Case study on zinc oxide (ZnO)
The solubility of ZnO varies strongly in different media. nano-ZnO as well as micro-ZnO dissolves in water (pH about 7) up to 13 μg/ml. Up to 4.5 mg/ml ZnO dissolved in artificial lysosomal fluid (pH 4.5) during 24 hours at room temperature, while little was dissolved in artificial interstitial fluid (Gamble´s solution, pH 7.4) under the same experimental condition (Cho et al, 2012). Cho et al (2012) also examined ZnO particle and Zn2+ ion induced pulmonary toxicity after intratracheal instillation. The authors found qualitatively comparable effects after instillation of ZnO particles and dissolved Zn2+ ion. In vitro, lysosomal instability and cell death was increased in macrophage culture after treatment with nano ZnO. In a rat instillation study, Hirano et al (1989) reported a clearance half-time of 14 hours after intratracheal instillation of ZnO to rat (100 μg Zn/rat) and examination of Zn content in lung after 8 h, 1, 2, 3, 5, 7, 14 and 21 days. The instilled Zn was recovery solely in the supernatant of the bronchoalveolar lavage fluid.
These data reported by two research groups demonstrate biological solubility of ZnO. Based on the short half-time, ZnO is considered to be biologically soluble.