Discharges to the air are also governed by regulations, because air pollution can lead via precipitation to water pollution. Mercury, for example, now enters the environment chiefly as a result of the burning of fossil fuels. In the past mercury compounds were often used in pesticides and in the production of chlorine. In moist environments micro-organisms transform inorganic mercury into organic mercury, which is more poisonous. Organic mercury compounds, such as methyl mercury, can accumulate in the food chain and are dangerous for the central nervous system.
In the 1950s and 1960s very persistent (poorly degradable) pesticides such as lindane and DDT were widely used. A chemical such as DDT can spread through water over the whole world. DDT itself has been found in the ice of Antarctica. Other poorly degradable compounds such as PCBs are also still found in the environment, although its use was restricted long ago. PCBs were used in electrical equipment and as lubricants, among other applications.
One of the characteristics of these compounds is that they can accumulate in the food chain. This means that the levels in higher organisms, including fish, molluscs and crustacea, are many times higher than the concentrations in water.
In Directive 93/351/EEC the European Commission has set the maximum levels of mercury that may be found in samples of halibut, tuna, wolf fish and eels at 1 mg per kilogram. For other fish the limit is 0.5 mg/kg. This directive has been incorporated in the Netherlands in the Commodity Act. The Commodity Act also set maximum levels for other heavy metals such as mercury, cadmium and lead, as well as for substances such as PCBs and radioactive substances. The residues provisions of the Pesticide Act contain limits for pesticides.
European Directive 91/493/EEC and Directive 91/492/EEC require each Member State to draw up a programme in which fishery products and molluscs and crustacea are checked for the presence of environmental contaminants such as heavy metals and organic halogen compounds. The Netherlands has complied with this requirement only partly. The Dutch fish monitoring programme focuses on eel, which can be seen as the most sensitive fish with regard to the accumulation of residues.
According to Directive 96/23/EC, fish from fish farms will also have to be monitored for the presence of a number of substances, including veterinary drugs in the near future.
At present about 100,000 synthetic substances are made or used in fields as diverse as the petrochemical industry, the production of synthetic chemicals or iron and steel manufacture. The use of a few of these, such as DDT and PCBs, has been forbidden or strictly regulated in recent years. There are many other chemicals for which it is still not clear whether they are harmful for humans.