Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 847

1949

PCBs extracted from air were analyzed by Jacobs (1949)

before the advent of gas chromatography.

1950

In the 1950–1952 time frame, an episode of PCB poison-

ing occurred when 7 of 14 people exposed to vapors from

a leaking PCB-filled heat exchanger developed chloracne.

[Meigs, 1954].

1952

Askarel used in capacitors up to 1952 was Aroclor 1254.

After 1952 the type of askarel was changed to Aroclor 1242.

1953

Badische Anilin had an industrial accident on 17th November,

1953 and is one of seven similar accidents which have been

recorded. The process involved the manufacture of trichloro-

phenol from tetrachlorobenzene (Figure 2).

The Seveso incident, which occurred in 1976, is per-

haps the best documented industrial accident of this type.

Thermographic records at the Seveso plant showed an uncon-

trolled rise in the temperature of the reaction vessel, following

failure of a safety shutdown device. The mixture was heated

at 230° to 240°C for 4 to 5 hours and 3 to 4 atmospheres

before a safety disc ruptured and released nearly the entire

mixture (Figure 3).

1954

Meigs et al. estimated that about 1 year was required to

develop chloracne after the beginning of exposure to air with

a concentration of 0.1 mg PCB/m^3. PCB was used as a heat

exchange fluid for a reaction chamber. There was leakage of

vapour which exposed a group of 7 to 14 workers.

1956

A Monsanto accident in the manufacture of chlorophenols

occurred in the Monsanto Nitro Plant during 1949. A similar

accident also occurred at Badische Anilin in Germany on

17th November, 1953.

Monsanto described their experience in a 1956 memo-

randum with personnel from Badische Anilin: “A thorough

systematic investigation has isolated impurities in the tri-

chlorophenol process (or residues) which will cause the

same effects in rabbits.” The materials mentioned include

“pentachlor naphthalene” and “chlorinated diphenyl oxide”.

PCDFs and PCDDs can be formed from chlorophenols

in three different ways as follows:

1) the dimerization of chlorophenates;

2) the cyclization of polychlorinated diphenyl ethers

(PCDPEs);

3) the cyclization of polychlorinated phenoxy phe-

nols, termed pre-dioxins.

1 The dimerization of chlorophenates has been

shown in the reaction shown above.

2 Polychlorinated diphenyl ethers are formed in the

polychloro-phenol synthesis according to the fol-

lowing reaction (Figure 4).

The formation of PCDFs from the cyclization

of polychlorinated diphenyl ethers is as follows

(Figure 5).

3 Polychlorinated phenoxy phenols are termed ‘pre-

dioxins’ for the reason that they are intermediates

in the cyclization sequence (Figures 6 and 7).

One or more of the steps in the formation of PCDDs

has a high activation energy. Activation energy in chemical

FIGURE 2 The intended reaction produces TCP.

FIGURE 3 The dimerization of chlorophenates yields PCDDs.

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