Gasoline properties have changed over the years. The first changes were made to comply with federal and later ASTM gasoline specifications. Beginning in 1960, limitations were placed on gasoline properties to help reduce air pollution. The state of California has led the nation in limiting gasoline properties to improve ambient air quality. The Clean Air Act Amendments of 1990 increased the federal government's role in controlling the properties of gasoline. Wintertime oxygenated gasoline was introduced in 1992 in specified areas with carbon monoxide concerns. Reformulated gasoline was introduced in 1995 in areas of the United States with severe ozone concerns. The strictest control of gasoline properties to date occurred in 1996 when California Phase 2 reformulated gasoline was required.