| N | Nitrogen; Normal |
| N₂O | Nitrous Oxide |
| N450 | Manganese Compounds |
| N458 | Mercury Compounds |
| N₅₀ | median dose |
| N982 | Zinc Compounds |
| NAA | Nonattainment Area |
| NAAQS | National Ambient Air Quality Standards |
| NAAS | Network Authorization and Authentication Services |
| NAC | NATIONAL AUDIOVISUAL CENTER |
| NADB | National Allowance Data Base |
| NADG | National Air Data Group |
| NAFTA | North American Free Trade Agreement |
| NAICS | North American Industrial Classification System; North American Industry Classification System |
| NAICS Code | North American Industry Classification System Code |
| NAO | North Atlantic Oscillation |
| NAPL | Non-Aqueous Phase Liquid; Nonaqueous Phase Liquid |
| NAPL-Contaminated Zone | Non-Aqueous Phase Liquid Contaminated Zone |
| NAPLs | Non-Aqueous Phase Liquids |
| NAS/NRC | National Academy of Sciences/National Research Council |
| NATA | National-Scale Air Toxics Assessments |
| NATTS | National Air Toxics Trends Station |
| NAVD88 | North American vertical datum of 1988 |
| NBAR | Non-Binding Allocations of Responsibility |
| NCA | Noise Control Act |
| NCAPS | National Corrective Action Prioritization System |
| NCBI | National Center for Biotechnology Information |
| NCD | NMIM County Database |
| NCore | National Core Monitoring Station |
| NCP | National Contingency Plan; National Oil and Hazardous Substances Pollution Contingency Plan |
| ncRNA | Untranslated RNA |
| NCWS | Non-Community Water System |
| NDCE Recovery Furnace | Nondirect Contact Evaporator Recovery Furnace |
| Near-Road NO2 Monitor | Near-Road Nitrogen Dioxide Monitor |
| NEI | National Emissions Inventory |
| NELAC | National Environmental Laboratory Accreditation Conference |
| NELAP | National Environmental Laboratory Accreditation Program |
| NEON | NEI on the NET |
| NEP | National Estuary Program |
| NEPA | National Environmental Policy Act |
| NEPA Process Completion | National Environmental Policy Act Process Completion |
| NER | Nucleotide Excision Repair |
| NERC Region | North American Electric Reliability Council Region |
| NESCAUM | Northeast States for Coordinated Air Use Management |
| NESHAP | National Emission Standard for Hazardous Air Pollutants; National Emission Standards for Hazardous Air Pollutants |
| NESHAPS | National Emissions Standards for Hazardous Air Pollutants |
| NETA | National Environmental Training Association |
| New CCR Landfill | New Coal Combustion Residuals Landfill |
| New IPP Facility | New Independent Power Production Facility |
| New MSWLF Unit | New Municipal Solid Waste Landfill Unit |
| New OCS Source | New Outer Continental Shelf Source |
| New SAPU | New Secondary Aluminum Processing Unit |
| NFFA | No Further Federal Action; No Further Federal Response Action |
| NFG | National Functional Guidelines |
| NFRAP | No Further Remedial Action Planned |
| NGL | Natural Gas Liquid |
| NGLs | Natural gas liquids |
| NGO | Nongovernmental Organization |
| NGVD29 | national geodetic vertical datum of 1929 |
| NH₃ | Ammonia |
| NHD | National Hydrography Dataset |
| NiCad Battery | Nickel Cadmium Battery |
| NIEHS | National Institute of Environmental Health Sciences |
| NIF 3.0 | NEI Input Format Version 3.0 |
| NIH Manuscript Submission System | National Institute of Health Manuscript Submission System |
| NIHMS System | NIH Manuscript Submission System |
| NII | National Information Infrastructure |
| NIMBY | Not in My Backyard |
| NiMH | Nickel Metal Hydride Battery |
| NIOSH | National Institute for Occupational Safety and Health; National Institute of Occupational Safety and Health |
| NIOSH 7402 | NIOSH Polarized Light Microscopy Method 7402 |
| NIOSH Ceiling REL | NIOSH Ceiling Reference Exposure Level |
| NIOSH IDLH | NIOSH Immediately Dangerous to Life; Health |
| NIOSH PCM 7400 | NIOSH Phase Contrast Microscopy Method 7400 |
| NIOSH REL | NIOSH Recommended Exposure Limit |
| NIOSH STEL | NIOSH Short-Term Exposure Limit |
| NIPDWR | National Interim Primary Drinking Water Regulations |
| NIR Reflective Glazing | Near Infrared Reflective Glazing |
| NIRS | Near-Infrared Spectroscopy |
| NIS | Non-Indigenous Species |
| NIST SRM | NIST Standard Reference Material |
| NIST/EPA-Approved CRM | NIST/EPA-Approved Certified Reference Material |
| NIV | Nivalenol |
| nm | Nanometers |
| NMD | Nonsense-Mediated Decay |
| NMHC | Non-Methane Hydrocarbon; Nonmethane Hydrocarbons Sum |
| NMHCE | Non-Methane Hydrocarbon Equivalent |
| NMIM | National Mobile Inventory Model |
| NMOC | Nonmethane Organic Compounds |
| NMOG | Non-Methane Organic Gases |
| NMR | Nuclear Magnetic Resonance |
| nmRNA | Non-Messenger RNA |
| NMVOC | Non-Methane Volatile Organic Compound |
| NMVOCs | Non-Methane Volatile Organic Compounds |
| NNC | Notice of Noncompliance |
| NNI | Neonicotinoids |
| NO | Nitric Oxide |
| No Organic HAP | No Organic Hazardous Air Pollutant |
| NO₂ | Nitrogen Dioxide |
| NOA | Nearest Onshore Area; Notice of Availability |
| NOAEC | No Observed Adverse Effects Concentration |
| NOAEL | No Observable Adverse Effect Level; No Observed Adverse Effect Level; No Observed Adverse Effects Level |
| NOD | Notice of Deficiency |
| NODI | No Data Indicator |
| NOEL | No Observed Effect Level |
| NOF | NEI Output Format |
| NOHSCP/NCP | National Oil and Hazardous Substances Contingency Plan |
| NOI | Notice of Intent |
| Noise | Environmental Noise |
| NOM | Natural Organic Matter |
| Non-Dedicated PAI Process Unit | Non-Dedicated Pesticide Active Ingredient Process Unit |
| Non-HAP Binder | Non-Hazardous Air Pollutant Binder |
| Non-HAP Coating | Non-Hazardous Air Pollutant Coating |
| Non-HAP Material | Non-Hazardous Air Pollutant Material |
| Non-HAP Solvent | Non-Hazardous Air Pollutant Solvent |
| Non-Mercury HAP Metals | Non-Mercury Hazardous Air Pollutant Metals |
| Nonmethane TOC | Nonmethane Total Organic Compound |
| NORM | Naturally Occurring Radioactive Materials |
| NOS | Nitric Oxide Synthase |
| NOS-1 | Neuronal Nitric Oxide Synthase |
| Notice-Level Operation | Mineral Extraction |
| NOV | Notice of Violation |
| NOx | Nitrogen Oxides |
| NOX SIP Call | Nitrogen Oxides State Implementation Plan Call; NOX State Implementation Plan call |
| NP Diesel | Non-Petroleum Diesel Fuel |
| NPAH | Nitrated Polycyclic Aromatic Hydrocarbons |
| NPDES | Discharge Permits; National Pollutant Discharge Elimination System (Various); National Pollution Discharge Elimination System |
| NPDES Permit | National Pollutant Discharge Elimination System Permit |
| NPDES Permit ID | National Pollutant Discharge Elimination System Permit ID |
| NPDES State | National Pollutant Discharge Elimination System State |
| NPDWR | National Primary Drinking Water Regulation |
| NPFC | National Pollution Funds Center |
| NPL | National Priorities List |
| NPP | Net Primary Production |
| NPRM | Notice of Proposed Rulemaking |
| NPS | Non-Point Source; Nonpoint Source Emission; Nonpoint Sources |
| NPS pollution | Nonpoint Source Pollution |
| NPV | Net Present Value |
| NR | NONROAD; reactive nitrogen |
| NR Diesel Fuel | Nonroad Diesel Fuel |
| NRC | National Response Center; Nuclear Regulatory Commission |
| NRCS | Natural Resources Conservation Service |
| NRD | Natural Resources Damage |
| NRDA | Natural Resource Damage Assessment |
| NRE | Nonroad Engine |
| NRLM Diesel Fuel | Nonroad, Locomotive; Marine Diesel Fuel |
| NRR | Noise Reduction Rating |
| NRT | National Response Team; Nutrient removal technology |
| NSAID | Non-Steroidal Anti-Inflammatory Agent |
| NSCIU | Nonsignificant Categorical Industrial User |
| NSDWRs | National Secondary Drinking Water Regulations |
| NSF | National Strike Force |
| NSFCC | National Strike Force Coordination Center |
| NSI | National Security Information |
| NSPS | New Source Performance Standards |
| NSR | New Source Review |
| NTA | Nitrilotriacetic Acid |
| NTDE | national tidal datum epoch |
| NTI | National Toxics Inventory |
| NTNCWS | Non-Transient Non-Community Water Systems |
| NTRM | NIST Traceable Reference Material |
| NTU | Nephelometric Turbidity Unit |
| NTX | Neurotoxicity |
| NUE | nutrient use efficiency |
| NVOC | Non-Volatile Organic Compound |
| nvPM | Non-Volatile Particulate Matter |
| NWPA | Nuclear Waste Policy Act |
| NWW | Nonwastewater |
| nZVI | Nanoscale Zero-Valent Iron |