| I | Infrastructure |
| IA | Interagency agreement |
| IAP | Indoor air pollution |
| IAQ | Indoor air quality |
| IASSC | Interagency Agreement Shared Service Center |
| IBC | Intermediate bulk container |
| IBI | Index of biological integrity; Index of biotic integrity |
| IBR | Incorporation by reference |
| ICE | Internal combustion engine; Ion composition elucidation |
| ICF | Insulating concrete forms |
| ICI | Independent commercial importer; Invertebrate community index |
| ICIS | Integrated Compliance Information System |
| ICIS-NPDES | Integrated Compliance Information System-National Pollutant Discharge Elimination System |
| ICP | Inherent chemical property |
| ICP-AES | Inductively coupled plasma-atomic emission spectroscopy |
| ICP-MS | Inductivity coupled plasma mass spectrometry |
| ICR | Information collection request |
| ICRE | Ignitability corrosivity reactivity toxicity |
| ICS | Incident command system; Institutional control |
| ICV | Independent calibration verification |
| IDC | Initial demonstration of capability |
| IDL | Instrument detection limit |
| IDLH | Immediately dangerous to life and health |
| IDP | Initial demonstration of performance |
| IDW | Inverse distance weighting |
| IDx | Infective dosex |
| IECP | Integrated error correction process |
| IEM | Integrated environmental modeling |
| IEUBK | Integrated Exposure Uptake Biokinetic Model |
| IFD | Indentation force deflection |
| IFMS | Integrated Financial Management System |
| IFR | Internal floating roof |
| IFR Type A Failure | Internal floating roof type A failure |
| IFR Type B Failure | Internal floating roof type B failure |
| IGA | Intergovernmental agreement; International agreement |
| IGCC Electric Utility Steam Generating Unit | Integrated Gasification Combined Cycle Electric Utility Steam Generating Unit |
| IGCC Facility | Integrated gasification combined cycle facility |
| IGMS | Integrated Grant Management System |
| IJC | International Joint Commission |
| ILD | Interstitial lung diseases |
| i.m. | Intramuscular |
| I/M | Inspection and maintenance program |
| IMAT | Incident management assistance team |
| IMT | Incident management team |
| In Situ Leach Mining | In-situ solution mining |
| INAA | Instrumental neutron activation analysis |
| INAD | Investigational new animal drug |
| IND | Improvised nuclear device; Investigational new drug |
| Inorganic HAP | Inorganic hazardous air pollutant |
| iNOS | Inducible nitric oxide synthase |
| Intermediate PMN | Intermediate premanufacture notice |
| Intermittent HMIWI | Intermittent Hospital/Medical/Infectious Waste Incinerator |
| IO | Inorganic |
| IP | Induced polarization |
| IP address | Internet Protocol address |
| IP Stack | Internet Protocol Stack |
| IPC | Instrument performance check |
| IPCC | Intergovernmental Panel on Climate Change |
| IPCT | Industrial process cooling tower |
| IPM | Integrated pest management |
| IPP | Independent power producer |
| IPP Facility | Independent power production facility |
| IPR | Initial precision and recovery |
| IPT | Integrated project team |
| IQG | Information quality guidelines |
| IQR | Interquartile range |
| IR | Infrared spectroscopy |
| IR Radiation | Infrared radiation |
| IR-4 | Interregional Research Project No. 4 |
| IRB | Institutional Review Board |
| IRIS | Integrated Risk Information System |
| IRM | Information resources management |
| IRMS | Isotope ratio mass spectrometry |
| IRT | Interagency review team |
| IS | Internal standard |
| ISA | Integrated science assessment |
| ISDN | Integrated Services Digital Network |
| ISE | Ion-selective electrode |
| ISO | International Standardization Organization |
| ISO 10312 | International Organization for Standardization Method 10312 |
| ISO 9001-Registered Facility | International Organization for Standardization 9001-Registered Facility |
| ISO Standards | International Organization for Standardization standards |
| ISS | Identity self service |
| ISSN | International Standard Serials Number |
| IT | Information Technology |
| ITCZ | Intertropical Convergence Zone |
| ITEM | DISPOSITION ITEM; SCHEDULE ITEM |
| ITFS | Instructional television fixed service |
| ITR | Integrated transdisciplinary research |
| IU | Industrial user |
| IUR | Inhalation unit risk |
| IVD | Intake valve deposit |
| IVDS | Interactive video data service |
| IWS | Ionizing wet scrubber |