In our investigation of our subject we have perused the literature, analysed the data of other workers, conducted a number of animal experiments, considered the bearings clinical and pathological of a case of presumed toluene poisoning, and conducted an investigation into the blood state and symptoms of co-workers with this case and yet are unable to give a categorical answer to the question. What are the relative toxicities of toluene and benzene? We would almost feel inclined to echo the words of another worker in the experimental field with regard to benzene, Neumann (1915), who concludes: “The fact that my results so often do not correspond either with one another or with those of Selling, so often indeed do not correspond to expectation, justifies the conclusion that, although benzene is to be regarded as a powerful leucotoxin, it manifests great individual differences in its mode of action and it is very difficult to lay down exactly what action is to be expected.”

We should be inclined to put the case somewhat differently however and put forward the following view as our tentative finding.

The actions of benzene and toluene are very similar. That of benzene is the more powerful because it is a more volatile substance and can therefore reach higher concentration than toluene. This greater volatility at the same time is a reason why it is eliminated more quickly and its effects are less lasting than those produced by toluene. The action of both these substances is on young body cells, probably whatever they are and wherever they may be found, but is most easily discoverable because of its action on the young bone marrow cell and the consequent effect on the cells of the peripheral blood. It is not a specific action but operates, according to circumstances, on the stem cells of the myeloid leucocyte or the erythrocyte. This does not mean that the substances have little or no effect on the lymphocyte, for the lymphoid tissue in the body is so very abundant that it would only be in extreme cases that the effect on that cell would be manifest peripherally. Like many poisons, indeed like many of the drugs which are used in Medicine, the first effects of these substances are stimulating. Leucocytosis, erythrocytosis and possibly thrombocytosis are early manifestations or the manifestation of slighter action, with histological evidence of hyperplasia. This is succeeded by the truly toxic or destructive action and the production of leucopenia, anaemia, thrombopenia and the effects due to action on other organs. The ultimate result in clinical terms is an agranulocytic anaemia accompanied by purpura haemorrhagica and, as the pathological manifestation an aplastic bone marrow. The fatal termination is commonly an infection to which there is no resistance possible in an individual deprived of defence cells and possibly also of defensive substances in the body fluids. Powerful solvents of fat as both these chemical substances are, they produce an action through this means also, on nerve cells, capillary endothelium and parenchyma of organs with consequences represented in nervous symptoms and in haemorrhages. This then is our reading of the results of our investigation and explanation of the variability which may be found, a variability which is regarded mainly as one of phase, although we do not exclude idiosyncrasy altogether as explanation.