The Environmental Protection Agency (EPA), under the authority of Clean Air Act 111(b) and (d), proposes to regulate air pollutant emissions from new and existing municipal waste combustors (MWCs). MWC emissions cause, or contribute significantly to, air pollution that endangers public health and welfare. The Standards and Guidelines require the reduction of emissions to the air of organic compounds (principally dioxins and furans), metals (cadmium, mercury, chromium, lead, nickel, etc.), acid gases (sulfur dioxide, hydrogen chloride, etc.), nitrogen oxides, and carbon monoxide. The Standards identify these pollutants as MWC emissions and nitrogen oxides. MWC emissions consist of MWC organics, MWC metals, and MWC acid gases. MWC emissions is the designated pollutant that triggers application of 111(d); the Guidelines do not address nitrogen oxides. Currently, EPA new source review permitting directives to the states recommend control of most of these pollutants at new, large MWCs.