Hard metal dusts, typically WC/Co, but not pure WC or Co particles, cause the so-called ‘hard metal lung disease’ when inhaled over long periods of time at the workplace. In order to investigate the chemical nature of the dust which originates the disease, the surface behaviour of pure cobalt, pure tungsten carbide, an industrial hard metal dust and a mechanical mixture of cobalt and tungsten carbide have been compared. Electron microscopy reveals an intimate contact between metal and carbide in the mixed dusts. The mixed dust is more active than the single components in the adsorption of water vapour in both adsorbed amount and interaction energy (111 kJ mol ^-1 for the mixture, 95 kJ mol ^-1 for pure cobalt and 84 kJ mol ^-1 for pure WC). Both industrial and mechanical mixtures are more active than pure components in the catalytic decomposition of hydrogen peroxide. Incubation of the mixed dusts in phosphate buffered solutions causes a progressive release of cobalt(ii) ions in solution and the appearance of round smooth aggregates (diameter ca. 300–400 µm) at the expense of smaller particles. The mixed dusts, but not the pure components, promote the homolytic rupture of a carbon–hydrogen bond in aqueous suspension, as revealed by the formation of carboxylate radicals from formate ions. This is evidenced by the use of DMPO as a spin trap, which yields the DMPO–CO ₂ ^- adduct whose EPR spectrum intensity measures the amount of radicals generated. Radicals are only formed in aerated solutions indicating a crucial role of atmospheric oxygen in their generation. The hydroxyl radical, however, does not appear to be implied, for two main reasons: (i) no free oxygen radicals are detected in the absence of formate as target molecule; and (ii) free-radical release is insensitive to the addition of mannitol (an OH scavenger). The formation of the carboxylate radical CO ₂ ^- is an activated process: an induction time of ca. 30 min is required to produce detectable amounts of radicals, while radical generation continues for several hours. Samples withdrawn from the solution, washed, dried and re-employed are still active, as long as some metallic cobalt is present. A model is proposed whereby in the mixture electrons from oxidized cobalt are translocated at the carbide surface where they reduce atmospheric oxygen in a surface-active form which is responsible for the generation of carboxylate radicals from formate ions. The implication of this reaction in health related effects as well as possible hazards from particulates in enviromental pollution is discussed.