Effect of organic extraction on changes in cytotoxicity and number of viable cells. 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) LDH in the cell culture supernatants and b) the number of viable cells were analysed. Histograms represent means ± SEM of separate experiments (n = 4). * p<0.05 exposed vs. control, # p<0.05 washed particles vs. organic extracts of particles. Statistical analysis was in a) performed by repeated measures 2-way-ANOVA with Bonferroni post-tests on log transformed data and in b) by one-way-ANOVA with Bonferroni post-tests.