On 25 March 1957, six countries – Germany (GFR), Belgium, France, Italy, Luxembourg and the Netherlands – signed the Treaties of Rome establishing the European Economic Community (EEC) and the European Atomic Energy Community (Euratom). The legislation came into effect on 1 January 1958.
Regarding occupational safety and health more specifically, note that various research programmes had been carried out with the support of the European Coal and Steel Community (ECSC), established by the Treaty of Paris in 1951. But the EEC treaty showed more clearly the need for an overall approach. See Articles 117 & 118 in particular:
Member States hereby agree upon the necessity to promote improvement of the living and working conditions of labour so as to permit the equalisation of such conditions in an upward direction.
They consider that such a development will result not only from the functioning of the Common Market which will favour the harmonisation of social systems, but also from the procedures provided for under this Treaty and from the approximation of legislative and administrative provisions.
Without prejudice to the other provisions of this Treaty and in conformity with its general objectives, it shall be the aim of the Commission to promote close collaboration between Member States in the social field, particularly in matters relating to […] protection against occupational accidents and diseases, industrial hygiene, the law as to trade unions, and collective bargaining between employers and workers.
For this purpose, the Commission shall act in close contact with Member States by means of studies, the issuing of opinions, and the organising of consultations both on problems arising at the national level and on those of concern to international organisations. Before issuing the opinions provided for under this Article, the Commission shall consult the Economic and Social Committee.