Except for lung cancer the most important biological effects of diesel exhaust concern the airways and lungs including acute and chronic effects on lung function. According to most reports these effects are attributable to the soot particles. Hitherto vehicle certification regulations concern particulate matter, PM, i.e., all particles that can be collected from diluted exhaust. The regulation therefore does not discriminate between more or less inert and nuisance and harmful particles. Therefore determination of soot particles, e.g., by size and carbon content, in the exhaust is warranted. But studies of diesel exhaust from different fuels, or subjected to different exhaust after-treatments show that the biological effects of the particles could increase when a decrease was intended for. This indicates that it is not sufficient to identify and determine the concentration of soot particles. Amendments of regulations to guide development of the system ‘engine-fuel-exhaust gas after-treatment’ are required, in which the health effects of soot-particles are examined, preferably in biological models.