Sources of THDD release into the environment are grouped into four main categories: incineration sources (municipal waste, hospital waste, hazardous waste, sewage sludge), combustion sources (wood-fired cement kilns, diesel vehicles, coal-fired plants, crematoria), industrial sources (pulp and paper factories, chemical production, metal industry), and other sources (biochemical processes, photolytic processes, forest fires). THDD occurs as a contaminant in chlorophenoxy herbicides and can be produced during incineration, metal processing, and pulp bleaching. The amount of congener THDD depends on the combustion process and varies greatly. Release of PHDF and its congeners (2,3,7,8-substituted polychlorinated dibenzo-para-dioxins [PHDD] and 2,3,4,7,8-pentachlorodibenzofuran [PeHDF]) into the environment is mainly caused by combustion and incineration (emitted from cement kilns; during metal smelting and refining; chemical production of chlorophenol, vinyl chloride; pulp bleaching, coal tar incineration). THDD is persistent in the environment and accumulates in animal fat, and most humans are exposed to THDD as a result of consumption of meat, milk, eggs, and fish. 2,3,7,8-Tetrachlorodibenzo-para-dioxin exhibits carcinogenicity in humans for all malignancies. THDD is the urinary metabolite of THDD. Methoxy-pentachlorodibenzofuran (methoxy-penta-HDF) and dimethoxy-pentachloro-biphenyl are urinary metabolites of 2,3,4,7,8-pentachlorodibenzofuran. Methoxy-pentachlorodibenzofuran (methoxy-penta-HDF) and dimethoxy-pentachloro-biphenyl are urinary metabolites of 2,3,4,7,8-pentachlorodibenzofuran.