Most burned things release Very Bad Chemicals(tm) only when they are incompletely burned. It is possible to design burners that ensure a complete combustion, b

Most burned things release Very Bad Chemicals(tm) only when they are incompletely burned. It is possible to design burners that ensure a complete combustion, but they are much more expensive than simple burners. Upon a complete combustion, some things may still release volatile acid oxides, e.g. oxides of nitrogen or of chlorine, but those can be captured.