Representative histology of lung tissue (200X) 3 days after the second intratracheal instillation of PM.

<p>Lungs were excised and fixed with formaldehyde, sectioned in 5-μm-thick slices, and the slides were stained with H&E. (A) Vehicle (VEH) control tissue. (B) Tissue exposed to high-dose PM_2.5 showing focal aggregation of lymphocytes and foamy histiocytes in the alveolar space. (C) Tissue exposed to medium-dose PM₁₀ showing focal infiltration of lymphocytes and foamy histiocytes into the alveolar space. (D) Tissue exposed to high-dose PM₁₀ showing focal infiltration of neutrophils, lymphoplasma cells and foamy histiocytes.