Two main reasons are scratch damage and heat damage. Compact Disc is a 1.2 mm thick Polycarbonate transparent disc used for digital writing with the help of Laser beam. The poly carbonate plastic used in CD is Polymethyle Meta Acrylic. The surface of the CD is coated with a thin layer of aluminium to make it reflective, and is protected by a film of lacquer on top of the reflective layer, on which the label print is applied .In the CD, from the center outward there are areas like center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim. A standard CD has 120mm diameter and 15-20 gm weight. It can hold 80 minutes uncompressed (700 MB) data.
The writing side of the CD is polished on which the data is recorded as Bits (lands) and Pits (holes). Each pit is 100nm deep and 500nm wide and the pits are arranged spirally from the centre of the CD to its rim. Pits are formed during CD writing by burning the plastic with the help of pencil beam Laser. Pits are much closer to the label side of a disc, so that defects and dirt on the clear side can make the beam out of focus during playback. CD suffers more scratch damage on the label side. If the CD is exposed to heat, the lacquer coating melts and label detaches from the CD leaving small air gaps between the CD and label. This severely affects its working because the Laser beam passes out from the CD. Due to over use the dye’s physical characteristics may change, causing read errors and data loss.