OK you may have read the post in which the joys of PVC are mentioned, here is the flip side of the coin. PVC is not a perfect plastic, to be honest no plastic is perfect.
One of the problems with PVC is that in the event of a fire it forms a lot of hydrogen chloride, this is by a series of elimination reactions which form a polyenes molecule. This is not a nice clean synthesis of polyacetylene, insetad it makes a polyene which is a short length of polyacetylene which darkens the PVC and spoils it. These molecules can decompose, this can then made a range of organic products.