CD rot

Also known as DVD rot, CD rot is a term used to describe the slow deterioration of the aluminum layer on CD's, DVD's, and other discs over time. This occurs when the disc was not sealed properly, which allowed for moisture to enter through small holes and oxidize the aluminum or more commonly caused by improper care of the disc.
Although nothing lasts forever there are a lot of handling tips you can do to help prevent CD rot and keep your discs lasting much longer.

• Store discs vertically and not horizontally.
• Keep discs in a protective casing (jewel box) not a sleeve or other envelope type holder.
• Do not stick labels on them.
• If you're writing on the disc only use a marker.
• Keep out of the sun.
• Handle the discs with care. This includes the top and bottom of the disc.
• Keep in a room with a comfortable temperature (between 40 and 70) and with a low humidity.
• Keep out of rooms or away from locations where the temperature quickly changes (e.g. away from heaters, stoves, fireplaces, swamp coolers, etc.).

Unlike traditional CDs, DVDs, and mass manufactured discs. CD-R and other recordable discs do not use aluminum and often use other alloys that are not as susceptible to oxidation. However, all steps mentioned above in caring for the discs should still be followed. If the data is really important it's also always a good idea to make more than one copy.