The 19th century and first half of the 20th century is one of those periods in history of rapid economical, technical and social changes. There is an ongoing mechanization, followed by a movement to rationalize production and to make it cost effective. This has created a heavy burden of occupational deaths amongst workers. In this period occupational safety is developing into a professional field. Engineers are enclosing hazards and fencing heights, shaping up safety technique, and writing very practical publications on occupational safety. These publications, predominantly published in the United States, are leading to quite some safety related metaphors, with the iceberg and the domino's as the most famous ones. Sociologists, psychologists, and physicians are more concerned with questions related to accident causation, and these specialists are conducting research leading to two different safety theories. Causes of occupational accident are found either in the workers' capacity to handle hazardous situations, or in external causes, like very long working hours, dangerous machines and the increased pressures of work and speed of production. The Pittsburgh survey, the first extended analysis of occupational accidents in a steel district, strongly advocated the environmental hypothesis, while the so-called 'individual hypothesis' is favored by the American Safety First Movement, starting as a private initiative in 1906 by US Steel, and later spreading out over Western European countries. The British Industrial Fatigue Board has given the individual hypothesis its scientific justification. Despite scientific criticism just after World War II on the concept of accident proneness and 'unsafe acts', its popularity is not fading. Even nowadays the famous metaphors are still part of the vocational training of safety experts, also in The Netherlands. Apparently professional and scientific developments in occupational safety are belonging to two separate worlds. Before World War II, The Netherlands is not a leading country in occupational safety but a follower, first of France and the German speaking countries. After World War I its focus is directed towards the Anglo-Saxon countries.