Methods of active monitoring are standardised to an even higher degree as compared to passive biomonitoring: The culture of selected plant species takes place under homogeneous and standardised conditions, e.g. in the greenhouse. After a given culture period, the plants are exposed under defined exposure conditions for a limited time (mostly several weeks) on the site. Use of uniformely developed plants results in more precise information on the extent of air pollution impact and the time of its incidence. Additionally, the choice of the monitoring site is independent of the natural occurrence of certain plants, which is most important in urban or industrialized areas.