The experimental production of radiographic shadows by the inhalation of industrial dusts. Part II: Zircon (ZrSiO4).

A preliminary report on the toxicology of zircon (Harding, 1948) gave some details of the properties and uses of this substance and suggested that it was extremely inert within the animal body. At the time of the publication of this preliminary report an experiment was almost completed during which rats were exposed to afairly high concentration (725,000 particles per cc. of air) of zircon dust in a chamber. The period of exposure lasted just over three months, and the animals were killed at intervals up to six months thereafter. To our surprise we were unable to find any zircon in the lungs of these animals either by radiography or microscopically. Since it seemed possible that there had been an error which we were unable to identify, we repeated the experiment on two further occasions. Again no zircon, or only very small amounts, could be found in the lungs. The animals were exposed in a chamber with a dust feed of the type described by Lloyd Davies (1946): working with identical techniques before, between, and after these experiments we found no difficulty in demonstrating dust in the lungs of rats exposed to other dusts (e.g., iron oxide, manganese dioxide, beryllium oxide, silica, graphite, talc, cobalt metal) of similar particle size and in similar concentrations. Because of the difficulty we had experienced in demonstrating appreciable quantities of zircon in these animals, we set up a fourth experiment in which the concentration of dust in the chamber was increased considerably and the period of exposure was also lengthened. It is this fourth experiment that forms the basis of this paper.