Relationship between the particles' oxidative potential (DTT consumption) and cellular responses in RAW 264.7 macrophages. Cells were exposed to increasing concentrations of PM after which MTT-reduction activity and the release of pro-inflammatory markers was measured. For each cellular response parameter, the slope of the concentration-response curve is plotted against the corresponding DTT consumption of each PM sample. The solid lines represent correlation investigated for all sites, dotted lines without the underground train station site (wo underground). Panel A: MTT-reduction activity (n = 20 PM samples; 8 sites × 3 PM size fractions per site minus 4 samples that were not tested in the DTT-assay since there was not enough material). Panel B-D: release of pro-inflammatory markers (n = 15 PM samples, 20 minus 5 more samples that were excluded because of high endotoxin levels). Statistical analysis was performed by multiple linear regression and shown as effect estimate (β, slope) and belonging p-value.