Particle-induced release of cytokines. Co-cultures of pneumocytes (A549) and monocytes (THP-1) were exposed to 0 (control) or to 40 μg/cm2 of particles for 12 or 40h, before the levels of a) TNF-α, b) IL-8 and c) IL-6 in cell culture supernatants were determined by ELISA. Histograms represent means ± SEM of separate experiments (n=4). * p<0.05 exposed vs. control and # p<0.05 12 vs. 40 h exposure. Statistical analyses were performed by two-way ANOVA with Bonferroni post-tests on log transformed data.