This guideline VDI 4251 Part 1 presents the general requirements to be taken into account in planning plant-related plume measurements of microbial air pollutants. The purpose of the plume measurement here described is to determine the mean plant- and/or sourcerelated impact range of microbial air pollutants. As it has so far not been possible to set limit values based on dose-response relationships, the mean impact range is to be used as a criterion for assessing the environmental impact of a plant.