12. Polychlorinated Biphenyls (PCBs):
Uses and Environmental Releases

Heidelore Fiedler
Bavarian Institute for Waste Research - BIfA GmbH
Am Mittleren Moos 46a, D-86167 Augsburg, Germany

SEE CHAPTER 6.2

Introduction

Polychlorinated biphenyls (PCBs) comprise a class of 209 individual compounds. The chemical structure and the numbering of the C-atoms are shown in Figure 1.

 

 

 

 

Figure 1: Structure and numbering for PCBs

  1. Sources and Chemical Identity of PCBs

The two main sources of PCBs are:

PCBs are produced by chlorination of biphenyl; its commercial production started about 60 years ago. The total amount produced world-wide is estimated at 1.5 million tons [Ivanov and Sandell 1992, Rantanen 1992].

Depending on the degree of chlorination of the PCBs, their physico-chemical properties, like inflammability or electric conductivity, brought about a wide field of application. Thus, PCBs have been used as electric fluids in transformers and capacitors, as pesticide extenders, adhesives, dedusting agents, cutting oils, flame retardants, heat transfer fluids, hydraulic lubricants, sealants, paints, and in carbonless copy paper. Some of their applications resulted in a direct or indirect release of PCBs into the environment. Relatively large amounts were released due to inappropriate disposal practices, accidents and leakages from industrial facilities.

PCBs were marketed with respect to percentage of their chlorine content (by weight) and were available under several trade names, e.g. Clophen (Bayer, Germany), Aroclor (Monsanto, USA), Kanechlor (Kanegafuchi, Japan), Santotherm (Mitsubishi, Japan), Phenoclor and Pyralene (Prodolec, France). In 1972, PCB production plants existed in Austria, Federal Republic of Germany, France, Great Britain, Italy, Japan, Spain, USSR, and USA. In the USA, the Monsanto industrial Chemical Company (the market leader with 98% of PCB/PCT production) terminated production and export in 1977. In Japan, the production of PCBs was started by Kanegafuchi Chemical Co. Ltd. (Kaneka) in 1954 and marketed under the trade name "Kaneclor (KC)". PCB production, use and import were banned in Japan in 1972.

    1. Production of PCBs in Germany and the United States

The composition and degree of chlorination of commercial mixtures of PCBs are given in Table 1 (here: Aroclors, the commercial products of the Monsanto Company, USA) (WHO 1987). With the exception of Aroclor 1016, which contains 41% chlorine by weight, the last two digits of the numerical designation represents the weight percentage of chlorine in the mixture. Different Aroclors tended to be used in different years and for different purposes. In U.S. electrical equipment manufacturing, Aroclor 1260 was more used prior to 1950, as was Aroclor 1254, Aroclor 1242 was the dominant mixture used in the 1950s and 1960s. It was phased out in 1971 and replaced with Aroclor 1016, which was about the same in percentage chlorine but did contain less PCBs with four or more chlorine atoms. This variation in usage led to variation in the type of human exposure over time and to geographic differences.

     

Percentage of Congeners with Indicated

     

Number of Chlorine Atoms

Commercial

Percent

 

1

2

3

4

5

6

7

8

9

10

PCB Product

Chlorine

Number of Isomers Possible

3

12

23

42

46

42

24

12

3

1

Aroclor 1232

32

 

26

29

24

14

           
Aroclor 1016

41

 

2

19

57

22

           
Aroclor 1242

42

 

3

13

28

30

22

4

       
Aroclor 1248

48

   

2

18

40

36

4

       
Aroclor 1254

54

       

11

49

34

6

     
Aroclor 1260

60

         

12

38

41

8

1

 

In (West-) Germany Bayer AG produced PCBs under the trade name 'Clophen' (A30 to A60). In 1972, the Bayer AG restricted their supply of PCBs for use in closed systems (transformers, condensers, hydraulic fluid). Until then approximately a total of 23,000 t of PCBs were used in "open systems". In Table 2 the production of PCBs in Germany for the years of 1974 to 1983 is listed separately for the various degrees of chlorination. It shows that there was shift in production from higher chlorinated PCBs to lower chlorinated PCBs over the years. It also shows that the use of PCBs in Germany remained more or less constant up to 1978 and then decreased continuously. It is interesting to note that the production of PCBs increased until 1980 with an increased proportion going into export. In 1983, the last year of PCB production by Bayer AG, 90% of the PCBs produced was exported.

t/year

Degree of Chlorination (% Cl)

Total

Used in

Export

 

39

42.5

47

48.5

54

55

60

Production

Germany

 
1974 -- 2449 460 -- -- 1619 1810 6338 2920 3258
1975   1648 292     1466 2141 5447 3400 2047
1976   2170 454     970 1436 5030 2789 2241
1977 139 2500 -- 525 2516 --   5680 2910 2770
1978 690 4061   666 2223     7640 2824 4816
1979 937 3379   865 1963     7144 2446 4698
1980 799 4180   1127 1358     7464 2447 5017
1981 -- 4778   -- --     4778 1180 3598
1982   3734           3734 968 2766
1983   4355           4355 430 3925

In used in Germany in 1985.

Table 3

the distribution of PCBs for the three closed systems transformers, condensers and coal mining are shown. While the use of PCBs in condensers and transformers decreased continuously from 1974 to 1983 there was an increased use of PCBs in mining. This was due to technical developments in coal mining with an increased demand for fire-resistant hydraulic fluids. Part of this demand had to be satisfied by import of PCBs from the French company Prodelec. In coal mining new PCBs were last used in Germany in 1985.

Table 3: Use of PCB in Germany 1974-1984 (t PCB)

Year

Condensers

Transformers

Coal Mining

Total

1974 1075 1130 871 3076
1975 752 1656 818 3226
1976 649 1125 930 2704
1977 637 740 967 2344
1978 446 590 1158 2194
1979 306 392 1361 2059
1980 253 334 1587 2174
1981 113 305 1350 1768
1982 30 318 1482 1830
1983 6 40 1241 1287
1984 -- -- 607 607
Sum 1974-1984 4267 6630 12372 23269

 

No.

Structure No. Structure No. Structure No. Structure

Monochlorobiphenyls

Tetrachlorobiphenyls Pentachlorobiphenyls Hexachlorobiphenyls

1

2

52

2,2',5,5'

107

2,3,3',4',5

161

2,3,3',4,5',6

2

3

53

2,2',5,6'

108

2,3,3',4,5'

162

2,3,3',4',5,5'

3

4

54

2,2',6,6'

109

2,3,3',4,6

163

2,3,3',4',5,6
   

55

2,3,3',4

110

2,3,3',4',6

164

2,3,3',4',5',6
Dichlorobiphenyls

56

2,3,3',4'

111

2,3,3',5,5'

165

2,3,3',5,5',6

4

2,2'

57

2,3,3',5

112

2,3,3',5,6

166

2,3,4,4',5,6

5

2,3

58

2,3,3',5'

113

2,3,3',5',6

167

2,3',4,4',5,5'

6

2,3'

59

2,3,3',6

114

2,3,4,4',5

168

2,3',4,4',5',6

7

2,4

60

2,3,4,4'

115

2,3,4,4',6

169

3,3',4,4',5,5'

8

2,4'

61

2,3,4,5

116

2,3,4,5,6    

9

2.5

62

2,3,4,6

117

2,3,4',5,6 Heptachlorobiphenyls

10

2,6

63

2,3,4',5

118

2,3',4,4',5

170

2,2',3,3',4,4',5

11

3,3'

64

2,3,4',6

119

2,3,4,4',6

171

2,2',3,3',4,4',6

12

3,4

65

2,3,5,6

120

2,3',4,5,5'

172

2,2',3,3',4,5,5'

13

3,4'

66

2,3',4,4'

121

2,3',4,5',6

173

2,2',3,3',4,5,6

14

3,5

67

2,3',4,5

122

2',3,3',4,5

174

2,2',3,3',4,5,6'

15

4,4'

68

2,3',4,5'

123

2',3,4,4',5

175

2,2',3,3',4,5',6
   

69

2,3',4,6

124

2',3,4,5,5'

176

2,2',3,3',4,6,6'
Trichlorobiphenyls

70

2,3',4',5

125

2',3,4,5,6'

177

2,2',3,3',4',5,6

16

2,2',3

71

2,3,4',6

126

3,3',4,4',5

178

2,2',3,3',5,5',6

17

2,2',4

72

2,3',5,5'

127

3,3',4,5,5'

179

2,2',3,3',5,6,6'

18

2,2',5

73

2,3',5',6    

180

2,2',3,4,4',5,5'

19

2,2’,6

74

2,4,4',5 Hexachlorobiphenyls

181

2,2',3,4,4',5,6

20

2,3,3'

75

2,4,4',6

128

2,2',3,3',4,4'

182

2,2',3,4,4',5,6'

21

2,3,4

76

2',3,4,5

129

2,2',3,3',4,5

183

2,2',3,4,4',5',6

22

2,3,4'

77

3,3',4,4'

130

2,2',3,3',4,5'

184

2,2',3,4,4',6,6'

23

2,3,5

78

3,3',4,5

131

2,2',3,3',4,6

185

2,2',3,4,5,5',6

24

2,3,6

79

3,3',4,5'

132

2,2',3,3',4,6'

186

2,2',3,4,5,6,6'

25

2,3',4

80

3,3',5,5'

133

2,2',3,3',5,5'

187

2,2',3,4',5,5',6

26

2,3',5

81

3,4,4',5

134

2,2',3,3',5,6

188

2,2',3,4',5,6,6'

27

2,3',6    

135

2,2',3,3',5,6'

189

2,3,3',4,4',5,5'

28

2,4,4' Pentachlorobiphenyls

136

2,2',3,3',6,6'

190

2,3,3',4,4',5,6

29

2,4,5

82

2,2',3,3',4

137

2,2',3,4,4',5

191

2,3,3',4,4',5',6

30

2,4,6

83

2,2',3,3',5

138

2,2,3,4,4',5'

192

2,3,3',4,5,5',6

31

2,4',5

84

2,2',3,3',6

139

2,2',3,4,4',6

193

2,3,3',4',5,5',6

32

2,4',6

85

2,2',3,4,4'

140

2,2',3,4,4',6'    

33

2',3,4

86

2,2',3,4,5

141

2,2',3,4,5,5' Octachlorobiphenyls

34

2',3,5

87

2,2',3,4,5'

142

2,2',3,4,5,6

194

2,2',3,3',4,4',5,5'

35

3,3',4

88

2,2',3,4,6

143

2,2',3,4,5,6'

195

2,2',3,3',4,4',5,6

36

3,3',5

89

2,2',3,4,6'

144

2,2',3,4,5',6

196

2,2',3,3',4,4',5',6

37

3,4,4'

90

2,2',3,4',5

145

2,2',3,4,6,6'

197

2,2',3,3',4,4',6,6'

38

3,4,5

91

2,2',3,4',6

146

2,2',3,4',5,5'

198

2,2',3,3',4,5,5',6

39

3,4',5

92

2,2',3,5,5'

147

2,2',3,4',5,6

199

2,2,3,3',4',5,5',6
   

93

2,2',3,5,6

148

2,2',3,4,5,6'

200

2,2',3,3',4,5,6,6'
Tetrachlorobiphenyls

94

2,2',3,5,6'

149

2,2',3,4',5’6

201

2,2',3,3',4,5',6,6'

40

2,2',3,3'

95

2,2',3,5',6

150

2,2',3,4',6,6'

202

2,2',3,3',5,5',6,6'

41

2,2',3,4

96

2,2',3,6,6'

151

2,2',3,5,5',6

203

2,2',3,4,4',5,5',6

42

2,2',3,4'

97

2,2',3',4,5

152

2,2',3,5,6,6

204

2,2',3,4,4',5,6,6'

43

2,2',3,5

98

2,2',3,4,6

153

2,2',4,4',5,5'

205

2,3,3',4,4',5,5',6'

44

2,2,3,5'

99

2,2',4,4',5

154

2,2',4,4,5,6'    

45

2,2',3,6

100

2,2',4,4',6

155

2,2',4,4',6,6' Nonachlorobiphenyls

46

2,2',3,6'

101

2,2',4,5,5'

156

2,3,3',4,4',5

206

2,2',3,3',4,4',5,5',6

47

2,2',4,4'

102

2,2',4,5,6'

157

2,3,3',4,4',5'

207

2,2',3,3',4,4',5,6,6'

48

2,2',4,5

103

2,2',4,5',6

158

2,3,3',4,4',6

208

2,2',3,3',4,5,5',6,6'

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

2,3,4',4,5,6 Decachlorobiphenyl

51

2,2',4,6'

106

2,3,3',4,5     209 2,2',3,3',4,4',5,5',6,6'

As can be seen from Table 5, the number of possible isomers within the same degree of chlorination varies.

Table 5: Molecular formula, name, number of isomers, IUPAC number, molecular mass, percentage of chlorine and number of isomers identified

Molecular

Name:

Number of

IUPAC-No.

Molecular

% of

No. of Isomers

Formula

Chlorobiphenyl

Isomers

 

Mass

Chlorine

Identified

C12H9Cl Mono 3

1-3

188.65 18.79 3
C12H8Cl2 Di 12

4-15

233.10 31.77 12
C12H7Cl3 Tri 24

16-39

257.54 41.30 23
C12H6Cl4 Tetra 42

40-81

291.99 48.65 41
C12H5Cl5 Penta 46

82-127

326.43 54.30 39
C12H4Cl6 Hexa 42

128-169

360.88 58.93 31
C12H3Cl7 Hepta 24

170-193

395.32 62.77 18
C12H2Cl8 Octa 12

194-205

429.77 65.98 11
C12HCl9 Nona 3

206-208

464.21 68.73 3
C12Cl10 Deca 1

209

498.66 71.10 1

Some physical and chemical properties of the PCBs made them suitable for a broad range of applications. Important characteristics are:

In general, melting point and lipophilicity increase with increasing degree of chlorination; vapour pressure and water solubility decrease. Thus, all PCBs are lipophilic and poorly soluble in water. Water solubilities for Aroclors were determined in the range from 0.0027-0.42 ng/L.

IUPAC-No.

S a

S b

S c

S d

0

5.94-7.48

     
1

4.13-7.8

 

4.13

5.9

4

0.79-1.5

1.207

0.79

1.5

28

0.085-0.266

0.117

0.260

0.085

52

0.006-0.046

0.110

0.027

0.046

77

0.000569-0.175

0.00055

0.00075

0.175

101

0.00424-0.031

0.007

0.004

0.031

153

0.0012-0.0095

     
182    

0.00047

0.00048

209

0.000004-0.015

     

Water solubility will increase in the presence of organic solvents (see Table 7).

IUPAC Name Methanol Butanol Octanol Benzene Benzyl alcohol
3 4-MonoCB 0.74 0.45 -0.10 -0.75 -0.12
30 2,4,6-TriCB 0.89 0.74 -0.26 -0.57 -0.04
61 2,3,4,5-TetraCB 1.02 1.25 -0.28 -0.77 -0.60
155 2,2',4,4',6,6'-HexaCB 1.22 1.76 -0.33 -0.70 0.39

The vapour pressure for some environmental pollutants are given in Table 8 and the vapour pressure from the solid phase and from the sub-cooled phase for selected PCBs in  

 

Table 9

. Compounds can be classified into three classes:

Class of Compounds

Vapour Pressure at 25C (atm)

Halogenated C1 and C2 hydrocarbons

10-2-1

Alkylbenzenes (butylbenzene-benzene)

510-4-10-1

Chlorobenzenes (hexachlorobenzene-monochlorobenzene)

10-8-510-2

Phthalate esters

10-7-10-4

Polycyclic aromatic hydrocarbons (PAH)

10-11-10-3

Aliphatic hydrocarbons (C18-C5)

10-7-1

PCB

10-12-10-4

 

IUPAC-No.

Substitution

A

B

C

D

   

Solid

Subcooled Liquid

     
4 2,2' 1.8210-6 4.110-6 1.510-6 4.1810-6 3.3110-6
7 2,4 1.7910-6 9.8610-7 1.7310-6 2.0910-6  
9 2,5 1.9410-6 1.9510-6 2.2910-6    
11 3,3' 3.3110-7 4.0810-7 6.3810-7 9.110-7  
12 3,4 7.2610-9 1.2710-8 5.2510-7 7.7510-7  
15 4,4' 3.2410-8 5.4110-7 5.0110-7 7.410-7  
18 2,2',5 7.5210-7 1.1510-6 3.510-7 8.9210-7 7.5710-7
26 2,3',5 3.1910-7 4.5310-7 1.810-7 3.4810-7 4.0710-7
28 2,4,4' 1.4310-7 3.010-7 1.510-7 2.7310-7 3.3110-7
30 2,4,6 6.3610-7 1.510-6 9.3410-7 1.0910-6  
40 2,2',3,3' 1.0810-8 9.4410-8 4.510-8 1.110-7 8.6910-8
52 2,2',5,5' 1.2810-7 5.3910-7 8.910-7 1.910-7 1.8210-7
53 2,2',5,6' 6.6210-8 4.010-7 1.110-7 3.5110-7 2.6310-7
54 2,2',6,6' 2.2410-8 1.1610-6 6.510-7 5.5910-7  
77 3,3',4,4' 1.810-10 5.1910-9 1.3810-8 2.0910-8  
101 2,2',4,5,5' 5.210-9 1.7110-8 1.410-8 3.5310-8 3.5410-8
104 2,2',4,6,6' 4.2810-8   1.6810-7 4.2810-8  
128 2,2',3,3',4,4' 2.910-11 4.610-10 0.9710-9 3.5410-9 3.6210-9
153 2,2'4,4',5,5' 3.210-10 1.910-9 2.510-9 6.5410-8 6.9110-8
155 2,2',4,4',6,6' 4.7410-9 3.4910-9 4.3710-8    

Henry’s Constant (H) is an important parameter to describe environmental behaviour of atmospheric pollutants. Henry’s Constant gives the ratio between vapour pressure (saturated) and the solubility of the compound in water.

Class of Compounds Henry’s Constant (atmL/mol)
Halogenated C1 and C2 hydrocarbons

0.5-50

Alkylbenzenes (butylbenzene-benzene)

1-10

Chlorobenzenes (hexachlorobenzene-monochlorobenzene)

0.5-10

Phthalate esters

0.001-0.002

Polycyclic aromatic hydrocarbons (PAH)

0.005-1

Alipatic hydrocarbons (C18-C5)

10-10000

PCB

0.01-1

Henry’s constant for PCBs is in the range 0.8 atm L/mol for 2-monochlorobiphenyl and 0.018atm L/mol for decachlorobiphenyl and thus, in the same range like 2,3,7,8-Cl4DD. Henry’s Law Constant is temperature dependent: H increases 10-fold in the temperature range 14-50 C. Within the same degree of chlorination, H increases with the umber of Cl atoms in ortho position [see Fiedler et al. 1994].

  1. PCB Levels in the Environment
  2. PCBs have been identified in almost every environmental compartment or matrix. Detected levels depend on the nature and location of the particular environmental sample. However, congener-specific analytical procedures for qualitative and quantitative detection of PCBs have not been developed as far as, for example, analysis of PCDD/PCDF.

    When compared to other chemicals, PCBs have very high KOW values: log KOW are in the range from 4.5 for monochlorobiphenyls to >8 for higher chlorinated PCBs. Consequently, PCBs tend to adsorb to unpolar surfaces and accumulate in lipophilic matrices along the aquatic and terrestrial food-chain. Some physical-chemical characteristics are given in Table 11.

    Table 11: Influence of chlorine substituents on the chemical-physical properties of hydrocarbons
    S = Water solubility, KOW = Octanol/water partition coefficient

    Compound

    Number of Chlorine Atoms

    S (mg/L)

    log KOW

    Benzene

    0

    1,780 2.13
    Hexachlorobenzene

    6

    0.006 6.18
    Phenol

    0

    82,000 1.45
    Pentachlorophenol

    5

    14 3.7
    Biphenyl

    0

    5.9-7.5 3.89
    PCB 209

    10

    0.000004 8.23
    Dibenzo-p-dioxin

    0

    0.842 4.3
    2,3,7,8-Cl4DD

    4

    0.000008 7
    Cl8DD

    8

    0.0000004 8.2

    Commercial PCBs, as well as environmental extracts, contain complex mixtures of congeners. PCB mixtures found in environmental matrices usually do not resemble the commercial PCB mixtures. For example, a congener-specific analysis of PCBs showed remarkable differences between a commercial Aroclor 1260 mixture and human breast milk [Safe 1990 and 1994, Norn and Lundn 1991]. This difference is due to the fact that the most abundant PCBs in commercial mixtures are ortho-substituted congeners which are readily degradable. However, smaller amounts of the so-called „dioxin-like" PCBs, namely the coplanar (= non-ortho substituted) and mono-ortho substituted congeners, are present in the commercial mixtures as well. The latter are very stable and resistant to biodegradation and metabolism. Moreover, it is well known that lower chlorinated PCBs can volatilise and are, thus, more susceptible to atmospheric removal processes [Mackay et al. 1992].

    Generally, the PCB levels found in environmental matrices are higher than the levels of PCDD/PCDF. This is due to the fact that besides thermal formation significant amounts of PCBs were and still are released via diffuse emissions from industrial products.

    Input of PCBs into soil occurs - as for other lipophilic chemicals - either from spills, direct application of, e.g. sludges, or via dry and wet deposition. The organic carbon of soil is the natural sink for such unpolar lipophilic substances. Due to the strong affinity to organic carbon, PCBs are quite immobile in soils. In combination with the persistence of the PCBs, soils possess a memory effect and remember inputs long times ago as well as long-term diffuse inputs.

    Henry’s Law constants are 0-1 for mono- and dichlorobiphenyls, thus, these substances will be found preferentially in the gas phase and due to the low water solubility are not washed out with rainwater from the atmosphere. Higher chlorinated biphenyls are (completely) adsorbed to particulates and thus, can be removed from the atmosphere by capture of aerosols in rain drops. These two effects result in a relative accumulation of the lower chlorinated PCBs in the atmosphere [Duinker and Bouchertall 1989]. Air concentrations are in the pg/m to ng/m range with lower levels in remote and rural areas. Background air levels in the USA were constant in the range of 1 ng/m over several years with tri- and tetrachlorinated congeners dominating.

    Reports on the occurrence of PCBs in fish, mussels, seals, sea birds and birds of prey first appeared in 1966, and in 1967, PCBs were detected in human adipose tissue, albeit in low concentrations. In 1968, PCBs from a leaking cooling system contaminated a rice oil tank at a food factory in Japan. As a result of the consumption of the contaminated rice oil which had reached the stores, 1,000 people fell ill with a disease subsequently known world-wide as Yusho Disease.

    PCBs in the Great Lakes display a more complex behaviour. They were found to volatilise where a river discharges relatively high PCB loads into Green Bay, Wisconsin. Baker and Eisenreich [1990] calculated an average volatilisation rate of PCBs from Lake Superior which approximately equals their atmospheric deposition. His findings support the conceptual model that these compounds permanently cycle between atmosphere and natural waters [Mackay et al. 1986]. According to this model, PCBs dissolved in rain drops or sorbed to particulates are washed out of the atmosphere by rain. This input of PCBs into surface waters results in a fugacity gradient towards the atmosphere, which in turn drives volatilisation.

    Relatively new PCB data exist for sediments and suspended particles in German rivers. Along the river Saar it was found that close to locations with heavy industry (coal mining and steel industry) PCBs and Ugilec (a commercial mixture of tetrachlorinated 2-methyl-diphenylmethanes) levels were higher than normal. Differentiation in depth showed that Ugilecs were only found in more recent sediments whereas PCBs could be detected down to 1.2 m with higher concentrations in the older sediments [see Fiedler et al. 1994].

  3. Environmental Fate of PCBs
    1. Biodegradation

Biodegradation by microorganisms may occur via three different mechanisms [for summary, see Fiedler et al. 1994]:

      1. Aerobic Degradation

In general, bacteria cannot use chlorinated aromatic hydrocarbons as substrate. Present knowledge assumes that bacteria growing on non-chlorinated biphenyl are capable to cause chemical reactions on the chlorinated ring system as well. However, some microorganisms are capable to use lower chlorinated PCBs as C-source. Thus, Acinetobacter sp. P6, Achromobacter sp. B 218, and Bacillus brevis B 257 can grow on 4-chlorobiphenyl as the only carbon source. The main degradation product is 4-chlorobenzoic acid. In general, formation of chlorinated benzoic acids is the major degradation pathway for PCBs. Further microorganisms capable of the biodegradation of PCBs belong to the class of Acetobacter, Alcaligenes, and Pseudomonas.

Some general conclusions can be drawn [Rochkind et al. 1986; for summary, see Fiedler et al. 1994]:

      1. Anaerobic Degradation

Polychlorinated biphenyls are extremely resistant to conventional aerobic transformation, but they will undergo anaerobic reductive dechlorination. Studies of PCB contamination in Hudson River sediment demonstrate that anaerobic environments yield markedly lower levels of tri-, tetra-, and pentachlorobiphenyls and higher levels of mono- and dichlorobiphenyls. Many of the less chlorinated PCBs then are aerobically biodegradable because generally less toxic than highly chlorinated PCBs.

For PCBs, the degradation rate is inversely related to the degree of chlorination; thus, highly chlorinated congeners are more readily dechlorinated than lower chlorinated congeners.

There is evidence that not only the number of chlorine substituents determines degradation rates, but also their position. Reductive dechlorination predominantly reduces chlorine in meta- and para-positions, resulting in accumulation of the ortho-chlorinated congeners. Addition of organic substrates, such as methanol, glucose or acetone stimulated dechlorination, whereas little stimulation was observed in cases with no organic additives [see Fiedler et al. 1994].

    1. Metabolism
    2. PCBs do not have reactive functional groups; thus, these lipophilic molecules have to be hydroxylated first to make them more polar and consequently subject for excretion. The rate limiting step in the elimination of PCBs is that of metabolism, which primarily occurs by the hepatic P-450-dependent monoxygenase system. Hydroxylated products are the major PCB metabolites and, based on available studies, it can be concluded that hydroxylation mainly occurs at para or meta positions if these sites are unsubstituted. The chlorine content, the substitution pattern and the presence of certain isoenzymes of the cytochrom-P-450 system are important factors to determine the transformation rate of PCBs [see Fiedler et al. 1994]. In general, metabolisation of PCBs decreases with increasing number of chlorine atoms present and with decreasing number of adjacent unsubstituted carbon atoms. Isoenzymes capable to metabolise Phenobarbital (PB) were found to metabolise the not dioxin-like PCBs whereas coplanar, non-ortho or mono-ortho-substituted (dioxin-like) PCBs can induce isoenzymes capable of metabolising 3-methylcholanthrene (MC) [Safe 1994]. Commercial PCBs, such as Aroclor 1254, induce both, MC and PB-inducible monooxygenases. Besides hydroxylation and subsequent conjugation, sulfur-containing metabolites, e.g. methyl sulfones, and partially dechlorinated metabolites have also been identified. Methyl sulfones have been shown to selectively accumulate in the Clara cells of rat lung and in lung tissues of mice. Methyl sulfonyl metabolites of 2,4,5-, 2,2,4,5-, and 2,2,4,5,5-PCBs have also been found in liver, adipose and fetal tissues and have been identified in environmental samples and in human milk [Ahlborg et al. 1992].

      Due to the low transformation and excretion rates of PCBs, certain congeners accumulate in organisms. Persistent congeners, such as 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) were found to promote tumours in rats, whereas 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) is easily degraded.

    3. Bioconcentration

Substances of low biological degradability tend to accumulate throughout trophic levels of the food net. For example, concentrations of total PCB increases with the trophic level. Only concentrations of PCBs in sediments are higher than levels in the subsequent trophic levels. It has been demonstrated that chlorinated dibenzo-p-dioxin and dibenzofuran congeners accumulate with considerable species differentiation. Contribution of the dioxin-like PCB congeners # 77, 105, and 126 to the total TEQ is substantially greater than that of PCDD/PCDF (even in cases of known PCDD/PCDF contamination). In all cases an exchange between trophic levels (sediment algae plankton planktivores piscivorous fish piscivorous birds) resulted in an increase of both, total PCB concentration and dioxin-like TEQ (for TEF/TEQ of PCBs, see section 5) [Fiedler et al. 1994].

 

  1. Toxicity of PCBs
  2. Commercial mixtures, as well as the individual PCB congeners, elicit a broad spectrum of biochemical and toxic responses, some of which are similar to those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Due to the fact that dioxin-like compounds normally exist in environmental and biological samples as complex mixtures of congeners, the concept of toxic equivalents (TEQ) has been developed to simplify risk assessment and regulatory control. In applying this concept, relative toxicities of dioxin-like compounds in relation to the reference compound (2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7,8-Cl4DD) were determined on the basis of results obtained in in vivo and in vitro studies (Table 12). Three coplanar PCBs, namely 3,3',4,4'-tetrachlorobiphenyl, 3,3',4,4',5-pentachlorobiphenyl, and 3,3',4,4',5,5'-hexachlorobiphenyl, exhibit dioxin-like effects, such as Ah-receptor agonist activity [Safe 1990 and 1994, Ahlborg et al. 1992 and 1994]. PCB 170 and PCB 180 are included because they are active as inducers of EROD activity and are present in significant amounts in environmental samples. In a WHO-workshop in 1997, new TEFs were proposed based on new information [WHO 1997]; the newly proposed TEFs are shown in the three columns at right of Table 12. As can be seen, there were different Tefs proposed for humans, fish, and birds (the latter two for wildlife risk estimates).

    Table 12:: TEF for coplanar and mono- and di-ortho-substituted PCBs [Ahlborg et al. 1994] 1 and TEF for humans, mammals, fish, and birds proposed by WHO [WHO 1997] 2

    PCB Substitution

    IUPAC

    TEF 1

    TEF 2 (1997)

    Congener  

    No.

    (1994)

    Humans

    Fish

    Birds

    Non-ortho 3,3',4,4'-TetraCB 77 0.0005 0.0001 0.0005 0.1
    substituted: 3,4,4’,5-TetraCB 81   0.0001 0.0001 0.05
      3,3',4,4',5-PentaCB 126 0.1 0.1 0.005 0.1
      3,3',4,4',5,5'-HexaCB 169 0.01 0.01 0.00005 0.001
    Mono-ortho 2,3,3',4,4'-PentaCB 105 0.0001 0.0001 <0.000005 0.0001
    substituted: 2,3,4,4',5-PentaCB 114 0.0005 0.0005 <0.000005 0.0001
      2,3',4,4',5-PentaCB 118 0.0001 0.0001 <0.000005 0.00001
      2',3,4,4',5-PentaCB 123 0.0001 0.0001 <0.000005 0.00001
      2,3,3',4,4',5-HexaCB 156 0.0005 0.0005 <0.000005 0.0001
      2,3,3',4,4',5'-HexaCB 157 0.0005 0.0005 <0.000005 0.0001
      2,3',4,4',5,5'-HexaCB 167 0.00001 0.00001 <0.000005 0.00001
      2,3,3',4,4',5,5'-HeptaCB 189 0.0001 0.0001 <0.000005 0.00001
    Di-ortho 2,2',3,3',4,4',5-HeptaCB 170 0.0001

    0

    0

    0

    substituted: 2,2',3,4,4',5,5'-HeptaCB 180 0.00001

    0

    0

    0

    To illustrate the consequences of the recommended PCB-TEF, the contribution of dioxin-like PCBs and PCDD/PCDF to the total TEQ was calculated for some matrices (Table 13). As can be seen from Table 13, the contribution from the PCBs to the total TEQ for PCDD/PCDF plus PCB is between 50% and 200%.

    Table 13. Toxic equivalents (TEQ) calculated for fish, cow milk, and human milk samples using the interim WHO/ICPS TEF [Ahlborg et al. 1994]

    TEQ

    Mother’s Milk

    Cow Milk

    Salmon

    Sum of TEQ for non-ortho PCBs 10.3 2.4 67.7
    Sum of TEQ for mono-ortho PCBs 10.1 0.4 46.8
    Sum of TEQ for di-ortho PCBs 0.6 0.04 8.3
    Total TEQ for PCBs 21.0 2.8 122.8
    Total TEQ for PCDD/PCDF 20.6 5.6 56.0
  3. Global Distribution - Long-Range Transport
  4. Today it is known that many chlorinated organics and other stable compounds are distributed at a global scale through atmospheric transport. A general tendency in these transport patterns is that different substances are evaporated and spread to the atmosphere at latitudes with warmer climates and then condense and fall-out closer to the poles (= global condensation). Consequently, areas close to the North and the South pole receive a disproportionate share of this fall-out. An indication of this phenomenon is that several chlorinated pesticides, long banned in countries, such as Sweden, are found - although at relatively low levels - in environmental compartments of this country. Examples of these substances are chlordane, toxaphene, and hexachlorocyclohexane; thus, compounds that are still being produced in other countries. For PCBs, an annual fall-out of 4 tons was estimated for Sweden in 1994.

    The presence of persistent polychlorinated compounds in remote areas such as the Arctic and the Antarctic has been reported. PCBs, PCDD/PCDF, HCH, and HCB were found in marine organisms such as seal blubber and pinniped milk as well as in lake and sea sediments. The occurrence of mainly man-made organochlorines in regions far away from industrialised and densely populated areas indicates that atmospheric transport is an important route to disperse these compounds. All three groups of compounds, PCB, PCDD/PCDF, and HCB, have the same source areas: Densely populated and industrialised regions. From these source regions, the organochlorines are transported via various mechanisms. Norstrom (Environment Canada) summarised the present knowledge on the occurrence of persistent chlorinated organic compounds in the Arctic aquatic environment as follows [see Fiedler et al. 1994]:

    Chlorinated hydrocarbons (CHC), such as the pesticides chlordane and toxaphene (polychlorinated camphenes, PCC), hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB) and DDT, and industrial chemicals, such as polychlorinated biphenyls (PCBs) have been identified in air, snow, ocean water, and biota in the marine ecosystem. Although most open uses of these chemicals were curtailed in many industrial countries, a considerable fraction of these compounds is still cycling in the ecosphere. Thus, it was estimated that 20 % of the world production of PCBs, 230,000 tons, are present in the upper layers of the ocean, and 790 tons were in the open ocean atmosphere. Large quantities of chlorinated pesticides continue to be used in less developed countries, especially in the Southern hemisphere. Although there is little information on the production amounts and releases of organochlorines from Russia and China, these areas are undoubtedly major contributors to the environmental burden with CHC.

    Chlorinated pesticides and PCBs have sufficiently high vapour pressures that they readily volatilise when spread over a large surface area such as soil or water. Atmospheric residence time of PCBs has been calculated to be in the order of a few months. Henry’s Law constants of the above mentioned compounds are in the range of 0.1-50 Pam/mol and thus, will allow that these substances will evaporise and cycle back and forth between land or surface waters and air. These processes lead to a global distribution. The „cold finger effect" will result in the fact that the Arctic and the Antarctic regions will become the sinks for organochlorines due to the distillation of these compounds from warmer to colder regions.

    Average concentrations of the major classes of CHC in the marine environment of the Arctic are given in Table 14. Hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB) are the dominant CHC in air, followed by the PCC. DDTs, chlordane, and PCBs are one order of magnitude lower. Because of its higher Henry’s Law constant, HCB is less dominating in snow and seawater. The high levels of HCH in the Arctic Sea water support the distillation theory. Moreover, sources in Asia might have a contribution. The effect of the higher lipophilicity of compounds such as DDTs, chlordanes, and PCBs goes along with bioconcentration. The concentrations of these compounds increase along the aquatic food-chain from plankton to beluga whales. The only exception are the HCHs which do not tend to strongly bioconcentrate (low KOW).

    Generally, the concentrations in the Antarctic are lower than the levels in the Arctic. This finding is reasonable as more than 80% of the industrialised regions which act as sources for PCDD/PCDF and PCBs are located in the Northern hemisphere. The slow inter-hemispheric air exchange (about 1-2 years) reduces the atmospheric transport of semi-volatile compounds from the Northern to the Southern hemisphere. Oehme and co-workers analysed air samples from both regions and found some interesting differences: Levels of hexachlorocyclohexanes (HCH) and hexachlorobenzene (HCB) were lower in the Antarctic air. Whereas in the Arctic air, a –HCH was dominating, the g -isomer dominated in the Antarctic. This indicates that preferentially the pure g -HCH has been applied in the Southern hemisphere whereas in the northern hemisphere more of the technical mixture (80-85% a –HCH) was used. HCB is mostly of anthropogenic origin - from incomplete combustion processes and utilisation as pesticide - and its atmospheric half-life time is 1-2 years leads to an almost homogeneous distribution on both hemispheres. Four chlordane compounds were identified in similar concentrations at both hemispheres. The PCDD/PCDF pattern found in Antarctic fur seal blubber was significantly different from that found in Arctic ringed seals and harp seals. Reasons might be differences in the emission patterns in the Northern and the Southern hemispheres, different food habits or inter-species variations. Amongst the PCB pattern, PCB 77, PCB 126 and PCB 169 were similar in Antarctic fur seals when compared to the Arctic harp seal. However, the levels - in TEQ - were lower in the Antarctic seals by a factor of about 5. This is less than the 1-2 orders of magnitude reported in earlier studies [see Fiedler et al. 1994].

    Table 14: Average concentrations of organochlorines in the Arctic marine environment
    NA = Not available

    Matrix, Dimension

    PCB

    HCHs

    HCB

    DDT

    Chlordanes

    Air, ng/m 0.014 0.58 0.19 <0.001 0.006
    Snow, ng/L 0.086 1.72 <0.002 <0.01 0.06
    Seawater (surf.), ng/L 0.007 4.3 0.028 <0.001 0.004
    Seawater (deep), ng/L <0.014 0.51 0.01 <0.002 0.005
    Zooplankton, g/g lipid 0.11 0.08 0.02 0.06 0.06
    Amphipods, g/g lipid <0.44 0.5 0.17 <0.35 0.43
    Cod, g/g lipid 0.23 0.58 0.2 0.26 0.19
    Beluga, g/g lipid 3.79 0.25 0.5 2.82 1.76
    Ringed seal, g/g lipid 0.55 0.23 0.03 0.5 0.4
    Polar bear, g/g lipid 5.4 0.51 0.27 0.4 3.7
    Human milk, g/g lipid 1.05 NA 0.14 1.21 NA

    Last but not least, it should be mentioned that distribution of chlorinated organic compounds also occurs via economic pathways and international trade of goods. Examples are textiles, leather, wood, packaging materials, etc. Consequently, ban of a given chloroorganic compound by one or a few countries - as, e.g. for pentachlorophenol in Germany, DDT, PCB in several industrialised countries - does not prevent a country from „pollution" with the given compound or its contaminants (e.g. PCDD/PCDF). Moreover, it is difficult to control the concentrations of chemicals in these goods that pass the country’s borders.

    The same thoughts have to be applied for transboundary atmospheric „imports" of unwanted pollutants: Stringent emissions limits set by own authorities do not prevent the country from importing atmospheric contaminants from the neighbour. Thus, in many aspects, the spread of persistent chloroorganic chemicals ask for international solutions.

  5. PCB Exposure: Occupational, Accidental, and General
    1. Occupational
    2. No studies have been conducted on the emissions during the production of PCB. It is assumed, however, that they were low. Major quantities reached the environment in the past through, for example, improper disposal of PCB-containing products and transformer fires. Other emission sources are landfills, small PCB-containing capacitors in household appliances and PCB-containing sealants for buildings. Inputs of PCBs to the environment can also occur through application of sewage sludge and sediments of water bodies. Products from the reprocessing of waste oil are another source from which inputs occurred.

      When the regulations for handling and transport of PCBs and PCB-containing waste are strictly enforced (as required in the German regulation = TRGS), there is little or no increased occupational exposure to be expected.

    3. Accidental - Yusho and Yu-cheng Accidents
    4. In 1968, a mass poisoning, called Yusho , occurred in Western Japan. Yusho was caused by the ingestion of rice oil which was contaminated with polychlorinated biphenyls, (PCBs), polychlorinated dibenzofurans (PCDFs), polychlorinated quaterphenyls (PCQs), and a small amount of polychlorinated dibenzo-p-dioxins (PCDDs). The poisoning was caused by Kanemi brand rice oil and shipped by Kanemi Conpani on February 5 and 6, 1968, or shortly later. The total number officially registered as Yusho patients were 1,862; deaths among them were 149 in 1990. X-Ray fluorescence analyses have shown that only the sample produced and shipped in the beginning of February contained a large amount of chlorine (maximum 462 ppm) and none of the rice oil produced later contained more than traces of chlorine. The average concentration of TEQ found n rice oil was 0.98 ppm [Masuda 1994].

      A second mass poisoning, Yu-cheng, occurred in Taiwan in 1978/79, from food grade rice cooking oil contaminated by heat-degraded PCB and related compounds. Samples of the Japanese oil reportedly contained about ten times more PCB, PCQ, and PCDF than did the Taiwanese oil. The Taiwanese population, however, on average consumed about ten times more oil. Thus, the exposures were remarkably similar in terms of the amount of total exposure to the contaminants and the number of people exposed.

      The most notable manifestations of the Yusho and the Yu-cheng populations were dermal lesions such as chloracne and comedones. However, many other symptoms including babies born with unusually brown colored skin, nails and eye discharge continued. Subsequent testing has shown that these victim’s offspring suffer neurotoxic and developmental effects.

      In Taiwan, many of the exposed developed chloracne, hyperpigmentation, peripheral neuropathy and other symptoms. A registry developed and maintained by the Taiwan Provincial Department of Health includes 2,008 exposed subjects. The average serum PCB level among 1,246 exposed subjects between 1979 and 1983 was 54 ppb [Yu et al. 1994]. In 1991, an update on the health status was performed on 1,837 Yu-cheng patients (986 females; 851 males) and 5,247 controls. The overall mortality in the Yu-cheng cohort was almost twofold compared to the control group. Significantly increased mortality rates were observed for deaths due to disease of the circulatory system, the respiratory system, and the digestive system.

      In 1992, the Yu-cheng subjects (10, exposed to PCB and PCDF) had higher cytochrome P4501A2 activities than the Seveso subjects (80; exposed to 2,37,8-Cl4DD). Thus, the body burdens in 1992 of PCB/PCDF of the Yu-cheng exposed were more potent P4501A2 inducers than the 2,3,7,8-Cl4DD body burdens in the Seveso cohorts. This difference supports the observations that the Yu-cheng subjects had more toxic effects than have been reported for the Seveso subjects [Lambert et al. 1993].

      Blood samples from 56 of the 68 Yu-cheng mothers were analyzed for PCB and PCDF [Guo et al. 1994]. Average concentrations of 2,3,4,7,8-Cl5DF were 1,625 pg/g serum lipid and for 1,2,3,4,7,8-Cl6DF were 3,820 pg/g serum lipid. Average PCB concentrations were 11,590 pg/g whole serum weight. The PCDF concentrations were still 60 to 190-times higher than the controls and PCB concentrations were 7-times higher than controls.

      The average concentration and estimated intakes of Yusho patients are summarized in Table 15 [Masuda 1994]

      Table 15: Estimated intakes of rice oil and TEQ by Yusho patients
      TEQ are calculated from 0.98 ppm in the rice oil and 0,92 of oil density
      TEF for PCDD/PCDF according to NATO/CCMS (I-TEF) and for PCB according to Ahlborg et al. 1994 [by WHO/IPCS]
      Mean latent period was 71 days; for smalles daily intake, the mean latent period was 135 days.

       

       

       

      Rice Oil

      TEQ

      Average total intake per capita

      699 mL (195-3,375)

      0.62 mg (0.18-3.04)

      Average intake during latent period

      506 mL (121-1,934)

      0.457 mg (0.11-1.74)

      Average daily intake

      0.171 mL/(kgd)
      (0.031-0.923)

      154 ng(kgd)
      (28-832)

      Smalles intake during latent period

      121 mL

      0.11 mg

      Smallest daily intake during latent period

      0.031 mL/(kgd)

      28 ng/(kgd)

      At the 15-year follow-up study of the Japanese Yusho cohort of 1,761 subjects, 6-fold and 3-fold increased liver cancer mortalities were found in exposed men and women, respectively. A significant excess of lung cancer deaths was seen in men as well. The fact that mortality to liver cancer was increased but not statistically significant in the Yu-cheng group, can be explained by the relatively young age of the people exposed (58% was younger than 30 years in 1979) and 13 years later, only a small number of liver deaths was reported. Another possibility is that liver cancer is the number 1 cause of cancer death in Taiwan with a high background rate which is very unlikely to show a significant increase within 13 years [Yu et al. 1994].

    5. Exposure of the General Population

    In Germany mother’s milk has been analysed on a large scale (about a thousand analyses per year) for various toxic substances like hexachlorobenzene, DDT, and PCBs. Due to the continuous change in analytical procedures for PCB-determination it is difficult to compare values obtained for example for mother's milk before 1984 with those of recent years. Regardless of this analytical problem it is evident that since about 1980 the maximum values for PCB concentrations have been decreasing constantly. The range of PCB-concentrations in mother’s milk in the last ten years is between 0.5 and 2.5 mg/kg milk fat. The average concentrations have been decreasing constantly since 1984. This is shown in Figure 2 [NRW 1996]. However, as can be seen from Figure 3, there was a major decrease in the mother’s milk concentrations in the mid and late 1970s whereas in the 1980s the concentrations remained on the same level. More recently, the concentrations seem to further decrease.

    Figure 2: PCBs in mother’s milk - Mean concentrations [NRW 1996]

    Figure 3: Time trend of PCB concentrations in mother’s milk [Brune and Fiedler 1996]

    From the possible transfer paths of human exposure through direct skin or air contact with technical PCB mixtures, meat and meat products, milk and milk products, fish and fish products, ambient and indoor air, the direct uptake of technical PCBs can be considered as negligible. In Germany, indirect exposure pathways via the terrestrial food-chain dominate the human intake. PCBs emitted into the air from combustion sources or re-entrained from reservoir sources (contaminated land, vegetation, surfaces in general) deposit on grass and other fodder plants which are eaten by cattle and thus, contaminating milk (and products) and meat (and products). Due to the many PCB contaminations of sediments from the years before 1980, PCBs can still be found in fish. The terrestrial and the aquatic food-chains are the most important routes for human exposure to PCBs.

    The question arises to what extent there exist primary sources of PCB input into the environment by de novo synthesis of PCBs by thermal sources, analogous to dioxin synthesis. At least for Germany we estimate that the PCB uptake via meat and milk and respective products originates to a major extent by the latter source [Hagenmaier 1996].

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Ahlborg U.G., A. Hanberg, and K. Kenne (1992): Risk Assessment of Polychlorinated Biphenyls (PCBs). Institute of Environmental Medicine, Karolinska Institutet Stockholm, Sweden, Nord 26

Baker J.E. and S.J. Eisenreich (1990): Concentrations and Fluxes of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls aross the Air-Water Interface of Lake Superior. Environ. Sci. Technol. 24, 342-352

Ballschmiter K. and Zell M. (1980): Analysis of Polychlorinated Biphenyls (PCBs) by Glass Capillary Gas Chromatography. Fresenius Z. Anal. Chem. 302, 20-31

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Guo Y.L., J.J. Ryan, B.P.Y. Lau, M.M.L.au, and C.-C. Hsu (1994): Blood Serum Levels of PCDFs and PCBs in Yucheng Women 14 Years after Exposure to a Toxic Rice Oil. Organohalogen Compd. 21, 509-512

Ivanov V. and E. Sandell (1992): Characterization of Polychlorinated Biphenyl Isomers in Sovol and Trichlorodiphenyl Formulations by High-Resolution Gas Chromatography with Electron Capture Detection and High-Resolution Gas Chromatography - Mass Spectrometry Techniques. Environ. Sci. Technol. 26, 2012-2017

Lambert G.H., P. Mocarelli, C.C. Hsu, L.L. Needham, J.J. Ryan, L. Guo, P. Brambilla, S. Signori, D.G. Patterson, T.J. Lai, F. Garcia, E. Ferrari, and D.A. Schoeller (1993): Cytochrome P4501A2 Activity in Dioxin Exposed Seveso Subjects as Compared to Polychlorinated Biphenyl and Polychlorinated Dibenzofuran Exposed Yucheng Subjects. Organohalogen Compd. 14, 253-256

Li A., J. Doucette J., and A.W. Andren (1992): Solubility of Polychlorinated Biphenyls in Binary Water/Organic Solvent Systems. Chemosphere 24, 1347-1360

Mackay D., Shiu W.Y. and Ma K.C. (1992): Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals. Volume I+II. Lewis Publishers Inc., Boca Raton, FL, USA

Masuda Y., A. Schecter, and O. Ppke (1997): Concentrations of PCBs, PCDFs and PCDDs in the Blood of Yusho Patients and Their Toxic Equivalent Contributions. Accepted for publication in Chemosphere

Masuda Y. (1994): Approach to Risk Assessment of Chlorinated Dioxins from Yusho PCB Poisoning. Organohalogen Compd. 21, 1-10

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