The chemical composition of particulate matter (PM) is a dynamic variable impacted by different urban emission sources. We conducted this study to investigate the impact of urban emission sources on the chemical composition of ambient PM and the corresponding toxicological properties. To this end, we collected six sets of PM samples in five urban location sites around the globe, intentionaly selected for their PM to be dominated by unique emission sources, including 1) PM2.5 generated primarily by traffic emissions in central Los Angeles, United States (US); 2) PM2.5 dominated by biomass emissions in Milan, Italy; 3) PM2.5 formed by secondary aerosols in Athens, Greece; 4) PM10 emitted by refinery and dust resuspension in Riyadh, Saudi Arabia (SA); 5) PM10 created by dust storms in Riyadh, SA, and 6) PM2.5 generated primarily by industries and traffic emissions in Beirut, Lebanon. The samples were then chemically analyzed for the content of redox-active components and the corresponding oxidative potential, measured by the dithiothreitol (DTT) assay. The highest oxidative potential was observed in Milan and Athens which were rich in water-soluble organic carbon (WSOC), released by biomass burning activities and secondary atmospheric reactions. The samples in areas characterized by elevated metal emissions induced the lowest oxidative potential as evaluated by the DTT assay. Furthermore, our results showed that the DTT consumption rate correlated the highest with WSOC, polycyclic aromatic hydrocarbons (PAHs) and organic carbon (OC), highlighting the impact of these components in driving PM toxicity.