IAQ Word Cloud

Indoor Air Quality Glossary of Terms

ACH (Air Changes per hour)

Air changes per hour or ACH is a common term used when discussing indoor air quality and ventilation. Air changes per hour measures how many times the air in a space – either single room or whole house – is replaced by outdoor air. Indoor air often leaks in through cracks or is removed by a ventilation system. The measurement generally is depicted as a fraction of “one air change per hour, such as .35 ACH,” states the U.S. Environmental Protection Agency.

Air Cleaning

Air cleaning is the process of removing airborne particulates and gases from the indoor atmosphere. Air cleaning may be accomplished by the running of a device designed specifically to remove particulates and/or gases from the air. Three types of air cleaning are particle filtration, electrostatic precipitation and usage of a gas absorbing material. Examples of airborne particles include pollen, dust mites, mold spores, animal dander and microorganisms such as bacteria.

Air Handling Unit

In regards to HVAC systems and other ventilation equipment, the air-handling unit is any item with a fan or blower component as well as elements such as heating and cooling coils. Other parts of the AHU may include condenser drip pans, air filters and controls for the unit. The EPA does not include ductwork, registers, boilers and chillers as parts of the AHU.

Animal Dander

Similar to human dandruff, animal dander is tiny (almost microscopic) skin cells shed by warm-blooded animals. Pet dander can contribute to poor indoor air quality. Animal dander is an allergen. People suffering from pet allergies are not allergic to the animal’s fur, states the Allergy and Asthma Foundation of America (AAFA.) Animal fur holds dander and the dander triggers the allergy and in some cases asthma. An air filter with a MERV rating of 11 or higher may be best for homes with pets.


The AAFA states an allergen is any substance that triggers or causes an allergic reaction. Allergic reactions may include but are not limited to sneezing, running nose, itchy/watery eyes, nasal congestion and itchy rashes. Common allergens that contribute to poor indoor air quality include pollen, mold spores, pet dander and dust mites. Inhaled allergens, in some cases, can trigger severe allergic reactions or asthma systems. More than 50 million Americans suffer from allergies.


An antimicrobial has the ability to destroy or slow down the growth of microorganisms that cause disease. Ethyl alcohol, hydrogen peroxide and chlorine bleach are examples of antimicrobials. Sometimes antimicrobials are referred to as an antibacterial, disinfectant or sanitizer. Hard surfaces cleaned with antimicrobials may help reduce the spread of the common cold, influenza and other viruses.


Back-drafting happens when the exhaust from combustion appliances (furnaces, hot water heaters, boilers and fireplaces) is transported down the flue and emitted into the home. This is caused by negative pressure and can release deadly gases such as carbon monoxide into the home. Carbon monoxide restricts a body’s ability to transport oxygen through the blood cells. It is colorless and odorless.

Biological contaminants

Biological contaminants or pollutants, produced by living things, can be found in items such as humidifiers, condensation pans and unvented bathrooms. However, some also can be found in carpets, bedding and other upholstery. To thrive, biological contaminants typically need food and moisture. Examples of common biological contaminants are bacteria, mold, viruses, pet dander, insect droppings, saliva as well as rodent urine and/or droppings. Humans respond to some biological contaminants with allergic reactions, while others may suffer illnesses that are more serious.

Carbon dioxide

Also known as CO2, carbon dioxide has a carbon atom bonded to two oxygen atoms. It is colorless, odorless and does not have any taste. Joseph Black originally identified carbon dioxide in the 1750s. Black was a physician as well as chemist and professor at the University of Glasgow when he discovered CO2. Carbon dioxide is non-flammable at room temperatures. The Centers for Disease Control and Prevention reports it is dangerous to life at 100,000 ppm (parts per million).

Carbon monoxide

Colorless, odorless, tasteless and toxic, carbon monoxide is a deadly gas. Carbon monoxide or CO fumes kills about 500 people annually. It can enter your home via unvented space heaters, automobile exhaust leaking in from attached garages, leaking chimneys and from back-drafting furnaces. Inhaling carbon monoxide fumes can cause dizziness, headaches, nausea and disorientation. A carbon monoxide detector can help protect everyone in your home from carbon monoxide poisoning.

Chemical pollutants

Organic and inorganic chemical pollutants are substances that adversely affect an environment. Organic forms have a relation to living organisms and include substances such as petroleum and crude oil, acetone, benzene, alcohols, phenols, PCBs and some detergents, plastics, pesticides and herbicides. Inorganic chemical pollutants include but are not limited to metals, sulfides, ammonia and inorganic fertilizers. HVAC air filters can help reduce the presence of chemical pollutants, improving a home’s IAQ.

Cubic feet per minute (CFM)

Cubic feet per minute is a measurement used to calculate the airflow in a space. According to the EPA, cubic feet per minute measures as the amount of air (in cubic feet) flowing through a space in one minute. The correct amount of airflow in your home is essential for efficient heating and cooling. Too much airflow can make the home or room feel drafty and uncomfortable. Not enough airflow and the space may take a long time to cool or heat, affecting the HVAC system’s overall efficiency.


The damper within a HVAC system controls the airflow. Typically, a damper is an adjustable plate that opens and closes, controlling airflow through ductwork, air outlets or inlets. Some dampers can be adjusted manually, while others are integrated into an automated system and connected to a thermostat. Larger HVAC systems may have more than one damper, each connected to its own “zone,” which in turn can be controlled by a multi-zone thermostat. By controlling the airflow to a room, the damper helps maintain a home’s heating and cooling efficiency.

Dust spot efficiency

MERV ratings are in part determined by dust spot efficiency, which is a measurement of an air filter’s ability to remove dust from the air. Dust spot efficiency numbers are expressed as a percentage. For example, an air filter with a dust spot efficiency between 25 and 30 percent (MERV 7) will catch mold spores, while a filter with a dust spot efficiency between 90 and 95 percent (MERV 14) will trap most tobacco smoke.

Exhaust ventilation

Exhaust ventilation is simply the act of removing air from a room, area or whole house using a piece of specialized equipment. The term also is used as the description of that equipment, which allows the air to vent. Exhaust ventilation is an important component in an HVAC system. The HVAC system must be able to remove quickly any concentrated air contaminants to keep the indoor air quality safe and clean.


Fungi are darkness-dwelling, parasitic plants that help some degrading of natural materials such as wood and various plant parts. Examples of fungi include mold, yeast and mushrooms. Fungi release spores into the air, which can enter a home through cracks in the foundation, windows or even through open garage doors. Fungi, like mold, need moisture to grow. You may see fungi growth in crawl spaces, leaky basements or in areas without adequate exhaust ventilation.

Humidifier fever

A flu-like illness, humidifier fever typically strikes a few hours after someone has been exposed to any amoebae, bacteria and/or fungi found in a humidifier reservoir, air conditioner or an aquaria, reports the EPA. Symptoms include but are not limited to fever, chills, headache and exhaustion. To rid a humidifier of harmful bacteria and other contaminants, it must be cleaned thoroughly using a disinfectant such as bleach.

Indoor air quality (IAQ)

The U.S. Environmental Protection Agency defines indoor air quality or IAQ as the quality of air “within and around buildings and structures.” IAQ specifically refers to the way the air affects both the health and the comfort of any human in the building or structure. IAQ can be affected adversely by air borne pollutants such as pollen, mold spores, dust mites, animal dander, VOCs (volatile organic compounds) and microorganisms including bacteria and fungi.

Indoor Air Quality Association

Established in 1995, the Indoor Air Quality Association (IAQA) promotes industry standards plus protocols regarding indoor air quality. The IAQA is a non-profit organization, consisting of members from the American Council for Accredited Certifications and the Indoor Environmental Standards Organization as well other air quality industry professionals. In addition to creating industry standards, the IAQA also promotes indoor air quality research and development.

Indoor air pollutant

Contaminants found in air that affect the room, area or home’s indoor air quality are considered indoor air pollutants. Indoor air pollutants include dust mites, pollen, mold spores and fungi. Household products, in particular cleaning agents and pesticides, enter the air as mists, gases or vapors and are considered indoor air pollutants as well. Toxic gases such as radon and carbon monoxide plus tobacco smoke are considered indoor air pollutants. Other examples include building materials such as asbestos, sometimes found in insulation.

Local exhaust

In your home, several types of local exhaust items work daily to keep the indoor air quality at healthy levels. Local exhaust is the fans used to move polluted air and moisture-filled air at or away from their source, states the EPA. Without the bathroom exhaust fan, the bathroom would be susceptible to mold and mildew growth. In the kitchen, the range hood provides a local exhaust to vent cooking odors as well as fumes. Other home areas with local exhaust may include utility rooms and garages.

Mechanically ventilated crawlspace system

A stagnant crawlspace can create several problems within a house. Without proper circulation and ventilation, moisture in the crawlspace can lead to mold and excess humidity (a cause of warped floors.) A mechanically ventilated crawlspace system increases ventilation and creates higher air pressure, in relation to the soil’s air pressure. It also can do the converse, generating lower air pressure in relation to the living space’s air pressure. Typically, this type of ventilation system uses a fan.

Negative pressure

Negative pressure occurs when a room or space receives less air than it expels or exhausts. When this happens, the air pressure surrounding the room is more than the air pressure within the room, generating negative pressure. Negative pressure can cause airflow from the surrounding spaces to enter the room through cracks or openings, contributing to poor indoor air quality.

Natural ventilation

Any air movement into and out of a home or other structure through holes and cracks in the foundation, walls, doors, windows or other locations is natural ventilation. Natural ventilation also includes air movement through open windows and doors. The air movement involved with natural ventilation comes from wind and outside temperature variances. The wind and temperature shifts cause the stack effect-where the warm air rises and expands pushes out of the home’s upper levels through cracks and holes.

National Institute for Occupational Safety and Health (NIOSH)

NIOSH is the only federal institute responsible for the research and recommendations regarding the prevention of work-related health hazards, including injuries and illnesses. NIOSH, part of the Centers for Disease Control and Prevention (CDC), provides both national and global leadership in the area of work-related health hazards, including the facilitation of research, protocols and on-site evaluations.

Organic compounds

Chemicals containing carbon are organic compounds. These include VOCs, also known as volatile organic compounds, which turn to into gases at room temperature. In general, organic compounds can be found throughout your home in common household cleaning products and various building materials that may contain phenols, polymers, benzene, alcohols and ketones. Sometimes exposure to certain organic compounds can affect your quality of health.

Outdoor air quality

The condition of the air outdoors and its effect upon the health and comfort of humans, animals and plant life is the outdoor air quality. Poor outdoor air quality contains pollutants and contaminants, which may cause health conditions such as respiratory issues, headaches and more. Children, adults ages 65 and older, people with compromised immune systems or respiratory diseases especially may be affected by air pollution or poor outdoor quality. You can check your local outdoor air quality index at http://www.airnow.gov/.


A particle is a very small piece of matter. In the air, particles or particulate matter can be a combination of solids and liquids, varying in size. Particles smaller than 10 microns in diameter are a health hazard-a strand of human hair is about 50 microns. Particles smaller than 10 microns can enter the body through the nose or throat, causing irritation. Fine particles are any smaller than 2.5 microns. The EPA provides an Air Quality List for Particle Pollution.

Positive pressure

Airflow is a key component to the health of all occupants of a home or other structure. A positive pressure system in a home also may be known as a supply-only system. This means the system pulls outdoor air into the home through a duct while forcing indoor air out of the home or building through structural cracks or “holes.” A positive pressure system also can be defined as a balanced system, utilizing both fans and ductwork for the exchange of indoor and outdoor air.

Radiant heat transfer

The expansive difference between the temperatures of two surfaces, not touching but exposed to the other, creates radiant heat transfer. The HVAC system in a home or other building uses energy to cool rooms and maintain comfort levels. That energy actually adds heat to the conditioned air-a type of radiant heat transfer.

Static pressure

The EPA presents the formula for static pressure as “the total pressure minus velocity pressure” in regards to airflow. According to the formula, static pressure is the pressure that pushes or presses as the same amount or equally in every direction. A HVAC technician can measure the static pressure in a system to help diagnose potential defects in ductwork that may contribute to performance issues in the system. When your HVAC system’s airflow is compromised, your IAQ is negatively affected.

Unit ventilator

When a home or other structure needs an additional ventilation system in order to meet ASHRAE standards, a unit ventilator may be installed. It is a unit consisting of heating coils, a fan, filter, dampers and controls set inside a metal cabinet. Traditionally, unit ventilators have been used in school classrooms, installed on an outside wall. Outdoor air enters the unit through a vent on the outside wall. The unit ventilator mixes the outdoor air with room, heating if needed and then dispersing it into the room through a different vent, usually atop the unit ventilator.

Volatile organic compounds (VOCs)

VOCs are gases released from specific solids or liquids. They are considered volatile because many can have serious health effects, some short-term effects and others resulting in chronic conditions. Your exposure to VOCs is higher indoors than outdoors-VOCs adversely affect indoor air quality. Examples include chemicals found in household items such as interior paint, cleaning products, flooring and furnishing finishes, pesticides and even craft materials such as some adhesives. Benzene, toluene, chlorine, hexane and styrene are VOCs.

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