proper storage can extend tape life by years or even decades.