| Absolute Humidity | The weight of water in a given volume of air. |
| Absorbents | Granular substances that help clean up stains and spills without solvents or chemicals. |
| Accidental Pressure | An air-pressure difference between the indoors and the outdoors. |
| Accidental Ventilation | Random air movement into and out of a house. |
| Activated Alumina | A form of aluminum oxide used as an adsorption material in air filters, more correctly called activated alumina impregnated with potassium permanganate. |
| Activated Carbon | A form of carbon used as an adsorption or pollutant-reduction material in air or water filters, often derived from coconut, wood, or coal. |
| Active Ventilation | Ventilation caused by a fan. |
| Adsorption | The adhering of a gas onto the surface of a substance. |
| Adsorption Filter | A filter that removes gases from the air by the process of adsorption. |
| Air Changes per Hour (ACH) | The number of times the volume of air in a house is replaced with outdoor air during an hour. |
| Air Cleaner | There are several different types, but these useful appliances are no substitute for good cleaning and other preventive measures. |
| Air distribution | Movement of air throughout a house with a ventilation system or heating/cooling system. |
| Air Filter Arrestance | Also known as a dust weight-arrestance test, it measures only for larger and heavier particles. |
| Air Filter Efficiency | Describes the ability of an air filter to remove very fine particles of solid or liquid found in the air, such as aerosols, smoke, fumes, dust and ash. |
| Air Handler | A cabinet containing a fan. |
| Air Silencer | A device, similar to an automotive muffler, that can be used to absorb some of the sound generated in a ventilation system. |
| Air-Pressure Difference | The dissimilarity in air pressures between two locations that can cause air to move between them. |
| Air-to-Air Heat Exchanger (AAHX) | A balanced ventilation device capable of transferring heat (and sometimes moisture) between two airstreams. |
| Airflow Grid | A device that mounts inside a duct. |
| Airtight Construction | Building techniques that result in a house with a small effective leakage area (ELA) to conserve energy. |
| Airtight Drywall Approach (ADA) | Building techniques that result in a house with a small effective leakage area (ELA) to conserve energy. |
| All-Cotton | Understand the claims on the labels of your all-cotton items. |
| All-Natural | Be discerning. All-natural does not necessarily mean all-safe. |
| Allergen Slipcover | These bedding accessories are a must for people with severe asthma or allergic sensitivity to dust mites. |
| American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) | A professional organization that sponsors research and sets voluntary standards for the ventilation industry. |
| Antibacterial Products | The scientific debate continues over whether antibacterial soaps, sprays and wipes may end up doing more harm than good. |
| Apparent Sensible Effectiveness (ASEF) | A measurement useful in determining the temperatures of airstreams in a heat or energy recovery ventilator. |
| Arsenic | Highly toxic and a known carcinogen, sources of arsenic contamination include industrial and workplace exposure, ingestion of food and water containing arsenic-related substances and more. |
| Asbestos | A wide range of building materials may contain asbestos, particularly where insulation or heat and fire resistance are essential to safe construction.
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| Atmospheric Draft | The negative pressure in a chimney caused by rising, warm air and combustion by-products. Often referred to simply as draft. |
| Atmospheric Dust Spot Test | A method of testing medium-efficiency air filters. |
| ATP | Adenosine Triphosphate is the main energy transfer molecule present in all living (and once living) biological cells including animal, plant, bacterial, yeast, and mold cells. |
| Axial Fan | A type of ventilating fan having propeller-like blades. |
