During their peak use period, PBBs represented under 1% of the total sales of fire retardant chemicals, and very probably would have escaped intensive study if they had not been mixed accidentally with animal feed preparations. Instead, international attention was drawn to PBBs by the state-supervised killing of over 35,000 cattle which had been contaminated with PBBs. Interestingly, low doses of PBBs exert a broad spectrum of toxicological, pharmacological, and biochemical effects despite low acute toxicity. These effects and the intensive bioaccumulation of PBBs derive from their structure and their consequent resistance of biotransformation and high solubility in fat. In rodents, PBBs are teratogenic, immunosuppressive, and potentially carcinogenic. In bovine, rodent, and avian species, PBBs reduce feed intake and induce mixed function oxidases of liver microsomes. The latter effect may be responsible for steroid level changes which underline hormonal toxicities observed in cows, mink, rats, and chickens. The effects of PBBs on humans are controversial, but data suggestive of immunological, skin, and liver disorders continue to accumulate. Concern about the clinical effects of PBBs is heightened by the knowledge that these compounds readily enter the fetus by crossing the placental barrier and can be transferred to newborn children after extensive passage into breast milk.

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The vacuum-ultraviolet photolysis of acetylene has been investigated at the Kr (1236 Å, 1165 Å) and Xe (1470 Å, 1295 Å) resonance lines. Evidence is presented for production of a long-lived excited state of acetylene. Decomposition of the excited state at low pressure leads to the formation of molecular hydrogen and excited C2. At the Kr resonance lines and at the 1295-Å Xe line, an emission consistent with Swan bands (A 3πg→X 3πu) of C2 is observed at low pressures. No emission is observed in the photolysis of acetylene at 1470 Å. Intensity of the emission, the quantum yield for hydrogen production, and the quantum yield for acetylene disappearance are all greatly reduced by the addition of a few millimeters of a quenching gas. Decomposition to H2 and C2 also occurs in the solid phase. The over-all photolysis is explicable in terms of an excited molecule mechanism. Formation of C2 in excited A 3πg state depends on the energy of the incident photon: C2H2+hv→C2H2*,C2H2*+M→C2H2+M,C2H2*+C2H2→C4H4*,C4H4*→C2H2‡+C2H2‡,C2H2*→C2(A 3πg)+H2,C2(A 3πg)→C2(X 3πu)+hv,C2H2*→C2+H2,C2H2*→C2H+H,H+C2H2→C2H3,C4H4*+C2H2→C6H6*,etc.,C4H4*+C2H2→C4H4(vinyl acetylene)+C2H2,C6H6*+C2H2→C6H6+C2H2,2C2H→C4H2(diacetylene)

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