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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.

An old and a new diesel engine and three diesel engine fuels were compared regarding the concentration of particles in diluted exhaust immediately after dilution and after 40 seconds of aging. The flow of dilution air was adjusted to give a particle concentration of about 2 mg/m3. Filters were weighed and analysed with a Scanning Electron Microscope. The mean volume of soot particles were in the range 0.01-0.05 ·m 3, the mean weight, 10-40 femtogram. The mean density of the particles was found to be 1.7. In the different experiments the densities of the particles varied considerably indicating complicated agglomeration and adsorption-desorption processes. The new engine emitted significantly lower concentrations of particles than the old engine when measured in the dilution tunnel. The new engine was not better than the old and worn engine when results obtained after aging of the particles in the chamber were compared.