Background
Microplastics are pervasive environmental pollutants that pose potential risks to human health, particularly through inhalation. Despite growing concerns, limited data exist on how environmental aging, such as ultraviolet (UV) irradiation, affects the pulmonary toxicity of inhaled nano- and microplastics. This study evaluated the influence of UV-driven surface oxidation on the inflammatory potential and lung clearance kinetics of polystyrene (PS) particles. Spherical PS particles (50, 200, and 400 nm) were synthesized, selectively oxidized by UV irradiation, and thoroughly characterized for surface chemistry and intrinsic reactive oxygen species (ROS) generation.

Results
Mice exposed to these particles via pharyngeal aspiration (75 µg/mouse; n = 4 per group) exhibited significantly greater acute pulmonary inflammation from UV-oxidized particles compared to pristine particles, with smaller ones (50 nm) displaying slightly higher inflammogenicity. Although inflammation largely resolved by four weeks post-exposure (75 µg/mouse; n = 4 per group), mild neutrophilic inflammation persisted. Notably, particle-induced ROS generation and subsequent cellular oxidative stress in alveolar macrophages showed strong correlations with acute inflammatory endpoints. Additionally, the particle dispersion method significantly affected lung clearance rates: particles dispersed in distilled water (DW) containing 10% ethanol exhibited shorter clearance half-lives (3–8 days) than those dispersed in 5% mouse serum (~ 18 days) (75 µg/mouse; n = 4 per group). These results highlight the dispersion medium as an important experimental variable influencing pulmonary clearance and toxicity interpretation.

Conclusions
These findings suggest that the surface oxidation of nano- and microplastics through environmental aging can increase associated health risks. However, within the tested size range (50–400 nm), neither surface oxidation nor particle size markedly altered the overall lung clearance pattern.