Nearly all plastics are non-biodegradable. This means that they will not rot away naturally either through the weather or by bacteria in soil, and can therefore cause a long term litter problem.
There is another pollution problem produced when all plastics burn. As all plastics contain carbon, they can produce the toxic gas carbon monoxide (CO) when they are burned. However some plastics can give off other toxic gases when they burn, depending on which elements are present in the formula of the plastic.
Below is a table showing plastics and the toxic gas produced when the plastics in question are burned.