What are the little Black spots on a DVD-R (RW)?

I am starting to notice a number of very small black spots appearing under the clear plastic layer of my burned discs. They are mostly located on my DVD-RW discs, but I am spotting them on some of my DVD-R discs too. I don't seem to see it on any of my burned CD-R or CD-RW discs. I am using only Sony DVD-R and Sony DVD-RW discs and seeing these annoying spots.

It also seems that some of the DVD-RW discs that have a lot of this spotting are having problems transfering files/documents from these discs. Many times the disc files are completely useless.

The DVD discs I am refering to have only been burned for about 4 years now, and the DVD-RW's maybe reburned four -six times each at best (less in most cases). The DVD's have been stored in their jewel cases (vertically) on bookcase shelves, in a normal house setting for their entire life.

I am concerned that burning any type of disc (mostly DVD or DVD-RW's) are no longer considered SAFE for even a short limited time, more less the 100 year period most manufacturers say is their lifetime.

I have important graphic files that are being un-recoverable due to these bad discs.

Is this something the entire DVD/CD burning community should be talking more and more about? I see very little discussion, on google, of what is causing this black spotting on burned DVD's.

Any discussions would be appreciated here.