PEX or Cross-linked polyethylene, is a form of polyethylene that has been cross-linked to improve overall strength. There are three common ways of performing the cross-linking, Saline (PEX-b), Engel (PEX-a) or Irradiation (PEX-c). It is formed into tubing, and is used predominantly in domestic water piping, hydronic radiant heating and cooling systems.