Leipzig/Berlin. Residents of large German cities have it above all in their own hands how high the concentrations of ultrafine dust are in their homes. The level of pollution in the home depends only partially on the air quality outside. However, it also depends very much on activities inside the home, such as cooking activities or heating of solid fuel. This study was led by the Leibniz Institute for Tropospheric Research (TROPOS) commissioned by the Federal Environment Agency (UBA). To this end, fine and ultrafine aerosol particles were measured inside and outside for about two weeks in each of 40 apartments in Leipzig and Berlin during different seasons of the year.