Relative levels of BALF pro-inflammatory cytokines. BALF pro-inflammatory cytokines were expressed as fold increases of exposed groups compared to controls. Samples from all rats of the same exposure group were pooled together. The ratio for each cytokine was calculated as described in materials and methods. Results with ratios ≥1.2 were considered to represent a slight inflammation. (A) Aerosols with a primary NP size of 5 nm. (B) Aerosols with a primary NP size of 10–30 nm. (C) Aerosols with a primary particle size of 50 nm.