If your disk is scratched deeply enough to let air into the foil layer then it will almost certainly be dead. The air more commonly gets to the foil through the inner and outer edges of the disk when the two plastic layers are imperfectly sealed together during manufacturing.
and about the air and rusting.. during humid days in summers seasons when disc retail stores like HMV would turn off the air-conditioner at night , and in the morning when they'd turn it back on.... wouldn't the rapid change in humidity ruins the disc as well? because it would generate some moisture in the metallic layer inside the disc?
Rapid changes in humidity don't create moisture; they just cause it to be precipitated out of the air. Hence, if there is no air/moisture inside the disk to start with, pressure/humidity changes won't have any effect.
most people have cds in their car's audio player or the case storage compartment, with frequent traveling.... wouldn't there also be more constant exposures and transitions between hot and cold temperatures? wouldn't taking disc along like that poses a greater risk of killing off those favorite cds a lot faster than if it were to just left indoors?
eg: sun's heat = cd warms up
turn on airconditioner =cd cools down
engine off = gets hot from sun again
Changes in temperature and physical stress will cause disks to wear more rapidly, but since it is the plastic layer(s) that bear the brunt of the damage this has little effect on the lifetime of the disk; after all think how long it takes for a plastic bag to degrade.