| R T | Solids Turnover Ratio |
| RA | Reasonable Alternative; Regional Administrator; Regulatory Authority; Remedial Action; Return Air; Risk Assessment |
| RAA | Relative Accuracy Audit |
| RAB | Restoration Advisory Board |
| RACM | Reasonably Available Control Measures; Regulated Asbestos-Containing Material |
| RACT | Reasonably Available Control Technology |
| RAD | Radiation; Roentgen Absorbed Dose |
| RAGS | Risk Assessment Guidance |
| RAIC | RECORDS ADMINISTRATION INFORMATION CENTER |
| RAL | Remediation Action Level |
| RAM | RANDOM ACCESS MEMORY |
| RAMP | Remedial Action Master Plan |
| RAM-W | Risk Assessment Methodology for Water Utilities |
| RAP | Regulated Air Pollutant; Remedial Action Plan |
| RATA | Relative Accuracy Test Audit |
| RB | Red Border |
| RBC | Rotating Biological Contactor |
| RBCA | Risk-Based Corrective Action |
| RBCs | Risk-based Concentrations |
| RBDM | Risk-Based Decision Making |
| RBDs | Risk-based Doses |
| RBOB | Reformulated Gasoline Blendstock for Oxygenate Blending |
| RCC | Resource Conservation Challenge |
| RCM | Radiative-Convective Model |
| RCO | Responsible Corporate Officer |
| RCP | Representative Concentration Pathway |
| RCP Program | Radon Contractor Proficiency Program |
| RCRA | Resource Conservation & Recovery Act |
| RCRA Section 6002 | Section 6002 of the Resource Conservation and Recovery Act (RCRA) |
| RCRAInfo | Resource Conservation and Recovery Act Information System |
| RCS | Environmental Reactivity; Reusable Component Services |
| RCWP | Rural Clean Water Program |
| R&D | Research & Development |
| RD | Remedial Design; Remedy Design |
| RDD | Radiological Dispersion Device; Regional Deposited Dose |
| RDDR | Regional Deposited Dose Ratio |
| RDF | Refuse-Derived Fuel |
| RDF Stoker | Refuse-Derived Fuel Stoker |
| rDNA | Recombinant DNA; Ribosomal DNA |
| RD/RA | Remedial Design and Remedial Action; Remedial Design/Remedial Action |
| RE | Rule Effectiveness |
| READ | Registry of EPA Applications, Databases, and Models |
| REC | Renewable Energy Certificate |
| RECs | Renewable Energy Certificates |
| Recycled Onsite HAP Solvent | Recycled Onsite Hazardous Air Pollutant Solvent |
| redox | oxidation-reduction |
| Redox Property | Oxidation-Reduction Property |
| REF | Reference |
| REFS | Reference Files System |
| Regulated CCR Unit | Regulated Coal Combustion Residuals Unit |
| REI | Restricted Entry Interval |
| REL | Recommended Exposure Limit |
| RELs | Recommended Exposure Limits |
| rem | Roentgen Equivalent Man |
| RENC | Regional Exchange Network Coordinator |
| Renewable LNG | Renewable Liquefied Natural Gas |
| REPI | Renewable Energy Production Incentive |
| RESS | Rechargeable Energy Storage System |
| REST | Representational State Transfer Web Service |
| REST APIs | Representational State Transfer Application Programming Interface |
| RETIREMENT | RECORDS RETIREMENT |
| ReVA | Regional Vulnerability Assessment |
| RF | Radio frequency; Reactivity Factor |
| RFA | RCRA Facility Assessment; Regulatory Flexibility Act |
| RfC | Inhalation Reference Concentration; Request for Correction |
| RfD | Oral Reference Dose |
| RFG | Reformulated Gasoline |
| RFG Covered Area | Reformulated Gasoline Covered Area |
| RFG Opt-in Area | Reformulated Gasoline Opt-in Area |
| RFI | RCRA Facility Investigation |
| RFID | Radio Frequency Identification Tracking |
| RFP | Reasonable Further Progress |
| RFP Baseline | Reasonable Further Progress Baseline |
| RFP Plan | Reasonable Further Progress Plan |
| RFR | Request for Reconsideration |
| RfV | Reference Value |
| RGB | Red, Green, and Blue |
| RGDR | Regional Gas Dose Ratio |
| RGM | Research Gas Mixture |
| RGM (Mercury) | Reactive Gaseous Mercury |
| RI | Remedial Investigation |
| RICE | Reciprocating Internal Combustion Engine |
| RI/FS | Remedial Investigation and Feasibility Study; Remedial Investigation/Feasibility Study |
| RIN | Regulatory Identification Number; Renewable Identification Number; Request Identification Number |
| RIN Generator | Renewable Identification Number Generator |
| RIN Verification Status | Renewable Identification Number Verification Status |
| RIN-Generating Foreign Producer | Renewable Identification Number Generating Foreign Producer |
| RIN-less RNG | Renewable Identification Number-less Renewable Natural Gas |
| RLO | RECORDS LIAISON OFFICER |
| RM | Raw Material; Records Management; Risk Management |
| RMANs | Recovered Materials Advisory Notices |
| RMCL | Recommended Maximum Contaminant Level |
| RME | Reasonable maximum exposure; Responsible Management Entity |
| RME Risk | Reasonable Maximum Exposure Risk |
| RMO | RECORDS MANAGEMENT OFFICER |
| RMP | Risk Management Plans |
| RMP Facility Proximity | Risk Management Plan Facility Proximity |
| RMP Program Applicant | Radon Measurement Proficiency Program Applicant |
| RMS | Resource management system |
| RMSE | root mean square error |
| Rn | Radon |
| RNA | Ribonucleic Acid |
| RNG | Renewable Natural Gas |
| RNG as Feedstock | Renewable Natural Gas as Feedstock |
| RNG Importer | Renewable Natural Gas Importer |
| RNG Producer | Renewable Natural Gas Producer |
| RNG Production Facility | Renewable Natural Gas Production Facility |
| RO | EPA Regional Office; Rated Output; Responsible Official; Reverse Osmosis |
| ROD | Record of Decision |
| ROI | Radius of Influence |
| ROI (Chemistry) | Reactive Oxygen Intermediate |
| ROM | READ ONLY MEMORY |
| ROP | Rate of Progress |
| ro/ro | Roll-on/Roll-off |
| RO/RO Cargo | Roll-on/Roll-off Cargo |
| ROS | Reactive Oxygen Species |
| RP | Responsible Party |
| RPD | Relative percent difference |
| RPF | Relative Potency Factor |
| RPM | Remedial Project Manager |
| RPO | Regional Planning Organization |
| rPR | Rated Pressure Ratio |
| RPS | Renewable Portfolio Standards |
| RQ | Reportable Quantity |
| RR | Relative Risk; Residual Risk; Risk Ratio |
| rRNA | Ribosomal Ribonucleic Acid; Ribosomal RNA |
| RRT | regional response team |
| RS | Reconstructed Source; Responsiveness Summary |
| RSASTP | Random Sampling Abatement System Testing Program |
| %RSD | Percent Relative Standard Deviation |
| RSD | Relative standard deviation; Risk Specific Dose |
| RTO | Regenerative Thermal Oxidizer |
| RTP | Real-Time Electricity Pricing |
| RT-PCR | Reverse Transcriptase-Polymerase Chain Reaction |
| RTU | Roof Top Unit |
| RUP | Restricted Use Pesticide |
| RV | Residual Volume |
| RY | Reporting Year |