A paradoxical response of the rat organism to long-term inhalation of silica-containing submicron (predominantly nanoscale) particles of a collected industrial aerosol at realistic exposure levels

- When instilled intratracheally, submicron particles containing amorphous silica, induced a response testifying for their high cytotoxicity.
- In a long-term inhalation test they proved to be of very low systemic toxicity and pulmonary fibrogenicity.
- Low SiO2 retention in the organism may be explained by relatively high solubility of these particles.
- The most dangerous effect of long-term exposure was systemic genotoxicity.
- We also demonstrated penetration of nanoparticles into the brain.

While engineered SiO2 nanoparticle toxicity is being widely investigated, mostly on cell lines or in acute animal experiments, the practical importance of as well as the theoretical interest in industrial condensation aerosols with a high SiO2 particle content seems to be neglected. That is why, to the best of our knowledge, long-term inhalation exposure to nano-SiO2 has not been undertaken in experimental nanotoxicology studies. To correct this data gap, female white rats were exposed for 3 or 6 months 5 times a week, 4 h a day to an aerosol containing predominantly submicron (nanoscale included) particles of amorphous silica at an exposure concentration of 2.6 ± 0.6 or 10.6 ± 2.1 mg/m3. This material had been collected from the flue-gas ducts of electric ore smelting furnaces that were producing elemental silicon, subsequently sieved through a < 2 μm screen and redispersed to feed a computerized “nose only” inhalation system. In an auxiliary experiment using a single-shot intratracheal instillation of these particles, it was shown that they induced a pulmonary cell response comparable with that of a highly cytotoxic and fibrogenic quartz powder, namely DQ12. However, in long-term inhalation tests, the aerosol studied proved to be of very low systemic toxicity and negligible pulmonary fibrogenicity. This paradox may be explained by a low SiO2 retention in the lungs and other organs due to the relatively high solubility of these nanoparticles. nasal penetration of nanoparticles into the brain as well as their genotoxic action were found in the same experiment, results that make one give a cautious overall assessment of this aerosol as an occupational or environmental hazard.