The particle size distribution is normally a straight line in a log-log graph. The PDM equipment produces such a graph as visual result of a measurmeent. In this graph three area’s of interest can be found. The aera around small particles such as 5 tot 15 micrometer. If this group of particles dominates it is advisable to look into the airflow of the cleanroom. Airflow is of high influence on this group and a remedy to reduce this group. The group of particles 0f 100 micrometer and larger does say something about the cleaning of the room. If this groups shows up as a tail in the log-log graph intensivation of the cleaning programme will be an action to take. Lastly the middle group around 30 micrometer. This group indicates human activity and can be reduced by improvement of the cleanroom garment and behavior. By means of measurements the experience will be build up over time.