Road dust is a complex mixture of particles from organics, metals, other inorganics, mold spores, pollen, animal dender etc. Its sources are traffic, industrial activities, powerplants, fossil fuel burning, waste incineration, construction and demolition activities, and resuspension [90]. Topsoils and particularly roadside dusts in urban areas are indicators of airborne particulate matter metal pollution. Most trace metals settle down as surface dust from atmospheric depositions before full incorporation into the soil matrix. Thus, the extent of atmospheric contamination may be better revealed by road dust than by soils [91,92].

Though this data compilation refers to deposited dust only in terms of mg/kg, the amount of re-suspendable particulate matter should be considered in terms of inhalation risk assessment, in particular the grain-size fractions below 2.5 µm aerodynamic diameter (PM_2.5) and below 10 µm (PM₁₀). In the US, the PM_2.5 has been estimated at about 12% of the total dust, unless detailed investigations are available. Therefore, a detailed study of grain-size distributions was done in various roadside environments in Albuquerque NM, Atlanta GA, Birmingham AL, El Paso TX, Los Angeles CA, New York NY, and adjacent New Jersey, after sampling by a high-volume sampler and separation by a cyclone dust trap. The average composition of paved road dust from industrial sites differed from other urban regions by containing a larger fraction of elemental carbon and less-iron crustal material (Al-Ca-Si), due to coking facilities or high local emissions of trucks, trains and ships. There was slightly more diversity among urban regions than among urban site types. The portion of silicates + carbonates, as well as the PM_2.5 content, was greatest in arid regions. Among 51 species determined by XRF and ion chromatography of water-solubles (detailed data not given), nine variables were identified as most important in distinguishing samples: Cr, Ni, Al, Cl, Ca, Si, Fe, Cu, and nitrate. Paved road dust had a higher content of potentially bioreactive metals and higher elemental C at urban than at rural sites [93].

Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action. In addition to chemical toxicity, however, some particles promote catalytic reactions at their surface, which is not traceable by chemical analysis only. Direct health implications are due to the inhalable fraction, which is rather floating in the air than depositing at surfaces. Deposited dust directly imposes health hazards via consumption of unwashed food, particularly for babies who like to lick around, for dogs which like to lick and sniff at dusty surfaces, and finally the dust-washout might harm green plants in parks, as well as river systems [94].

Permanent changes in the technologies for house construction, traffic and heating during cold seasons, as well as changes and improvements in the purification of industrial emissions, change the composition of urban dust on a long term. Within a first step, representative sampling grids have to be established to find the hotspots of contaminations, prior to rather laborious and expensive size-fractionated sampling and microlocal analysis. Contrary to direct air dust sampling, deposited dust yields integrated contamination values over a long period of time, usually back to the last rainfall or street cleaning action [3].

The extent of contaminant input from traffic activities is generally determined by comparing concentration data of road deposited sediments with those of nearby uncontaminated soils [36]. Comparisons between road dust samples from these largely different urban areas might reveal common global urbanization effects, and specific rather local influences. This can be ascertained from additional data per gram dust, available from other densely populated areas. The effect of urbanization might be traceable from differences between road dust samples from Budapest and from more rural Hungary [59].

Vehicular traffic contributes to dust emission by release of tire and brake pads wear, releasing fibers (like copper, steel, potassium titanate, glass, organics), fillers (barium and antimony sulfate, Mg and Cr oxides, ground slag), lubricants, and abrasives. However, street dust can also contain up to 60% of particles originating from soil, like quartz, feldspars, clay minerals, chlorite and muscovite. Whereas main sources of Zn and Fe are tire wearing, brake wearing release in non-asbestos brake pads reaches up to 15%, containing Ba, Cu, Fe, Pb, Zr and Sn. Ni and Cr can originate from corrosion of cars. Technogenic magnetic particles from high temperature combustion processes have characteristic spherical shape, while those from traffic emissions and iron smelting form irregular non-spherical aggregates [25].

A compilation of road dust data easily shows that some elements are always enriched versus upper crust mean values, like As, Cd, Hg, Mo, Sb, and Sn. Though levels in road dust may be higher than background soils, Ba, Mn, P, Tl and V were found within the fluctuation of geochemical composition. Largely, but not always higher than upper crust values are Cr, Cu, Pb and Zn, and occasionally higher are Co and Ni.