All countries have their own legal drinking water standards. These prescribe which substances can be in drinking water and what the maximum amounts of these substances are. The standards are called maximum contaminant levels. They are formulated for any contaminant that may have adverse effects on human health and each company that prepares drinking water has to follow them up. If water will be purified to make it suitable to drink it will be tested for a number of dangerous pollutants, in order to establish the present concentrations. After that, one can determine how much of the contaminants have to be removed and if necessary purification steps can be progressed.