The Environmental Protection Agency (EPA) is promulgating revisions to the National Emission Standard for Hazardous Air Pollutants (NESHAP) that sets limits on radon emissions from phosphogypsum stacks, codified as subpart R of 40 CFR part 61. The Agency is taking today's action in response to a petition for reconsideration from The Fertilizer Institute (TFI), which critiqued the risk assessment EPA performed in support of the version of subpart R promulgated in 1992. Today's action raises the limit on the quantity of phosphogypsum that may be used for indoor research and development from 700 to 7,000 pounds, eliminates current sampling requirements for phosphogypsum used in indoor research and development, and clarifies sampling procedures for phosphogypsum removed from stacks for other purposes.