Metal fuel borne catalysts (FBCs) can be used with diesel fuels to effectively reduce engine out particle mass emissions. Internationally, metal FBCs are used for both on-road and off-road applications, although current regulations in the U.S. restrict their on-road use. Although metal FBCs are intended to be used with specialized diesel particulate filters (DPFs) where they are effectively trapped to oxidize additional soot and regenerate the filter, they are sometimes used without DPFs, and, under these conditions, contribute to ambient air emissions and potential health effects. This paper explores potential environmental and health aspects of FBC in diesel fuels, when burned in diesel engines without DPF's. However, rather than examine data from a range of diesel engines, the work focuses on the impact of systematic changes in the fuel composition on emissions from a single, small prototype diesel generator. Experiments using ferrocene as a diesel fuel additive, with varying fuel Fe concentrations from 0 to 200 ppm, indicate similar to 30-40% decreases in particle mass, total particle volume, and black carbon emissions, and increases (approaching a factor of 5) in particle number concentrations associated with 10-30 nm Fe particles liberated during soot oxidation. Fe concentrations in overall particle emissions increase from 0.1% to 7.5% as the Fe catalyst is increased from 0 to 200 ppm. The Fe is emitted primarily in the elemental form. While polycyclic aromatic hydrocarbons (PAHs) are reduced with increasing Fe, emissions of alkanes and organic acids show no clear trend. These experimental results can be interpreted in the light of a mechanism whereby the Fe acts to oxidize soot-related PAH species, but does not affect Organic compounds associated with unburned fuel and lubrication oil that avoid flame processes. Calculations performed to predict the evolution of the particle size distribution (PSD) associated with the emitted particles suggest that once diluted to simulate behavior along a highway, the Fe-rich nuclei mode is likely to persist for some time. This has health effect implications related to potential exposures associated with these particles. (C) 2013 Elsevier Ltd. All rights reserved.