Inhalation toxicity, pulmonary clearance, and biotransformation of ferric oxide nanoparticles in rats

Ferric oxide (Fe₂O₃) nanoparticles are widely used in industrial and biomedical applications, raising concerns regarding inhalation exposure, particularly in occupational settings. Although the inhalation toxicity of magnetite (Fe₃O₄) was reported, no studies have addressed the pulmonary clearance, biotransformation, or systemic distribution of Fe₂O₃ nanoparticles. In this study, male rats were exposed nose-only to aerosols of γ-Fe₂O₃ nanoparticles at concentrations of 0.56, 1.63, and 4.92 mg/m³ for 6 h/day, 5 d/week, over 28 d, followed by recovery periods of 7 and 28 d. Particulate and ionic iron were separately collected via proteinase K digestion and quantitatively and qualitatively evaluated using elemental quantification and electron microscopy.

The toxicity study showed no treatment-related effects, including clinical signs, body weight, hematology, serum biochemistry, or histopathology. Lung burden analysis revealed a clearance half-life of 3.6-3.91 d, with ~ 25% of the initially deposited iron (Fe) retained after 28 d of recovery. At this time point, the retained Fe persisted as particulates biotransformed into fragments of a few nanometers, with a smaller portion present as ionic Fe. The extrapulmonary distribution showed ionic Fe in the liver and spleen, whereas particulate Fe was confined to the lung-associated lymph nodes. This study provides the first inhalation toxicity and biodistribution data for γ-Fe₂O₃ nanoparticles obtained under regulatory testing conditions, demonstrating subacute inhalation toxicity outcomes and revealing their persistence in the lungs without systemic translocation of intact particles. This study is the first subacute inhalation toxicity study conducted under regulatory testing conditions and includes a comprehensive in vivo biokinetic and biotransformation analysis, revealing the long pulmonary biopersistence of γ-Fe₂O₃ nanoparticles and the absence of systemic translocation of intact particles, with only limited extrapulmonary distribution of their ionic biotransformation products.

Although γ-Fe₂O₃ nanoparticles persist in the lungs without overt toxicity, their prolonged retention and biotransformation into smaller particles may ultimately lead to overload-driven oxidative stress and chronic pulmonary effects.