Inflammatory cell response in lung lavage 24 hr after intratracheal instillation of fine (~ 250 nm) and ultrafine (20–30 nm) TiO2 expressed by different dose metrics [particle mass (A,B), number (C,D), and surface area (E,F)] and different response metrics [number of PMNs (A,C,E) and PMN/macrophage ratio (B,D,F)].