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Radiation Protection

Subpart R: National Emission Standards for Radon Emissions From Phosphogypsum Stacks

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Rule Summary

Subpart R protects the public and the environment from the hazards of radioactive materials found in phosphogypsum. Phosphogypsum is a  waste product that results from processing phosphate ore to make phosphoric acid that is later used in fertilizer. Because the phosphate ore contains uranium and radium, phosphogypsum also contains these radionuclides. The radium is of particular concern because it decays to form radon, a cancer-causing, radioactive gas.

Additional Resources

Under certain conditions, phosphogypsum may be removed from stacks for outdoor agricultural purposes or indoor research and development. Any other use of phosphogypsum requires advance permission from EPA. See: Subpart R: Resources.


EPA requires that phosphogypsum be managed in engineered stacks, which are designed to limit emissions of radon and other radionuclides.

Owners and operators must monitor and report radon emissions from inactive stacks and take measures to keep radon emissions within the regulatory limit of 20 picocuries per square meter per second.

All facilities subject to any National Emissions Standards for Hazardous Air Pollutants (NESHAP) must meet 40 CFR Part 61, Subpart A: General Provisions.