The toxicity of ozone in the presence of oxides of nitrogen.

In a previous paper we (Diggle and Gage, 1954) have referred to published statements that the toxicity of ozone is increased if oxides of nitrogen are present and we have suggested the hypothesis that if, in fact, this is so, then the increased toxicity is likely to be due to the presence of nitrogen pentoxide, to which the lower oxides of nitrogen are converted by ozone. As a first step to establish this, we undertook a study of the toxicity of nitrogen pentoxide and we demonstrated that it is a " lung irritant " gas of high potency. Since the appearance of our paper, a careful review of the literature on the toxicity of ozone has been made by Stokinger (1954) from which it is evident that although there is general agreement that ozone produces pulmonary oedema and emphysema, there are wide variations in the concentrations which are regarded as necessary to produce these symptoms. A direct comparison of the results of various investigators is rendered difficult by the use of different experimental conditions and species, by different degrees of purity of the ozone, and by deficient or faulty analytical control of the test atmospheres. In the investigation described in the present paper, we have measured the toxicity of ozone to rats and the effect thereon of the presence of added amounts of nitrogen dioxide, and we have studied the extent to which this can be attributed to the analytically determined amounts of nitrogen pentoxide present. We have also made analytical measurements on an industrial atmosphere where conditions are favourable for the production of ozone together with oxides of nitrogen.