To evaluate the effect of carbonaceous materials on the pathogenic activity of quartz dusts, mixtures of carbon soot (1 and 10%) and quartz (Min-U-Sil) were prepared and then milled so to attain an intimate association of carbon and the quartz surface. Both cellular and cell-free tests show that carbon associated to quartz completely inhibits the typical free radical generation of quartz dusts (through Fenton activity and homolytic cleavage of a C-H bond) and suppresses the oxidative stress and inflammation induced by quartz alone on MH-S murine macrophage cells (lipid peroxidation, nitric oxide release, and tumor necrosis factor-α synthesis). The cytotoxic response to quartz is also largely reduced. An extremely pure quartz milled with 10% of soot showed inactivating effects on the adverse reactions to quartz similar to Min-U-Sil quartz. None of these effects takes place when the same experiments are carried out with mechanically mixed samples, which suggests that carbon acts not just as a radical quencher but because of its association to the quartz surface.