Burning most kinds of plastics or metals is a quick path to creating a significant hazard for those near-by, or even farther afield if there’s wind. The potential for adverse health effects goes up because these are inherently toxic and the gases that are produced in the chemical reaction of the fire are equally as dangerous, if not more so. 
Some of the worst items to burn in terms of toxic emissions include manmade chemicals like Teflon or  other plastics, used motor oil and rubber such as that found in tires all of which I have seen on allotment bonfires.

There are no safe bonfires.