The Issue Thousands of high-rise building facades worldwide are protected by extremely flammable panels known as cladding or aluminum composite material (or ACM) with combustible cores such as polyethylene (PE) or polyurethane (PUR). These panels can burn rapidly, causing large areas of a building to go up in flames. The most famous example is the burning of the Sears Tower in Chicago during a wind storm in October 1989. There have been at least 10 other cases around the world where ACM panels have caused buildings to burn down. ACM is more fire resistant than steel but it will still burn if enough oxygen is present. The secret behind its flame resistance is the carbon filler in the panel's core. The more carbon, the better the panel will resist burning. Most commonly, ACM is made from two layers of sheet metal with an aluminum core sandwiched in between. But it can also be made from one piece of aluminum with no core material inside. ACM is used instead of wood or stone because it is much lighter and can be used in larger sizes. The problem is that it is very difficult to find places that will recycle this material. So almost all of it ends up in landfills or incinerators after use. Burning plastic and rubber creates gases that can cause things to explode.