The primary federal law governing efforts to control air pollution. Federal legislation addressing air pollution was first adopted in 1955 (Air Pollution Control Act, P.L. 84-159) to provide research and technical assistance. Subsequent amendments, most notably the Clean Air Act Amendments of 1970 (P.L. 91-604), 1977 (P.L. 95-95), and 1990 (P.L. 101-549), strengthened the federal role. The Clean Air Act seeks to protect human health and the environment from emissions that pollute the air. The Environmental Protection Agency is required to establish minimum National Standards Ambient Air Quality Standards (NAAQS), while states are assigned primary responsibility for developing compliance. Areas not meeting the standards (nonattainment areas) are required to implement specific control measures. There is no direct federal regulation of agriculture under the Clean Air Act. Two of the NAAQS (for particulates and ozone) could affect agriculture: particulates, because certain agricultural practices, such as prescribed burning and tilling, create airborne particles that might be targeted for control in State Implementation Plans; and ozone, because concentrations of ozone above the standard can adversely affect crop yields. Ozone is formed in the atmosphere when nitrogen oxides and volatile organic compounds (from manufacturing, transportation, and utilities) react in the presence of sunlight (agriculture rarely if ever represents significant sources of ozone precursors).