Introduction
The effective application of EU law cannot be ensured by the EU and its courts alone. It depends strongly on domestic courts and individuals that initiate proceedings before these courts to enforce their rights under EU law (Case 26/62 Van Gend & Loos). Therefore, the judges of domestic courts are at the same time European judges. They not only apply domestic law but also EU law. The CJEU cooperates with domestic courts in the framework of the preliminary ruling procedure. An additional source for the interpretation of EU law can be found in judgments delivered as a result of infringement procedures, in particular in the field of the environment. In air quality legislation, see Cases C-365/10 Commission v Slovenia, C-479/10 Commission v Sweden, C-34/11 Commission v Portugal, C-638/18 Commission v Romania (exceeding the limit values for concentrations of PM10 in ambient air) and other case-law referred below.
The national judges should not put the EU requirements aside because of the difficulties related to the transposition and effective application for the Member States (such as socio-economic situation, large-scale investments and structural changes or technical difficulties). According to the CJEU, it is irrelevant whether the failure to fulfil obligations is the result of intention or negligence on the part of the Member State responsible, or of technical difficulties encountered by it. A Member State which encounters temporarily insuperable difficulties preventing it from complying with its obligations under European Union law may plead force majeure only for the period necessary in order to resolve those difficulties (Case C-68/11 Commission v Italy, Case C-488/15 Commission v Bulgaria, Case C-336/16 Commission v Poland concerning limit values for concentrations of PM10 in ambient air).