Antimony in brake pads - a carcinogenic component?

Antimony trisulfide (Sb2S3) is used as a lubricant in friction material. X-ray diffraction analysis revealed Sb in 3/3 disc brake pads (range 41,000-46,000 mg/kg) and in 2/2 disc brake dust samples (21,000 and 17,000 mg/kg) from trucks. Considerably lower concentrations were found in drum brake pads (3/5, 59-6400 mg/kg) and in drum brake dust (4/18, 78-2800 mg/kg). Other toxic metals were also detected in pads and dust. The health risk of Sb in particulate brake emissions should be dependent on particle size and chemical entity, in particular solubility. A significant amount of the emitted dust was shown by size-fractionated optical particle counting to be inhalable in environmental ( > 90% mass) and occupational ( > 50% mass) exposure situations. Differentiation via selective solubility showed a considerable amount of Sb2S3 to oxidize during the braking process, likely leading to the formation of Sb2O3, a suspected human carcinogen. Sb in brake dust was soluble in calf serum (8.5 +/- 1.2%, 8.9 +/- 1.7 mg/l at 37degreesC for 3 d). Hence, the use of Sb in friction material should be suspected to pose a human cancer risk and be deterred. To design healthy and environmentally sound alternatives, we propose to apply the Intelligent Product System that distinguishes products of consumption and products of service. Brake pads are true products of consumption, because they are released to biological cycles, and should thus consist of materials with positively defined health and environmental properties.