Currently, PCDDs are primarily released to the environment during combustion of fossil fuels (coal, oil, and natural gas) and wood, and during incineration processes, municipal and medical solid waste and hazardous waste incineration. While incineration may be the primary current source of release of PCDDs into the environment, the levels of PCDDs produced by incineration are extremely low. PCDDs are associated with ash generated in combustion and incineration processes. Emissions from incinerator sources vary greatly and depend on management practices and applied technologies. PCDDs also have been detected at low concentrations in cigarette smoke, homeheating systems, and exhaust from cars running on leaded gasoline or unleaded gasoline, and diesel fuel. Burning of many materials that may contain chlorine, such as plastics, wood treated with pentachlorophenol (PCP), pesticidetreated wastes, other polychlorinated chemicals (polychlorinated biphenyls or PCBs), and even bleached paper can produce PCDDs and even home frying.
PCDDs enter the environment as mixtures containing a variety of individual components and impurities. In the environment they tend to be associated with ash, soil, or any surface with a high organic content, such as plant leaves. In air and water, a portion of the PCDDs may be found in the vapour or dissolved state, depending on the amount of particulate matter, temperature, and other environmental factors.
PCDDs are known to occur naturally. They are naturally produced from the incomplete combustion of organic material by forest fires or volcanic activity. PCDDs are not intentionally manufactured by industry, except in small amounts for research purposes.