1.1. Identity, physical and chemical properties, and analytical methods. Polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) are almost planar tricyclic aromatic compounds. Theoretically, 75 PBDDs and 135 PBDFs are possible. In addition, a large number of mixed halogenated congeners - 1550 brominated/chlorinated dibenzo-p-dioxins (PXDDs) and 3050 brominated/chlorinated dibenzofurans (PXDFs) - are theoretically possible. Because of the complexity of the analytical procedures and paucity of analytical reference standards, it has been possible to characterize and determine only a small number of these compounds. The most toxic congeners are those substituted at positions 2, 3, 7, and 8. There are 7 2,3,7,8-substituted PBDDs and 10 2,3,7,8-substituted PBDFs, as well as 337 possible 2,3,7,8-substituted PXDDs and 647 possible 2,3,7,8-substituted PXDFs. PBDDs/PBDFs have higher molecular weights than their chlorinated analogues, high melting points, low vapour pressures, and low water solubilities. They are generally soluble in fats, oils, and organic solvents. There are very few experimental data on the physical and chemical properties of PBDDs/PBDFs. Photolysis occurs at a more rapid rate for PBDDs/PBDFs than for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PBDDs/PBDFs are thermostable. The temperatures of formation and destruction of PBDDs/PBDFs depend on several conditions, including the presence or absence of oxygen, polymers, and flame retardant additives, such as antimony trioxide (Sb2O3). In the presence of excess chlorine, bromine is substituted by chlorine to give PXDDs/PXDFs. Because of the toxic nature of these compounds and their photolytic properties, care must be taken during sampling and analysis. Highly sensitive, selective, and specific analytical methods (gas chromatography/mass spectrometry, or GC/MS) are required because of the large number of PBDD/PBDF congeners. Sampling procedures are identical for all polyhalogenated dibenzo-p-dioxins (PHDDs) and polyhalogenated dibenzofurans (PHDFs), but separation and determination of PBDDs/PBDFs (and PXDDs/PXDFs) differ slightly from those of their chlorinated analogues. PBDDs/PBDFs have higher molecular weights and longer GC retention times than the chlorinated analogues, as well as different MS isotopic cluster patterns and interference compounds. Exact identification of specific brominated congeners is very limited owing to the small number of reference standards currently available. For the same reason, determination of mixed halogenated congeners is almost impossible. 1.2. Formation and sources of human and environmental exposure. PBDDs/PBDFs are not known to occur naturally. They are not intentionally produced (except for scientific purposes) but are generated as undesired by-products in various processes. They can be formed by chemical, photochemical, or thermal reactions from precursors and by so-called de novo synthesis. PBDDs/PBDFs have been found as contaminants in brominated organic chemicals (e.g. bromophenols) and, in particular, in flame retardants, such as polybrominated diphenyl ethers (PBDEs), decabromobiphenyl (decaBB or DBB), 1,2-bis(tribromophenoxy)ethane, tetrabromobisphenol A (TBBPA), and others. They have been detected in distillation residues of some bromophenols and bromoanilines and in wastes from chemical laboratories. PBDFs and, to a lesser extent, PBDDs have been detected as photochemical degradation products of brominated organic chemicals, such as PBDEs and bromophenols. Laboratory thermolysis experiments showed the formation of PBDDs/PBDFs from bromophenols, PBDEs, polybrominated biphenyls (PBBs), and other brominated flame retardants (pure or in a polymer matrix). There was a broad range of yields, from zero to maximum values (reached from PBDEs) in the g/kg range. Generally, PBDFs were much more abundant than PBDDs. The optimum PBDF formation temperature of a series of pure flame retardants was in the range of 600-900°C. The presence of polymers or synergist (e.g. Sb2O3) resulted in a decrease in the optimum formation temperature (down to 400°C). In addition to temperature and the presence of polymer matrix or synergists, several other factors, such as metals, metal oxides, water, oxygen, and the type of combustion apparatus used, influenced the yield and pattern of PBDDs/PBDFs. In ternary mixtures of PBDE, polymer matrix, and Sb2O3, tetrabromodibenzofurans (tetraBDFs or TeBDFs) were frequently the most abundant homologue group. 2,3,7,8-Substituted PBDDs/PBDFs (tetra to hepta) were found at varying concentrations; for example, 2,3,7,8-TeBDF was found at up to 2000 mg/kg in pyrolysates of polymers containing octabromodiphenyl ether (octaBDE or OBDE). In the manufacture of plastics, elevated temperatures (150-300°C) occur during several processes. Studies of the exhaust streams from machines processing polymers - such as acrylonitrile-butadiene-styrene (ABS) and polybutylene terephthalate (PBT) - containing different types of brominated flame retardants showed that PBDDs/PBDFs (di to octa) can be formed at these temperatures. OBDE and decabromodiphenyl ether (decaBDE or DBDE) produced the highest amounts of PBDDs/PBDFs, the major portion consisting of PBDFs. Levels observed with TBBPA or bis-tetrabromophthalimide ethylene (TBPI) were several orders of magnitude lower. No PBDDs/PBDFs were detected during processing of ABS flame-retarded by brominated styrene or 1,2-bis(tribromophenoxy) ethane. 2,3,7,8-Substituted congeners were not determined (processing of DBDE), were detected at trace levels (processing of OBDE), or were not detected (processing of TBBPA and TBPI). Various plastic materials at several processing stages were analysed for PBDDs/PBDFs.