Chapter 5.10 Polychlorinated biphenyls (PCBs) General description Physical and chemical properties Polychlorinated biphenyls (PCBs) are aromatic, synthetic chemicals which do not occur naturally in the environment. They consist of the biphenyl structure with two linked benzene rings in which some or all of the hydrogen atoms have been substituted by chlorine atoms. The basic molecular structure, including the conventional numbering of the substituent positions, is shown in Fig. 1. Fig. 1. Structural formula of PCBs The chemical formula of PCBs is C12H10-nCln, where n ranges from 1 to 10. Theoretically, 209 different congeners are possible, but only about 130 of these have been identified in commercial products. Ballschmiter and Zell (1) proposed a numbering system for the PCB congeners which has been adopted by the International Union of Pure and Applied Chemists (IUPAC) (Table 1). All congeners of PCBs are lipophilic and their lipophilicity increases with increasing degree of chlorination. However, they have very low water solubilities. Congeners with a lower degree of chlorination are more volatile than those with a higher degree. Pure individual PCB congeners are colourless and often crystalline. Commercial PCB mixtures are clear to light yellow oils or resins and they do not crystallize, even at low temperatures. PCBs are practically fire resistant because of their high flash points (170–380 °C). They form vapours which are heavier than air, but are not explosive. They have low electrical conductivity, high thermal conductivity and high resistance to thermal degradation. On the basis of these properties they have been used as dielectric isolators in electrical equipment. Like many organochlorine compounds, many of the congeners are highly persistent and accumulate within food chains. Investigations in many parts of the world have revealed widespread distribution of PCBs in the environment. The universal distribution of PCBs throughout the world, suggests that PCBs are transported in air (2). The ability of PCBs to co-distil, volatilize from landfills into the atmosphere (adsorption to aerosols with a particle size of < 0.05–20 µm), and resist degradation at low incinerating temperatures, makes atmospheric transport the primary mode of global distribution. In a study in the USA, 92% of the PCBs detected were in the vapour phase (2). In a German study, congeners with a low degree of chlorination were dominant in filtered air, whereas those with a high degree dominated in aerosols and rainfall (2). WHO Regional Office for Europe, Copenhagen, Denmark, 2000 1 Chapter 5.10 PCBs Air Quality Guidelines – Second Edition Table 1. IUPAC numbers and chlorine atom positions of all PCB congeners (1) a No. Structure No. Structure No. Structure No. Structure 1 2 56 2,3,3',4' 111 2,3,3',5,5' 166 2,3,4,4',5,6 2 3 57 2,3,3',5 112 2,3,3',5,6 167** 2,3',4,4',5,5' 3 4 58 2,3,3',5' 113 2,3,3',5',6 168 2,3',4,4',5',6 4 2,2' 59 2,3,3',6 114** 2,3,4,4',5 169* 3,3',4,4',5,5' 5 2,3 60 2,3,4,4' 115 2,3,4,4',6 170*** 2,2',3,3',4,4',5 6 2,3' 61 2,3,4,5 116 2,3,4,5,6 171 2,2',3,3',4,4',6 7 2,4 62 2,3,4,6 117 2,3,4',5,6 172 2,2',3,3',4,5,5' 8 2,4' 63 2,3,4',5 118** 2,3',4,4',5 173 2,2',3,3',4,5,6 9 2,5 64 2,3,4',6 119 2,3',4,4',6 174 2,2',3,3',4,5,6' 10 2,6 65 2,3,5,6 120 2,3',4,5,5' 175 2,2',3,3',4,5',6 11 3,3' 66 2,3',4,4' 121 2,3',4,5',6 176 2,2',3,3',4,6,6' 12 3,4 67 2,3',4,5 122 2',3,3',4,5 177 2,2',3,3',4',5,6 13 3,4' 68 2,3',4,5' 123** 2',3,4,4',5 178 2,2',3,3',5,5',6 14 3,5 69 2,3',4,6 124 2',3,4,5,5' 179 2,2',3,3',5,6,6' 15 4,4' 70 2,3',4',5 125 2',3,4,5,6' 180*** 2,2',3,4,4',5,5' 16 2,2',3 71 2,3',4',6 126* 3,3',4,4',5 181 2,2',3,4,4',5,6 17 2,2',4 72 2,3',5,5' 127 3,3',4,5,5' 182 2,2',3,4,4',5,6' 18 2,2',5 73 2,3',5',6 128 2,2',3,3',4,4' 183 2,2',3,4,4',5',6 19 2,2',6 74 2,4,4',5 129 2,2',3,3',4,5 184 2,2',3,4,4',6,6' 20 2,3,3' 75 2,4,4',6 130 2,2',3,3',4,5' 185 2,2',3,4,5,5',6 21 2,3,4 76 2',3,4,5 131 2,2',3,3',4,6 186 2,2',3,4,5,6,6' 22 2,3,4' 77* 3,3',4,4' 132 2,2',3,3',4,6' 187 2,2',3,4',5,5',6 23 2,3,5 78 3,3',4,5 133 2,2',3,3',5,5' 188 2,2',3,4',5,6,6' 24 2,3,6 79 3,3',4,5' 134 2,2',3,3',5,6 189** 2,3,3',4,4',5,5' 25 2,3',4 80 3,3',5,5' 135 2,2',3,3',5,6' 190 2,3,3',4,4',5,6 26 2,3',5 81 3,4,4',5 136 2,2',3,3',6,6' 191 2,3,3',4,4',5',6 27 2,3',6 82 2,2',3,3',4 137 2,2',3,4,4',5 192 2,3,3',4,5,5',6 28 2,4,4' 83 2,2',3,3',5 138 2,2',3,4,4',5' 193 2,3,3',4',5,5',6 29 2,4,5 84 2,2',3,3',6 139 2,2',3,4,4',6 194 2,2',3,3',4,4',5,5' 30 2,4,6 85 2,2',3,4,4' 140 2,2',3,4,4',6' 195 2,2',3,3',4,4',5,6 31 2,4',5 86 2,2',3,4,5 141 2,2',3,4,5,5' 196 2,2',3,3',4,4',5,6' 32 2,4',6 87 2,2',3,4,5' 142 2,2',3,4,5,6 197 2,2',3,3',4,4',6,6' 33 2',3,4 88 2,2',3,4,6 143 2,2',3,4,5,6' 198 2,2',3,3',4,5,5',6 34 2',3,5 89 2,2',3,4,6' 144 2,2',3,4,5',6 199 2,2',3,3',4,5,6,6' 35 3,3',4 90 2,2',3,4',5 145 2,2',3,4,6,6' 200 2,2',3,3',4,5',6,6' 36 3,3',5 91 2,2',3,4',6 146 2,2',3,4',5,5' 201 2,2',3,3',4',5,5',6 37 3,4,4' 92 2,2',3,5,5' 147 2,2',3,4',5,6 202 2,2',3,3',5,5',6,6' 38 3,4,5 93 2,2',3,5,6 148 2,2',3,4',5,6' 203 2,2',3,4,4',5,5',6 39 3,4',5 94 2,2',3,5,6' 149 2,2',3,4',5',6 204 2,2',3,4,4',5,6,6' 40 2,2',3,3' 95 2,2',3,5',6 150 2,2',3,4',6,6' 205 2,3,3',4,4',5,5',6 41 2,2',3,4 96 2,2',3,6,6' 151 2,2',3,5,5',6 206 2,2',3,3',4,4',5,5',6 42 2,2',3,4' 97 2,2',3',4,5 152 2,2',3,5,6,6' 207 2,2',3,3',4,4',5,6,6' 43 2,2',3,5 98 2,2',3',4,6 153 2,2',4,4',5,5' 208 2,2',3,3',4,5,5',6,6' 44 2,2',3,5' 99 2,2',4,4',5 154 2,2',4,4',5,6' 209 2,2',3,3',4,4',5,5',6,6' 45 2,2',3,6 100 2,2',4,4',6 155 2,2',4,4',6,6' 46 2,2',3,6' 101 2,2',4,5,5' 156** 2,3,3',4,4',5 47 2,2',4,4' 102 2,2',4,5,6' 157** 2,3,3',4,4',5' 48 2,2',4,5 103 2,2',4,5',6 158 2,3,3',4,4',6 49 2,2',4,5' 104 2,2',4,6,6' 159 2,3,3',4,5,5' 50 2,2',4,6 105** 2,3,3',4,4' 160 2,3,3',4,5,6 51 2,2',4,6' 106 2,3,3',4,5 161 2,3,3',4,5',6 52 2,2',5,5' 107 2,3,3',4',5 162 2,3,3',4',5,5' 53 2,2',5,6' 108 2,3,3',4,5' 163 2,3,3',4',5,6 54 2,2',6,6' 109 2,3,3',4,6 164 2,3,3',4',5',6 55 2,3,3',4 110 2,3,3',4',6 165 2,3,3',5,5',6 a Marked congeners have been assigned "toxic equivalency factors" (TEFs): * non-ortho congener; ** mono-ortho congener; *** di-ortho congener.