At the 6th meeting of the IRG/WP plenary group held on 20 June, 1974, in Vienna, Mr Richardson expressed some concern about dioxins which might be contained as impurities in pentachlorophenol and in sodium pentachlorophenoxide or which might be formed during the combustion of treated wood. He mentioned the malformations that have been noted in Vietnam due to the teratogenic action of the 2, 3, 7, 8 tetrachlorodibenzo-p-dioxin contained in certain preparations of the 2, 4, 5 trichlorophenol used in defoliants. He also raised the question about analogous risks that might come from the presence of octachlorodibenzo-p-dioxin in pentachlorophenol. I specified that work has been carried out on this subject and that the octachlorodibenzo-p-dioxin could in no way be compared to tetra and presented practically no danger considering the doses at which it could be found in pentachlorophenol; consequently no action needed to be taken against it. As stated in point 31 of the minutes of the plenary group meeting, it seemed necessary to me to take stock of the question and to evaluate the possible influence of the presence of chlorodioxins in pentachlorophenol or its sodium salt or even in the combustion products of treated wood. This is the subject of this communication. It is seen that the conclusions are reassuring and they can be summarised in 3 points: (1) Pentachlorophenol does not contain 2, 3, 7, 8 tetrachlorodibenzo-p-dioxin which is by far the most toxic and dangerous chlorodioxin. (2) The chlorodioxins that might be found in pentachlorophenol are the octachlorodibenzo-p-dioxin which has no teratogenic action and is practically non-toxic at the doses at which it is found in pentachlorophenol, and, in much smaller quantities the hexachlorodibenzo-p-dioxin which is 1,000 to 100 times less toxic and less teratogenic than the 2, 3, 7, 8 tetrachlorodibenzo-p-dioxin. (3) The combustion of wood treated with pentachlorophenol or with sodium pentachlorophenoxide does not lead to the- formation of chlorodioxin, but on the contrary, tends to decrease the quantity in relation to that contained in the wood before burning.

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