Indoor Air Quality Glossary of Terms

This Indoor Air Quality Glossary of Terms and Abbreviations is designed to be a practical reference you can actually use, whether you are purchasing a new furnace, reading an inspection summary, comparing filtration options, shopping for a room air purifier, or simply trying to connect the dots between headaches and the symptoms of poor indoor air quality.

It is organized into two sections: 1) IAQ Abbreviations and 2) IAQ Terms & Definitions

• Activated Carbon Filters – Air filters made with highly porous activated carbon that traps chemical fumes, cooking smells, smoke, and volatile organic compounds (VOCs) that standard furnace filters miss. Performance varies widely based on the amount of carbon, type of carbon, the contact time allowed by the filter design (speed and depth), and which gases are being targeted, since not all VOCs and gases behave the same way.
• Air Changes per Hour (ACH) – How many times the air in a space is replaced in an hour, either with outside air or supply air from the furnace or air handler. A higher ACH can reduce pollutant concentration, provided the replacement air is clean and well filtered. ACH is influenced by ventilation rates, air sealing and leakage, and mechanical systems. Ideal target: 4 to 6 air changes per hour for residential living spaces.
• Allergen – Any airborne particle, such as dust mite debris, ragweed pollen, mould, or animal hair, that triggers allergic reactions, sneezing, or asthma symptoms in occupants.
• Animal Dander – Tiny, microscopic flecks of shed skin from household pets such as cats, dogs, and birds. Dander floats easily in the air and settles into carpets and upholstery, making it a primary indoor allergen trigger.
• Bioaerosols – Biological organisms floating in indoor air, including mould spores, pollen, bacteria, viruses, and dust mite fragments.
• Bypass Filter – A secondary air cleaner installed on a side circuit of the central duct system. It allows a portion of home air to pass through an ultra-dense filter, such as a HEPA filter, without overloading the main furnace blower.
• Carbon Dioxide (CO2) – A gas released when people breathe indoors or when gas appliances run. CO2 is not usually the primary toxic threat indoors at typical levels; it is mainly used as a ventilation proxy, showing how "rebreathed" the air has become. CO2 concentration is not a measure of indoor air quality as it cannot be filtered from the air.
• Carbon Monoxide (CO) – A toxic, odourless gas caused by fuel-burning appliances such as furnaces, water heaters, and fireplaces. Target range: 0 ppm indoors. Functioning carbon monoxide alarms are required on every level of a Canadian home.
• Clean Air Delivery Rate (CADR) – A performance metric for air purifiers indicating how much cleaned air, for specific particle types, a unit delivers. Think of it as a measure of how much useful particle-cleaning a purifier can provide in a real room. A practical rule of thumb is to choose a CADR appropriate for your room size and run the purifier at a fan speed you can comfortably live with day to day.
• Cubic Feet per Minute (CFM) – The measure of how many cubic feet of air your furnace or air purifier moves every minute. A higher CFM means faster room air cleaning.
• Electrostatic Precipitators – Electronic air cleaners installed in ductwork that give passing dust particles an electric charge, then pull them onto oppositely charged collector plates downstream, much like a magnet. The collector plates are cleaned periodically to maintain effectiveness.
• Energy Recovery Ventilator / Heat Recovery Ventilator (ERV / HRV) – An ERV exchanges indoor and outdoor air while transferring some heat and moisture. An HRV exchanges air and transfers heat, typically with less moisture transfer than an ERV.
• Formaldehyde – A common household VOC gas given off by pressed-wood furniture, laminate flooring, glues, and cabinet materials. It can cause eye and respiratory irritation. Target range: Health Canada recommends keeping long-term exposure below 40 ppb.
• Heating, Ventilation, and Air Conditioning (HVAC) – HVAC affects IAQ through filtration, airflow patterns, humidity control, and the introduction of outdoor air, where the system is designed to do so.
• High-Efficiency Particulate Air Filters (HEPA) – Ultra-dense pleated air filters designed to trap 99.97% of airborne particles as small as 0.3 microns. Note: standard furnace blowers usually cannot handle full-sized HEPA filters without a bypass system, due to the high airflow resistance involved.
• Humidity – The amount of water vapour in indoor air. Target range: maintain between 30% and 50%. In cold Canadian winters, keep humidity around 30% to 35% to help prevent heavy window condensation, icing, and mould growth on drywall.
• Indoor Air Quality (IAQ) – Indoor air quality describes the condition of the air inside buildings as it relates to health, comfort, and cleanliness. IAQ is affected by pollutant sources (cooking, candles, cleaning products, combustion, and building materials), ventilation (how much outdoor air replaces indoor air), filtration and air cleaning (HVAC filters and purifiers), and moisture control (leaks, humidity, and condensation).
• Indoor Environmental Quality (IEQ) – A broader concept than IAQ that can include lighting, acoustics, thermal comfort, and ventilation, not just pollutants.
• Ionizers – Air purifiers that emit positive and/or negatively charged ions into room air. The ions attach to dust and allergens, causing them to clump together get caught in the return air filters. However, they can also settle onto ductwork, walls, curtains, floors, or furniture. Some products are significantly undersized for their intended application, so evaluate carefully and look for independent testing. Ions also can initiate chemical reactions transforming common indoor pollutants or gases into secondary chemical byproducts.
• Microgram per Cubic Metre (µg/m³) – The standard unit for measuring airborne particle weight in a given volume of indoor air. You will see this unit when reading the results from a home air quality monitor.
• Minimum Efficiency Reporting Value (MERV) – A scale ranging from MERV 1 to 16 that measures how well a home air filter catches particles. Homeowner tip: standard fibreglass filters are MERV 1 to 4 and mainly protect the furnace rather than your lungs. MERV 8 to 11 filters can offer a good balance of allergen removal and healthy furnace airflow. However, a MERV 13 is recommended to capture fine smoke particles, small allergens and bioaerosols. But confirm your system can handle the added air resistance. Higher MERV generally means better particle capture, but it can also mean higher airflow resistance (pressure drop), which can strain some systems if they were not designed for it. If you are upgrading to a higher MERV filter, confirm your HVAC system can handle it without reducing airflow too much. Note: polarized media air cleaners break the usual link between airflow resistance and filter efficiency. By design, they provide a low pressure drop while also capturing a broad range of particle sizes, including fine particles.
• NO2 (Nitrogen Dioxide) – A gas associated with combustion including gas appliances and traffic infiltration. It can irritate the lungs and worsen asthma symptoms.
• Off-gassing – The release of gases, often VOCs, from materials such as new carpet, paint, pressed wood, furniture foam, and some adhesives.
• Ozone (O3) – A highly reactive lung-irritating gas composed of three oxygen atoms. While naturally present in the upper atmosphere, ground-level indoor ozone can cause coughing and asthma flare-ups. Typical outdoor ozone levels are 30 to 50 ppb. Avoid air purifiers that raise indoor ozone levels in living spaces by more than 1 to 2 ppm.
• Particulate Matter (PM2.5 / PM10) – Microscopic solids or liquid droplets floating in the air. PM2.5 and PM10 refer to particulate matter with particle sizes less than or equal 2.5 and 10 micrometers. PM10 includes dust and pollen, while PM2.5 includes smoke, cooking emissions, and wildfire haze that can penetrate deep into human lungs. Target range: keep PM2.5 under 12 µg/m³ as an annual average (Health Canada recommendation).
• Parts per Billion (ppb) – A unit used to measure very trace concentrations of gases or chemical pollutants in household air. One part per billion means one single part of a substance for every one billion total parts of a mixture.
• Parts per Million (ppm) – A standard unit for measuring indoor gas concentrations, such as carbon dioxide (CO2). One part per million means one single part of a substance for every one million total parts of a mixture.
• Passive Electrostatic Filter – A replaceable air filter made from synthetic fibres that are manufactured with an initial static charge. As air flows through them, they attract dust without the use of external electricity. With these filters, efficiency starts out high because of the charge, falls as particles build up and cancel out the static charge, then rises again as the filter openings become blocked with particles.
• Passive Filter – A standard mechanical air filter, such as pleated cotton or fibreglass, that catches dust using a physical barrier rather than any electrical charge or chemical coating. Over time, passive filters accumulate larger particles on their surface, which increases efficiency (more particles get caught) but also increases the fan energy required to push air through the filter.
• Photocatalytic Oxidation (PCO) – An air-cleaning technology that uses UV light reacting with a catalyst (such as titanium dioxide) to chemically alter VOCs and other contaminants in the air. The specific reaction depends on the chemicals present, and because it is a surface-based process, contaminants must come into contact with the catalyst to be affected. Some products are significantly undersized for their intended application, so evaluate carefully and look for independent testing.
• Polarized Media Air Cleaner (PMAC) – An electronic filter installed in a furnace filter rack that uses a mild electric field to polarize the filter fibres, capturing tiny particles like a high-end filter without restricting furnace airflow. These filters break the usual trade-off between filter efficiency and power consumption: a polarized media air cleaner has a low pressure drop (less fan power consumed) alongside high efficiency (fine particle capture). They also capture particles through the full depth of their media instead of only on the surface, which can extend filter life up to eight to twelve times that of passive media.
• Radon (Rn) – A radioactive gas that can enter buildings from the soil typically accumulating in basements. It is invisible and odourless, so testing is the only way to know whether it is present. Residential radon levels should not exceed 200 Bq/m³ (Health Canada). Radon is a noble gas and not dangerous in itself. However, one of the naturally occurring radioactive isotopes, 222Rn, will decay into other elements known as decay products or radon daughters. It is these decay products which have been shown to cause lung cancer (US EPA).
• Radon Mitigation – Techniques used to reduce radon levels, most commonly through sub-slab depressurization (a vent pipe and fan system). The key ideas: test first (short-term and/or long-term), mitigate by venting soil gases safely outdoors if levels are elevated, and retest after mitigation to confirm the results.
• Relative Humidity (RH) – The percentage of moisture in the air relative to what it can hold at the current room temperature. Many IAQ issues, including mould, dust mites, and comfort, are strongly tied to RH. Typical comfort and IAQ goals often land around 30% to 50% RH; avoid sustained levels above 60% RH, since this can support mould growth in vulnerable areas.
• Room Air Purifiers – Portable, floor-standing units designed to filter the air in a single room, such as a bedroom, using internal fans, UV lights, HEPA filters, and carbon packs.
• Sick Building Syndrome (SBS) – A situation in which occupants experience chronic fatigue, headaches, coughing, or dry eyes that ease shortly after leaving the building, usually caused by a severe lack of clean air combined with a build-up of indoor chemicals.
• Smoke – A complex mix of particulate matter (especially PM2.5), VOCs, and other by-products. Indoor smoke can come from cooking, fireplaces, candles, cigarettes, and wildfire infiltration.
• SO2 (Sulphur Dioxide) – A common outdoor contaminant found near certain industrial sources that can also infiltrate indoors. It is less common in typical homes than carbon monoxide, nitrogen dioxide, or VOCs.
• Source Control – The most effective way to improve indoor air quality: removing or blocking pollutant sources directly, such as by using low-VOC paints, removing mouldy carpets, or smoking strictly outdoors, rather than relying on air filters alone.
• Total Volatile Organic Compounds (TVOC) – The combined measure of all organic gases floating in your home's air with sources including cleaning products, personal care products, aerosol sprays, paints, and new furniture off-gassing. A TVOC reading can be useful for spotting trends, but it does not identify which specific chemicals are present or how toxic each one is on its own. TVOC measurements do not measure inorganic gases such as CO2, SO2, NO2.
• Ultra-Low Particulate Air Filters (ULPA) – Extreme-grade industrial air filters that catch 99.999% of particles down to 0.12 microns. These are unsuitable for residential furnaces due to their high airflow resistance.
• Ultraviolet (UV) – Light waves used inside HVAC ductwork to help neutralize surface bacteria and mould spores.
• Ultraviolet-C Light (UVC) – Germicidal ultraviolet lights mounted inside furnaces and ductwork to control mould, bacteria, and viruses. They work by using a specific wavelength of light to disrupt the DNA, preventing it from replicating. There are two type used in residential furnaces: 1) low-powered UVC lamps which are typically installed above the air conditioning coil to help control the growth of biologicals on system components; and 2) high-powered UVC lamps installed in the ductwork to sanitize the air as it is blowing by.
• Ultraviolet-V Light (UVV) – A specific UV wavelength tube designed to help break down cooking odours, pet smells, and chemical fumes inside central duct systems.
• Ventilation – Bringing clean outdoor air into the home while pushing out stale indoor air. Canadian homes often rely on HRVs (Heat Recovery Ventilators) or ERVs (Energy Recovery Ventilators) to exchange fresh air without losing home heating energy.
• Volatile Organic Compounds (VOCs) – A broad category of carbon-based chemicals that evaporate easily at room temperature. Common indoor sources include paints, stains, and varnishes; cleaners and fragrances such as air fresheners and sprays; new furniture and flooring; gasoline or solvents stored indoors; and some hobby materials, such as resins and glues. To keep volatile organic compounds as low as possible, choose low-emission building materials and minimize scented chemical products (Health Canada)