Screening studies are carried out in order to rapidly analyse which chemical substances constitute health or environmental problems. This can be done throug hexamining the presence and distribution of a particular chemical substance in on eor a number of materials – matrices. Screening can also be undertaken in order to ascertain which different chemical substances occur in a particular matrix, for example air, water that drains off from landfills, sludge from sewage treatment plants, or a particular cloth. In distinction from continuous environmental monitoring, where the same substances are examined every year, screening studies a recarried out on one or more occasions over a limited time. Screening studies are a prerequisite in being able to monitor the development towards a non-toxic environment. They are also necessary to enable follow-up of various international requirements concerning emissions, occurrence and effects of metals and organic substances.