The Piceance Basin of northwestern Colorado contains the greatest domestic concentration of oil shale deposits favorably located for economical recovery. These deposits are defined geologically to be at least 30 ft thick with at least 30 gal of oil per ton of shale; by conservative estimate, they contain nearly 300 billion barrels of recoverable shale oil. The technologies used to extract this oil will need to comply with federal and state environmental protection regulations, particularly those governing ambient air quality. The regulatory framework for preventing degradation of ambient air quality was amended on August 7, 1980, and these rules are especially applicable for controlling air pollution in oil shale recovery areas such as the Piceance Basin.