Scientific Facts:

Since the thalidomide disaster it has been common knowledge that chemicals which do not harm a pregnant woman may, during the first few months of pregnancy, cause malformations of the foetus. Commercial 2,4,5-T contains an impurity which is one of the most powerful poisons known and is especially potent in causing malformations to pregnant mammals. This impurity is called 2,3,7,8-tetrachlorodibenzo-para-doxin, commonly abbreviated "dioxin". To summarise briefly a lot of scientific research, it appears that, in the rat, the maximun safe daily dose of dioxin is in the region of 1/8 of one millionth of a gram per kilogram of body weight. Since drug dosages are usually quoted in terms of weight of drug per unit of animal body weight, let us use the abbreviation microg./kg. for "millionth of a gram per kilogram body weight." The hamster is even more sensitive than the rat to the foetus-deforming effects of dioxin: daily doses of 0.02 microg./kg. in early pregnancy cause foetal damage in the hamster.

Of course we do not know how potent dioxin is in pregnant women. It might be less potent than in animals. On the other hand, it might be more potent. Thalidomide is 700 times more potent [unclear: a] at causing malformations in humans than in hamsters. Since we cannot deliberately dose women with such dangerous drugs, we must (as our minister of agriculture pointed out last year) calculate the maximum safe human dose from experiments on animals. In such calculations, the World Health Organisation recommends a safety factor of 2,000; this is reasonable, in view of the possibility that humans are far more sensitive to a particular drug than are the animals which have been tested. Applying this safety factor, and using the results from rats, we would conclude that the most dioxin a pregnant women should take is down around 0.000063 microg./kg. daily. On the other hand, if we calculate from the hamster results, we would conclude that even a daily dose as low as 0.00001./kg. would be dangerous in a pregnant woman.