Encyclopedia of Environmental Science and Engineering, Volume I and II

848 PCBs AND ASSOCIATED AROMATICS

thermodynamics is a concept which describes the energy flow

in a reaction from reactants to products. When there is a high

activation energy barrier, the reactants require more energy

to attain an activated state than when there is a low activa-

tion barrier. Once the reactants have gained enough energy

to form an activated intermediate they can either return to

the reactant form or proceed to a stable product. The bal-

ance between the forward and backward reaction directions

is an equilibrium condition which may be heavily weighted

in favor of either the reactants or products. The high activa-

tion energy in the formation of PCDDs contributes directly

to their low yields. The residue from the trichloro-phenol

distillation in the Monsanto synthesis was reported to be

20 times less effective in producing liver necrosis in rabbits

than the residue of the fraction remaining after heating to

above 230°C. The boiling points of many dioxin precur-

sors are lower than the reaction temperature range in which

PCDDs are formed and therefore there is a loss of precursor

molecules before reaction can occur.

The possibility of pressure differences in the formation

of the residues are only one of several key parameters which

make the comparison difficult. It is, nevertheless, clear that

the residue derived from the high temperature distillation

was substantially more toxic than the pentachloro naphtha-

lene studied. The health effects of PCNs had been described

in the literature some 17 years before but the effect observed

on rabbits left in the operating area after the BASF accident

was surprising—they died from liver necrosis after a one

week exposure!

1957

A review of characteristic chloracne lesions appeared in

1957 by Schwartz et al.

1962

Threshold limit values for PCBs in workroom air were

set in 1962 by the American Conference of Governmental

Industrial Hygienists. Under “Chlorodiphenyl—42%

Chlorine” it was stated that “Acne, systemic poisoning and

even death may result from exposure to chlorinated diphe-

nyls.” The references indicated were [Schwartz, L.: Am.

J. Pub. Health; 26 , 586 (1936) and Drinker, C.K., Warren,

M.F., Bennett, G.A.: J. Ind. Hyg. & Tox.; 19 , 283 (1937)].

A threshold limit value of 1 mg/m^3 was recommended to

offer good protection against severe systemic toxicity but

may not guarantee complete freedom from chloracne. In

other words, it was recognized that there was a risk of severe

systemic toxicity and that chloracne was a separate issue.

FIGURE 4 Formation of polychlorinated diphenyl ethers (PCDPEs) can lead to the formation of polychlorinated dibenzofurans

(PCDFs).

FIGURE 5 Loss of HCl produces cyclization of the diphenyl ether to a dibenzofuran.

FIGURE 6

FIGURE 7 Loss of H 2 from the pre-dioxin produces PCDD.

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