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PVC is a common plastic which because it is chemically unstable releases a chlorine gas. When this gas settles onto a surface it turns into hydrochloric acid. This acid will cause photographs to fade and discolour.
There are actually several kinds of PVC. The flexible material (like shower curtains are made of) does exactly what you indicate. The rigid material (for plumbing, siding, ceiling tiles), does not outgass, because it does not have the pasticizers and modifiers to make it flexible.