Reference

Indoor air glossary

Forty terms you will meet on spec sheets, in standards and in our guides — each explained in plain language, with links to the guide that covers it in depth.

Last updated 22 September 2026

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Air changes per hour (ACH): How many times per hour a volume of air equal to the room's volume is replaced or cleaned. For purifiers we speak of clean ACH: CADR divided by room volume. See room sizing.
Activated carbon: Carbon processed to create a vast network of pores, giving roughly 500–1,500 m² of internal surface per gram. Used to adsorb odors and many volatile organic compounds. Full guide.
Adsorption: The accumulation of gas molecules on a solid surface, as opposed to absorption into a material's bulk. The mechanism behind carbon filtration. Capacity is finite: once pores fill, the filter stops working.
AHAM AC-1: The American test standard that defines how CADR is measured for portable room air cleaners, using tobacco smoke, dust and pollen in a chamber of about 28.5 m³.
Allergen: A substance that triggers an allergic response — commonly pollen, dust mite protein, pet dander or mould spores. Filtration only affects the airborne fraction. Full guide.
ASHRAE Standard 241: A 2023 American standard on the control of infectious aerosols that introduced targets for equivalent clean airflow per occupant, combining ventilation and filtration.
Bypass leakage: Air that passes around a filter instead of through it, usually through gaps between the filter frame and the housing. Bypass is unfiltered, so a few percent of leakage can outweigh the difference between filter classes.
CADR (clean air delivery rate): The volume of completely particle-free air a purifier effectively produces per hour — approximately airflow multiplied by single-pass efficiency. Reported in m³/h or CFM. Full guide.
CARB certification: Certification required by the California Air Resources Board for indoor air cleaners sold in California, covering electrical safety and, for electronic devices, an ozone emission limit of 0.050 ppm. It is not a measure of cleaning performance.
CFM (cubic feet per minute): Imperial unit of airflow or CADR. 1 CFM = 1.699 m³/h; 1 m³/h = 0.589 CFM.
Carbon dioxide (CO₂): A gas exhaled by occupants, used indoors as an indicator of ventilation adequacy. Outdoor air is around 420 ppm. Filters do not remove it. Full guide.
Coanda effect: The tendency of a jet of air to stay attached to a nearby surface. Ceiling units exploit it so discharged air travels along the ceiling and spreads across the room. Where it matters.
CRI (colour rendering index): A 0–100 measure of how naturally a light source renders colours compared with a reference. Values above 90 are considered high quality for living spaces.
dB(A): A-weighted decibels — a sound level adjusted to approximate human hearing sensitivity. Logarithmic: +3 dB is double the sound energy, +10 dB is perceived as roughly twice as loud. Full guide.
Decay rate: How fast a pollutant concentration falls, expressed per hour. Total decay combines filtration, ventilation and settling. Time to remove 90% ≈ 2.3 ÷ decay rate (hours).
EC motor: An electronically commutated (brushless DC) motor. Compared with simple AC motors it is typically more efficient and allows smooth, continuous speed control — useful for running a purifier quietly.
EN 1822: The European standard that classifies EPA, HEPA and ULPA filters (E10–U17) by efficiency at the most penetrating particle size, and defines how they are tested. Class table.
EPA filter class (E10–E12): "Efficient particulate air" classes below HEPA under EN 1822, from 85% to 99.5% efficiency at MPPS. Not to be confused with the U.S. Environmental Protection Agency.
Formaldehyde: A small, volatile organic compound emitted by some wood products, adhesives and textiles. Poorly adsorbed by plain activated carbon; impregnated or specialist media is required. WHO indoor guideline: 0.1 mg/m³ over 30 minutes.
HEPA: High-efficiency particulate air filtration. In Europe defined by EN 1822 classes H13 and H14; in the U.S. commonly by a 99.97% efficiency at 0.3 µm definition.
"HEPA-type", "HEPA-like": Marketing terms with no standard behind them, usually describing media below true HEPA classification. Ask which standard and class was tested.
ISO 16890: The international standard for general ventilation filters, classifying them by their efficiency against ePM1, ePM2.5 and ePM10 fractions. Used for HVAC filters rather than HEPA.
ISO 29463: The international counterpart to EN 1822 for high-efficiency filters, using classes such as ISO 35 H (equivalent to H13) and ISO 45 H (equivalent to H14).
MERV: Minimum Efficiency Reporting Value, from the ASHRAE 52.2 standard, rating HVAC filters from MERV 1 to 16. The U.S. EPA suggests at least MERV 13 for home HVAC systems where the equipment allows it.
Mixing effectiveness: How evenly cleaned air distributes through a room. Laboratory tests use a mixing fan; real rooms mix imperfectly, so measured room performance is usually below the rated CADR. Full guide.
MPPS (most penetrating particle size): The particle size a given filter captures least efficiently — typically around 0.1–0.3 µm for HEPA media. Both larger and smaller particles are captured more efficiently, which is why EN 1822 rates filters at MPPS.
NDIR sensor: Non-dispersive infrared sensor, the reliable technology for measuring CO₂. "eCO₂" values estimated from a VOC sensor are not equivalent.
Ozone (O₃): A reactive gas and respiratory irritant that some electronic air cleaners emit. It also reacts with indoor chemicals to form secondary pollutants. What to check.
PCO (photocatalytic oxidation): A technology using UV light on a catalyst to oxidise organic molecules. Oxidation can be incomplete at short contact times, producing by-products such as formaldehyde.
PM2.5 / PM10: Particulate matter with diameters up to 2.5 µm and 10 µm respectively, reported as mass per cubic metre. WHO 2021 guideline levels for PM2.5: 5 µg/m³ annual, 15 µg/m³ over 24 hours.
Pre-filter: A coarse first stage that captures hair, lint and large dust, protecting the HEPA and carbon stages. Often washable, and the most frequent maintenance item. Schedule.
Pressure drop: The resistance a filter presents to airflow, rising as it loads with dust. Higher pressure drop means the fan must work harder: less airflow, more noise or more power.
Resuspension: Settled particles being lifted back into the air by walking, cleaning, bed-making or air currents — the reason dust mite allergen and coarse dust reappear in measurements.
Short-circuiting: When cleaned air from the outlet is drawn straight back into the intake without mixing with room air. It makes a purifier look busy while treating almost the same air repeatedly.
Sound power level (L_WA): The total acoustic energy emitted by a device, independent of distance. Always a higher figure than sound pressure for the same device — check which one a datasheet quotes.
Sound pressure level (L_pA): What a microphone or ear measures at a stated distance, usually 1 m. This is the number relevant to how loud a purifier feels in a room.
TVOC: Total volatile organic compounds — a single summed index from a sensor that responds to many compounds at once. Useful for spotting events, not for assessing the risk of a specific chemical.
ULPA: Ultra-low penetration air filters, classes U15–U17 under EN 1822, with efficiencies from 99.9995% upward at MPPS. Used in cleanrooms rather than homes.
UV-C / UVGI: Ultraviolet germicidal irradiation, typically at 254 nm, used to inactivate microorganisms. Effectiveness depends on intensity and exposure time; lamps emitting at 185 nm also generate ozone.
VOC (volatile organic compound): Organic chemicals that evaporate at room temperature, released by paints, adhesives, furniture, cleaning products and cooking. Controlled by source control, ventilation and sorbent filtration.