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 a-fairly high concentration (7-25,000 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.
ExperimentalTwelve rats, aged about 3 months, weighing 109-135 g. each, were exposed to a very high concentration of zircon dust for six hours daily, five days a week for 144 days, and then for eight and a half hours daily, five days a week for 20 days. Atmospheric dust counts with a thermal precipitator were frequently too high to be counted, but when they could be determined they varied between 37,000 and 130,000 particles per cc. of air, usually near the higher figure; 80% of the particles were below 1 ,u, and 30% below 0-2 ,u: only rare particles over 5 ,i were encountered in the counts. One rat died during the experiment and the others were sacrificed at intervals up to seven months after exposure to the dust. The animals were killed and their lungs preserved in an inflated state according to the technique previously described (Lloyd Davies and Harding, 1949). The lungs were later radiographed and sections made from them for microscopic examination. Sections were stained by haematoxylin and eosin, iron haematoxylin and van Gieson, and by Wilder's modification of Foot's reticulin stain.ResultsThe radiographs were examined by Dr. J. L. A. Grout who reported that they. showed a fine pinpoint mottling all over both lung fields. Although generalized, the mottling was more intense at the mid-zones and towards the apices, with a nodular character similar to that seen in well-established cases of silicosis in the human be...
