The U.S. Department of labour agency responsible for administering the Occupational Safety and Health Act (P.L. 91-596). According to OSHA, farming is the nation's most hazardous occupation. Agriculture is the largest occupational group in the U.S., with some 10 to 20 million people depending upon one's criteria of "agriculture." The intrinsically seasonal nature of many segments of agriculture not only causes the size of this workforce to vary temporally and often geographically via migrant work groups, but usually also has major effects on the nature and intensity of the work itself. OSHA has issued safety standards relating to agricultural operations.