Traditionally, industrial facilities have used fossil fuels such as coal, oil, and natural gas as their main source of energy. Over the years, however, facilities have sought to replace some or all of their fossil fuel with less expensive and more efficient alternative fuels from secondary materials. The Environmental Protection Agency (EPA) has regulated the combustion of such wastes under the Resource Conservation and Recovery Act (RCRA). Under RCRA, some of these secondary materials are classified as ''hazardous wastes,'' while others are referred to as ''non-hazardous secondary materials.'' Facilities that combust ''solid waste'' as defined by EPA under RCRA must be regulated by emission standards issued by EPA under section 129 of the Clean Air Act (CAA).