What Causes Carbon Spots?

Although it can significantly reduce the overall eye appeal of the coin, the value would be based on if it were environmental damage or just ugly toning. If it's just ugly toning, the price would be affected only upon what a certain person would be willing to pay for the piece. I have a few coins slabbed by PCGS that are problem free but have undesirable toning. They would still trade at fair market value to a prospective buyer.

As already noted it could be caused by almost anything. I wouldn't want to eat the lunch that caused that though, Thad.
People leave coins laying around in the oddest places. And they do have some strange things on their hands. Big reason doctors now stress cleaning your hands so much.
During Chemotherapy I was repeatedly told by everyone at the hospitals DO NOT shake hands with people.