Why is polystyrene not environmentally friendly?

Like many other plastics, polystyrene is a polymer derivative of oil. It can be, but often is not, recycled. This is because of a lack of plants capable of processing the material, and its low yield when processed from its expanded foam form. In the cases where it is recycled, it is often used in other plastics and materials. Furthermore, plastics generally decompose very slowly, and polystyrene is no exception. This means the material stays in the ground without breaking for a long time when placed in a landfill Historically, the material has been expanded using Chlorofluorocarbons (CFC's), increasing awareness and regulation have led to the use of other, more environmentally sound substances. In addition, some cities have imposed bans on the material for packaging. Polystyrene is a common and useful material, however, its long decomposition time and infrequent reprocessing, as well as its disruptive introduction into ecosystems contribute to its reputation for not being environmentally friendly. src: http://en.wikipedia.org/wiki/Polystyrene