[Current data and questions on the carcinogenicity of solid particles of diesel engine exhaust and other sources]

During the last few years, the findings could be confirmed which led to the conclusion that very fine solid particles which are not known to have an intrinsic toxicity, can induce lung tumours in rats. The carcinogenic potency seems to increase parallel to increasing specific surface area and decreasing particle size. However, many questions remain open. Organic compounds which contain many carcinogenic polycyclic aromatic hydrocarbons (PAH) can explain only less than 1% of the carcinogenic effect of diesel exhaust in the rat lung. Therefore, the carcinogenicity of diesel exhaust cannot be reduced by an oxidizing catalyst. It burns a part of the organic substances adsorbed on the surface of the carbonaceous core of diesel particles and thus helps to follow the low standard for particles in diesel emissions, but without reduction of the carcinogenic potential because it does not reduce the emission of the insoluble carcinogenic part of the particles, the elementary carbonaceous core. The search for a hypothetical threshold for the carcinogenicity of particles in the rat lung aims at the determination of mechanisms which are preconditions for the development of a tumour. If certain doses do not induce such lesions they are considered safe. Chronic inflammation, proliferation of epithelial cells and fibrosis are discussed to be preconditions. However, the pathways which lead to inflammation and proliferation could proceed independent of the molecular carcinogenesis. Therefore, a cancer risk from diesel particles and other very fine particles may occur under environmental conditions. If the positive epidemiologic studies are relevant, a lung cancer risk for humans can be calculated which is higher than that calculated from inhalation studies with rats.