Kinetic time courses of inhaled silver nanoparticles in rats

Silver nanoparticles (Ag NPs) are priority substances closely monitored by health and safety agencies. Despite their extensive use, some aspects of their toxicokinetics remain to be documented, in particular following inhalation, the predominant route of exposure in the workplace. A same experimental protocol and exposure conditions were reproduced two times (experiments E1 and E2) to document the kinetic time courses of inhaled Ag NPs. Rats were exposed nose-only to 20 nm Ag NPs during 6 h at a target concentration of 15 mg/m³ (E1: 218,341 ± 85,512 particles/cm³; E2, 154,099 ± 5728 particles/cm³). The generated aerosol showed a uniform size distribution of nanoparticle agglomerates with a geometric mean diameter ± SD of 79.1 ± 1.88 nm in E1 and 92.47 ± 2.19 nm in E2. The time courses of elemental silver in the lungs, blood, tissues and excreta were determined over 14 days following the onset of inhalation. Excretion profiles revealed that feces were the dominant excretion route and represented on average (± SD) 5.1 ± 3.4% (E1) and 3.3 ± 2.5% (E2) of the total inhaled exposure dose. The pulmonary kinetic profile was similar in E1 and E2; the highest percentages of the inhaled dose were observed between the end of the 6-h inhalation up to 6-h following the end of exposure, and reached 1.9 ± 1.2% in E1 and 2.5 ± 1.6% in E2. Ag elements found in the GIT followed the trend observed in lungs, with a peak observed at the end of the 6-h inhalation exposure and representing 6.4 ± 4.9% of inhaled dose, confirming a certain ingestion of Ag NPs from the upper respiratory tract. Analysis of the temporal profile of Ag elements in the liver showed two distinct patterns: (i) progressive increase in values with peak at the end of the 6-h inhalation period followed by a progressive decrease; (ii) second increase in values starting at 72 h post-exposure with maximum levels at 168-h followed by a progressive decrease. The temporal profiles of Ag elements in lymphatic nodes, olfactory bulbs, kidneys and spleen also followed a pattern similar to that of the liver. However, concentrations in blood and extrapulmonary organs were much lower than lung concentrations. Overall, results show that only a small percentage of the inhaled dose reached the lungs-most of the dose likely remained in the upper respiratory tract. The kinetic time courses in the gastrointestinal tract and liver showed that part of the inhaled Ag NPs was ingested; lung, blood and extrapulmonary organ profiles also suggest that a small fraction of inhaled Ag NPs progressively reached the systemic circulation by a direct translocation from the respiratory tract.