The toxicity of nitrogen pentoxide.

There is no evidence in the literature that nitrogen pentoxide (N205), the highest known oxide of nitrogen, has ever been the subject of a toxicological investigation. This gas rapidly breaks down to give nitrogen dioxide and oxygen, but is known to be stabilized by the presence of ozone, and several investigators (Schumacher and Sprenger, 1929; Johnston, 1951) have studied the complex system of equilibria which exists between ozone, oxygen, and the oxides of nitrogen. Gorodetskii (1936) has studied the gaseous products at silent and spark electrical discharges in air and has concluded that in the presence of excess ozone nitrogen pentoxide is the only oxide of nitrogen produced. Our interest in the toxicity of nitrogen pentoxide was aroused by our preoccupation with the atmospheric contamination in a high-voltage insulator testing installation. The evolution of ozone in the vicinity of the brush discharge which attends this testing is evident by the characteristic odour, and it might be assumed that this gas constituted the chief toxic hazard were it not for the work of Thorp (1941, 1950), who has shown that ozone prepared from pure oxygen has a far lower toxicity than ozone prepared from air, and that the higher toxicity of the latter must be due to nitrogen oxides. Thorp, however, admitted that the toxicity of air containing ozone and nitrogen oxides (calculated as nitrogen dioxide) was greater than might be expected from the known toxicities of pure ozone and nitrogen dioxide. In view of the observations of Gorodetskii (1936) and other workers it is an attractive hypothesis that the increase in ozone toxicity which accompanies the presence of nitrogen oxides is due to nitrogen pentoxide. The first step to establish this hypothesis is to study the toxicity of this substance. And this is the scope of the present investigation. The extent to which nitrogen pentoxide is responsible for the toxicity of ozone encountered in industrial atmospheres, and the steps which should be taken to control such atmospheres, will be the subject of a later communication. The toxicity of the lower nitrogen oxides, notably of nitrogen dioxide, has been studied by many investigators on account of the occurrence of these gases in " nitrous fumes " encountered in certain industries. There has been, however, little systematic work on the toxicity of pure nitrogen dioxide to the rat, the species selected for the nitrogen pentoxide experiments, and as concentrations of nitrogen pentoxide in air in the absence of ozone cannot be prepared free from nitrogen dioxide, a study of the toxicity of this gas is also included in the present investigation.