Comparative pulmonary toxicity study of nano-TiO2 particles of different sizes and agglomerations in rats: Different short- and long-term post-instillation results

Two intratracheal instillation experiments with nano-size titanium dioxide (TiO2) particles of different sizes and agglomerations were conducted in rats to compare the biological responses induced by the different particles. In experiment 1, 5 mg/kg of nano-TiO2 particles of different primary sizes was intratracheally instilled in rats. In experiment 2, a similar procedure was followed with 5 mg/kg of nano-TiO2 particles of the same primary sizes but different agglomerations in liquid. Following the instillations, body and lung weight measurements, bronchoalveolar fluid (BALF) cells and inflammatory biomarkers assessment, and histopathological evaluations of the lungs and other tissues were conducted. Pulmonary inflammatory responses until 1 week post-instillation differed among the TiO2 particle-exposed groups: that is, smaller particles induced greater inflammation in the short-term observations. With regard to the long-term effects (>1 week post-instillation), however, pulmonary inflammation remarkably recovered in all the TiO2 particle-exposed groups, with no differences between the groups regardless of particle size. On the other hand, no clear relationship was observed between the TiO2 particle-exposed groups with different agglomerations but the same primary size. These findings suggest that different evaluations can be derived on the basis of the observations up to 1 week post-instillation and those after 1 month post-instillation. In most of the current studies, the relationship between pulmonary responses and instilled particle sizes has been discussed only on the basis of the 24 h post-instillation results, which could be a misleading evaluation. Consequently, our findings indicate that both short- and long-term effects should be evaluated when assessing the toxicity of nanoparticles based on the results of intratracheal instillation studies.