The ash produced by incineration of mixed waste is generally regarded as more toxic than other forms of solid waste. Incinerator ash is a mixture of many different things, some of which are left over after combustion, and some of which are actually created by the burning process.
Toxic metals such as lead, cadmium, mercury, and chrome are elements. They cannot be destroyed by heating, so they wind up in the ash, where they become much more concentrated than they were in the original waste.
Most organic substances are burned away, but some remnants remain, mixing with different elements and creating complex toxins like dioxin, which are present in incinerator ash in much greater quantities than in the original waste.