Standards of Performance for the control of emissions from phosphate rock plants are being proposed under the authority of Section 111 of the Clean Air Act. These standards would apply to new, modified, and reconstructed phosphate rock dryers, calciners, grinders, and ground rock handling systems, and would limit emissions of particulate matter by 85 to 98 percent below the levels permitted by current typical state regulations. This document contains background information, environmental and economic impact assessments, and the rationale for the proposed standards. Volume II, which will be published when the standards are promulgated, will contain a summary of the public comments on the proposed standard and EPA's responses. A discussion of any differences between the proposed and promulgated standards will also be included.