Ultrafine particles form during combustion processes, for example, when wood or biomass is burned, as well as in power-generating and industrial processes. Alongside road traffic, large airports are a major source of these ultrafine particles, which are less than 100 millionths of a millimeter (100 nanometers) in size. Because they are so small, they can penetrate deep into the lower respiratory tract, overcome the air-blood barrier and, depending on their composition, cause inflammatory reactions in the tissue, for example. What's more, ultrafine particles are suspected of being capable of triggering cardiovascular diseases.