The fumes released from plastic combustion are very harmful, as they are very likely to contain the residue of the powerful solvents used in their manufacture, according to Reference.com. Fumes released by burning PVC plastics are especially dangerous as they contain a high concentration of dioxin, particulate matter and other contaminants that have negative environmental and health effects.
As Reference.com explains, the solvents used in manufacturing PVC and CPVC plastics usually evaporate during the manufacturing process, but many of the active agents in the solvents remain inside the plastic matrix and are released by combustion. It is for this reason that many plastic manufacturers mold their plastics in closely monitored conditions where the temperature is tightly controlled. Often, these molding chambers are filled with inert gas to drive out the oxygen and prevent residual heat from triggering combustion and open flames that quickly consume PVC material and fill the air with choking black smoke.
Reference.com notes that most organizations that handle or transport potentially hazardous material keep an inventory on hand in case of emergency. This inventory, or material safety data sheet, is helpful in identifying the exact chemicals that have been released when plastic burns. While many plastics melt safely, they release these potentially dangerous fumes when they burn.