A laser beam in the write mode can change single spots to either an amorphous or a crystalline state so that, again, a digital bit stream is created. Dust and dirt of all kinds. Humidity is, as with other data carriers, a most dangerous factor. In the case of optical media it has a hydrolytic action on components such as the protection layer of CDs and a corrosive influence on all metal components including metallic reflective layers. More importantly, it is responsible for dimensional changes which may be of concern, especially in the case of multi-layer media.
Mechanical integrity is of utmost, and underrated, importance.
Even microscopic scratches can hinder the reading laser beam, as do fingerprints and other foreign matter. Mechanical bending of discs cause microscopic cracks which again divert the laser. Dust and dirt prevents the proper reading of the recorded information. Cigarette smoke will accumulate on the disk surfaces and may hide information. The CD-family is again more exposed to this danger than those disks that are protected by cartridges.