You may remember that we've discussed rag board before. It is acid-free and must be used in the preservation of prints on paper and documents.
It is the acid in regular paper mats that causes prints to evenly discolor all over, get brown spots, (foxing) as if someone had taken a cigarette and randomly burned the print, and become brittle. Regular mats are made of wood pulp, full of acid, as are most paper products. Rag mats, however, are made of cotton, (hence "rag") which is acid-free. Think of fine stationery, such as Crane brand; you may know that it is 100% cotton.
How do you tell if your art, family documents, etc. have rag board? It's simple! Look at the beveled edges of the matting. If they are browning, they are regular mat boards and your art needs immediate attention. If the edges are white or off-white, they're rag mats.
Remember, too, that no art should be right up against the glass, as this traps moisture and tiny fungi will start to grow and get progressively worse. So, if your art has no mats at all, you need to take it to a good picture framer and re-frame it.