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