Human exposure to ambient metallic nanoparticles is an area of great interest owing to their potential health impacts. Ambient metallic nanoparticles found in the roadside environment are contributed by combustion engines and wear of brakes, tyres and road surfaces. Submicrometre atmospheric particles collected at two UK urban sites have been subject to detailed characterisation. It is found that many metallic nanoparticles collected from roadside sampling sites are rich in iron. The Fe -rich nanoparticles can be classified into (1) high Fe content (ca 90 wt%) with each alloying element less than 1 wff.; and (2) moderate Fe content (<75 wt%) with high manganese and silicon content. Both clusters contain a variable mix of minor constituents, Mn, S and Si being most important in the high -Fe group. The moderate Fe group also contains Zn, Cu, Ba, Al and Ca. The Fe -rich nanoparticles exhibit primary particle sizes ranging between 20 and 30 nm, although some much larger particles up to around 100 nm can also be observed, along with some very small particles of 10 nm or less. These tend to agglomerate forming clusters ranging from similar to 200 nm to 1 mu m in diameter. The iron -rich particles observed are oxides, taking the form of spheres or multifaceted regular polyhedra. Analysis by EELS shows that both high- and moderate -Fe groups include particles of FeO, Fe3O4, alpha-Fe2O3 and gamma-Fe2O3 of which gamma-Fe2O3 is the most prominent. Internal mixing of different Fe -oxides is not observed. (C) 2016 The Authors. Published by Elsevier Ltd.