Atmospheric aerosol particles cover a size range from few nanometers up to tenth of micrometers. While coarse particles larger than one micrometer are dominated by mineral dust, see salt and biological particles (like pollen, bacteria and spores) fine particles smaller than one micrometer are secondary material built from gases such as sulfates, nitrates and organic carbon or are originated from combustion processes such as elemental carbon (soot). The size range of ultrafine particles smaller than 0.1 micrometer is dominated by such soot particles, especially in urban areas. Furthermore, soot particles are believed to be dangerous since they carry toxic trace compounds such as heavy metals and polycyclic aromatic hydrocarbons (PAHs). The concentration of such particles is highly variable and shows a significant daily and weekly variation, especially close to traffic.
Ultrafine particles (UFP) are the smallest constituents of airborne particulate matter: they are smaller than 0.1 micrometres and invisible to our eyes. UFP can be inhaled more deeply into the lungs than larger particles, and are likely to have adverse health effects. However, they are not routinely monitored, and up to now there is not enough conclusive evidence on health impacts of UFP.