High concentrations of low-density particles may cause effects in acute inhalation toxicity studies which can be easily underestimated or misinterpreted following strictly the OECD TG 436, i.e., limited parameters as mortality and gross lesions will be evaluated only. Seven particle types (synthetic amorphous silica (SAS) HMDZ-SAS, silica gel, pyrogenic SAS, and precipitated SAS, calcium carbonate, aluminum oxide pyrogenic alumina, organic red pigment) were chosen at the highest technically feasible concentration of approximately 500 mg/m3 for acute inhalation studies with an expanded endpoint setup. Therefore additional parameters and a thorough histopathological evaluation of an extensive set of organs, including the respiratory tract emphasizing the nasal cavities were added. Six Crl:WI rats per study were exposed for four hours from which three animals were sacrificed after 24 hours and three animals after 14 days. HMDZ-SAS caused early death in all animals due to blockage of the nasal passages caused by its hydrophobicity. For all other Si-containing compounds, histology revealed minor inflammatory and reactive lesions in lungs after 24 hours that were still present after 14 days, except in silica gel-treated animals. After 14 days, for pyrogenic SAS, precipitated SAS, and pyrogenic alumina, granulomas formed in the BALT and lung-associated lymph nodes. In contrast, the calcium carbonate induced almost no findings, and the red pigment (also tested for the additional dose of 1000 mg/m3) stuck partially to the nasal mucosa without causing pathological damage and partly entered the lungs without showing any adverse effects. The results of the present study highlight the advantage of improving the rather simple study design of acute inhalation studies by implementing an extended study design.
Keywords: Acute inhalation toxicity; Aluminum oxide; Calcium carbonate; Hydrophobic surface; Nanoparticles; SAS.
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