| α | Alpha Particle |
| A-130 | OMB CIRCULAR NO. A-130, MANAGEMENT OF FEDERAL INFORMATION RESOURCES |
| AA | Allowance Allocation |
| AALAC | Ambient Aquatic Life Advisory Concentration |
| A/B | Afterburner |
| ABA | Auxiliary Blowing Agent |
| ABC | Association of Boards of Certification |
| ABL | Atmosphere Boundary Layer |
| ABS | Activity-Based Sampling |
| ABS Latex Resin | Acrylonitrile Butadiene Styrene Latex Resin |
| ABS Resin | Acrylonitrile Butadiene Styrene Resin |
| AC | Alternating Current |
| ACBM | Asbestos Containing Building Material |
| ACBM Removal | Asbestos-Containing Building Material Removal |
| ACC | Asphalt Cement Concrete |
| Accessible ACM | Accessible Asbestos-Containing Material |
| ACCESSION | ACCESSIONED RECORDS |
| ACE | Air Climate Energy |
| ACE Inhibitor | Angiotensin Converting Enzyme Inhibitor |
| ACGIH® STEL | American Conference of Governmental and Industrial Hygienists' Short-Term Exposure Limit |
| ACGIH® TLV® | American Conference of Governmental Industrial Hygienists' Threshold Limit Value |
| ACh | Acetylcholine; Air Changes per Hour; Automated Clearing House |
| AChE | Acetylcholinesterase |
| ACI System | Activated Carbon Injection System |
| ACL | Alternate Concentration Limit |
| ACM | Asbestos-containing material |
| ACO | Administrative Contracting Officer |
| ACP | Agricultural Conservation Program; Area Contingency Plan |
| ACQ | Aquaculture Production |
| ACTH | Adrenocorticotropic Hormone |
| Active Use SO2 Control | Active Use Sulfur Dioxide Control |
| AcTOR | Aggregated Computational Toxicology Resource |
| Actual HAP Emission | Actual Hazardous Air Pollutant Emission |
| Actual SO2 Emissions Rate | Actual Sulfur Dioxide Emissions Rate |
| Acute RfC | Acute Reference Concentration |
| Acute RfD | Acute Reference Dose |
| ACWM | Asbestos-Containing Waste Materials |
| ADAF | Age dependent adjustment factor |
| ADD | Average Daily Dose |
| ADF | Aircraft Deicing Fluid |
| ADHD | Attention Deficit Hyperactivity Disorder |
| ADI | Acceptable Daily Intake |
| ADP | AHERA Designated Person; AUTOMATIC DATA PROCESSING |
| ADR | Alternative Dispute Resolution |
| A/E Service | Architectural; Engineering Service |
| AEA | Atomic Energy Act |
| AECD | Auxiliary Emission Control Device |
| AERMOD | AMS/EPA Regulatory Model |
| AES | Atomic Emission Spectroscopy |
| AETB | Air Emission Testing Body |
| Affected EGU | Affected Electric Generating Unit |
| AFM | Atomic Force Microscopy |
| AFO | Animal Feeding Operation |
| AFS | Air Facility System |
| AFUE | Annual fuel utilization efficiency |
| AGC | Automatic Gain Control |
| AGR | Acid Gas Removal Unit |
| AGR Vent Emission | Acid Gas Removal Vent Emission |
| Ah Receptor | Aryl Hydrocarbon Receptor |
| AHERA | Asbestos Hazard Emergency Response Act of 1986 |
| AHH | Aryl Hydrocarbon Hydroxylase |
| AHJ | Authority Having Jurisdiction |
| AHR | Airway Hyperresponsiveness |
| AHU | Air Handling Unit |
| A.I. | Active Ingredient |
| AI | Artificial Insemination; Availability of Information |
| A&I Database | Abstract & Indexing Database |
| AIC | Akaike Information Criteria; Akaikes information criterion |
| AIDS | Acquired Immunodeficiency Syndrome |
| AIHA ERPG | American Industrial Hygiene Association's Emergenct Response Planning Guideline |
| AIIM | ASSOCIATION FOR INFORMATION AND IMAGE MANAGEMENT |
| AIP | Applicable Implementation Plan |
| AirToxScreen | Air Toxics Screening Assessment |
| AIS | AUTOMATED INFORMATION SYSTEM |
| AL | Action Level (regulatory); Aluminum |
| ALJ | Administrative Law Judge |
| Allowable HAP Loss | Allowable Hazardous Air Pollutant Loss |
| Allowable SO2 Emission Rate | Allowable Sulfur Dioxide Emission Rate |
| Alumina | Chemical Product |
| Aluminum Foundry HAP | Aluminum Foundry Hazardous Air Pollutant |
| ALVW | Adjusted Loaded Vehicle Weight |
| AM | Amplitude Modulation; arithmetic mean |
| AMBIENS | Atmospheric Mass Balance of Industrially Emitted and Natural Sulfur |
| AMD | Acid Mine Drainage |
| AML | Abandoned Mine Land |
| AMS | Aerosol Mass Spectrometry; Air Mass; Allowance Management System |
| AMSAN | Alpha Methyl Styrene Acrylonitrile Resin |
| ANC | acid neutralizing capacity |
| ANCOVA | Analysis of Covariance |
| Animal CNS | Animal Central Nervous System |
| ANOVA | Analysis of Variance |
| ANSI | American National Standards Institute |
| ANSI/ISO 14001:2004 | American National Standards Institute/International Organization for Standardization 14001:2004 |
| Anthropogenic Aerosol | Atmosphere Aerosol |
| AO | Administrative Order; Arctic Oscillation |
| AOC | Administrative Order on Consent; Administrative Orders on Consent; Approximate Original Contour; Area of Concern; Area of Observed Contamination |
| AOC Policy | Area of Contamination Policy |
| AOD | Argon-Oxygen Decarburization |
| AOD Vessel | Argon-Oxygen Decarburization Vessel |
| AOP | Adverse Outcome Pathway |
| AoR | Area of Review; Authorized Organizational Representative |
| AOS | Alternative Operating Scenario |
| AOX | Adsorbable Organic Halide |
| APA | Administrative Procedures Act |
| APCD | Air Pollution Control Device |
| AP-CIMS | Atmospheric Pressure Chemical Ionization Mass Spectrometry |
| API | Active Pharmaceutical Ingredient; Application Programming Interface |
| APIMS | Atmospheric Pressure Ionization Mass Spectrometry |
| APP | Affirmative Procurement Program |
| Applicable SIP | Applicable State Implementation Plan |
| Applied Coating VOC | Applied Coating Volatile Organic Compound |
| APPU | Process Unit |
| AQAD | Air Quality Assessment Division |
| AQAG | Air Quality Analysis Group |
| AQCR | Air Quality Control Region |
| AQI | Air Quality Index |
| AQL | Acceptable Quality Level |
| AQM | Air Quality Model |
| AQMG | Air Quality Modeling Group |
| AQPD | Air Quality Policy Division |
| AQS | Air Quality System |
| AR | Additional Risk; Applicable Requirement |
| ARA | Annual Reportable Amount |
| ARAR | Applicable; Relevant and Appropriate Requirement |
| ARARs | Applicable; Relevant and Appropriate Requirements |
| ARCHIVES | ARCHIVAL AGENCY; ARCHIVAL DEPOSITORY; ARCHIVAL REPOSITORY; NATIONAL ARCHIVES |
| ARD | Acid-Rock Drainage |
| A-RIN | A-Renewable Identification Number |
| ARM (Regional Method) | Approved Regional Method |
| ARM (Replicable Method) | Approved Replicable Methodology |
| ARP | Acid Rain Program |
| ARS | Autonomous Replicating Sequence |
| AS | Air Sparging; Area Source |
| ASA Resin | Acrylonitrile Styrene Acrylate Resin |
| ASC | Ascorbic Acid |
| ASCII | American Standard Code for Information Interchange |
| ASCS | Agricultural Stabilization and Conservation Service |
| ASD | Autism Spectrum Disorders |
| ASHARA | Asbestos School Hazard Abatement Reauthorization Act |
| ASHRAE | American Society of Heating, Refrigerating and Air-Conditioning Engineers |
| ASP | Active Server Page; Aspergillosis |
| ASPEN | Assessment System for Population Exposure Nationwide |
| ASR | Atmosphere-Supplying Respirator |
| Assigned RIN | Assigned Renewable Identification Number |
| AST | Above Ground Storage Tank; Aboveground Storage Tank |
| ASTER | Assessment Tools for the Evaluation of Risk |
| ASTM | American Society for Testing and Materials |
| AT | averaging time |
| Atmospheric Carbon Dioxide | Trace Gas |
| AtN | Atramentous Necrosis |
| ATP | Adenosine Triphosphate |
| ATS | Allowance Tracking System |
| ATSDR | Agency for Toxic Substances and Disease Registry |
| ATSDR MRL | Agency for Toxic Substances and Disease Registry Minimal Risk Level |
| ATSDR-HazDat | Agency for Toxic Substances and Disease Registry's Hazardous Substance Release/Health Effects Database |
| Attributional LCA | Attributional Life Cycle Assessment |
| ATU | Aerobic Treatment Unit |
| ATV | Advanced Television; All-Terrain Vehicle |
| au | Animal unit; Assessment Formation Union |
| AUM | Animal Unit Month |
| Available ADF | Available Aircraft Deicing Fluid |
| Average VO Concentration | Average Volatile Organic Concentration |
| AVHRR | Advanced Very High Resolution Radiometer |
| AVIRIS | Airborne Visible and Infrared Imaging Spectrometer |
| AVM | Avian Vacuolar Myelinopathy |
| AWC | Available Water Capacity |
| AWFCO System | Automatic Waste Feed Cutoff System |
| AWQC | Ambient Water Quality Criteria |
| AWT | Advanced Waste Treatment; Advanced Wastewater Treatment |