# Comparison of Genotoxicity and Pulmonary Toxicity Study of Modified SiO2 Nanomaterials
## 4 Discussion
However, Wistar rats which were intravenously injected with 15 and 55 nm silica showed positive results in the MN test [8]. The results indicate that different routes of administration might affect the genotoxicity of silica in the MN test. The data shown in the table are the ratio of RETs/NCEs and Mn-RETs/RETs, not the absolute count of Mn-RET. After being released into the peripheral blood circulation, the RET will mature and become NCE within 48 h. Therefore, the Mn-RETs we noticed at the 48 h timepoint may become Mn-NCEs at the 72 h timepoint. Also, RETs/NCEs at the 72 h timepoint were higher than 48 h timepoint. This is because the inhibition of RET production caused by the test article/positive control toxicity was declined after 48 h. Then, the bounced RET production was noticed at the 72 h timepoint. In summary, the decreased Mn-RETs/RETs at the 72 h timepoint was caused by (1) the Mn-RET noticed at 48 h timepoint maturing and becoming \"Mn-NCEs\" which were not counted as Mn-RETs at the 72 h timepoint and (2) the Mn-RETs maybe being diluted by the increased RET production noticed at the 72 h timepoint. To summarize the results of the genotoxicity tests, SB made the chromosomal aberration frequency of CHO-K1 cells significantly higher than the negative control. Though the SB group showed positive results in the Ames test and CA test, the genotoxicity of SB was reversed to produce negative results in these two tests after being metabolized with rat liver S9 extract. In the MN test, SB induced a weak positive result, which indicated that SB might cause gene damage in rodent hematopoietic cells. SB produced weakly positive or positive results in these genotoxicity tests. However, genotoxicity may be reduced after S9 metabolism or modification, which suggests that the genotoxicity of SB may be reduced by liver metabolism. In contrast, mSB did not induce significant genotoxicity in any of the three genotoxicity tests. The opposite result between SB and mSB indicated that the modification of SB might reduce genotoxicity. In the acute pulmonary toxicity test, the rats were given an intratracheal instillation of diluted samples and sacrificed after 1 or 14 days. High mortality in the mSA rats was observed at 1 day after IT. In contrast, the other samples did not induce death in rats. Histopathologic examination of the lungs of the SA, mSAL, and SB groups showed remarkable edema, hemorrhage, inflammation, and alveolar wall necrosis. Other pulmonary toxicity parameters that can represent pulmonary toxicity of samples in rats, such as relative lung weight, BALF WBC count, BALF neutrophil count, BALF protein concentration, and BALF TNF-\(\alpha\) concentration, showed similar results in the SA, mSAL, and SB groups. However, the SB group showed slight pulmonary damage in all of these parameters. After 14 days, aggregates of macrophages and granulomatous inflammation, which represent subacute pulmonary inflammation, were noticed in the lungs of the SA, mSA, and SB groups but not in the mSB group. The total histopathologic scores of the mSB group were significantly lower than the SB group at both 1 and 14 days, but there was no significant difference between the SA and mSA (low dose) groups. In summary, the modification of SA may increase pulmonary toxicity in rats 24 h after IT, based on the high mortality observed in the mSAH group, but there was no significant difference observed after 14 days. In addition, the modification of SB may reduce pulmonary toxicity in rats. According to the laws in Taiwan, the maximum permissible exposure dose of pure silica dust is 0.098 mg/m(Ministry of Labor, [PERSON], Taiwan, 2010). The exposure dose of rats was calculated using the formula below:  
Exposure concentration of nanoparticle (0.098 mg/m\({}^{3}\)) \(\times\) tidal volume of rat (2.1 mL) \(\times\) breathing frequency in rat (102 times/min) \(\times\) exposure hours in day (8 h) \(\times\) particle deposition efficiency (10%)/rat body weight (300 g) = the exposure dose of rat (0.0034 mg/kg) [9].