Air Quality Glossary

Every acronym and technical term you'll meet when shopping for a CO2 monitor or reading about indoor air quality, defined in plain English. New to the topic? Start with the beginner's guide to IAQ.

ABC (Automatic Baseline Calibration): A self-calibration method where the sensor assumes the lowest CO2 reading it sees each week equals fresh outdoor air (~420 ppm) and re-anchors itself to it. Works well in spaces that are regularly aired out; drifts high in continuously occupied rooms. Calibration guide →
ACH (Air Changes per Hour): How many times per hour a room's entire air volume is replaced with fresh air. Homes typically achieve 0.35-1 ACH; well-ventilated classrooms and offices target 4-6 ACH or more. Ventilation calculator →
AQI (Air Quality Index): A standardized 0-500 scale used by the US EPA to communicate outdoor air pollution levels, primarily driven by PM2.5 and ozone. 0-50 is good; above 150 is unhealthy for everyone. Note that AQI does not include CO2. AQI calculator →
ASHRAE: The American Society of Heating, Refrigerating and Air-Conditioning Engineers, whose Standard 62.1 defines ventilation requirements for acceptable indoor air quality — the basis for the common guidance of keeping indoor CO2 within ~700 ppm of outdoor levels.
CADR (Clean Air Delivery Rate): The volume of filtered air an air purifier delivers, measured in cubic feet per minute (CFM). The key sizing spec: a purifier's CADR should be at least two-thirds of the room's floor area in square feet. Purifier sizing calculator →
CFM (Cubic Feet per Minute): The standard US measure of airflow through fans, HVAC systems, and purifiers. Ventilation guidelines commonly recommend 15-20 CFM of fresh air per occupant.
CO (Carbon Monoxide): A toxic gas produced by incomplete combustion — completely different from CO2. CO binds to blood haemoglobin and is lethal at low concentrations. Detected only by a dedicated CO alarm, never by a CO2 monitor. CO2 vs CO explained →
CO2 (Carbon Dioxide): The gas exhaled by human respiration, used as the primary indicator of indoor ventilation quality. Outdoor baseline is ~420 ppm; indoor levels below 800 ppm indicate good ventilation. CO2 levels chart →
eCO2 (Estimated CO2): A CO2 value estimated from a VOC sensor's readings rather than measured directly. Notoriously unreliable — cooking or cleaning products can produce wildly wrong values. If a spec sheet says "eCO2" or "CO2 equivalent", the device does not contain a real CO2 sensor. Real vs estimated CO2 sensors →
ERV (Energy Recovery Ventilator): A mechanical ventilation system that exchanges stale indoor air for fresh outdoor air while transferring both heat and moisture between the streams, keeping energy costs low. The gold-standard fix for chronically high CO2. ERVs vs opening windows →
HRV (Heat Recovery Ventilator): Like an ERV but transfers heat only, not moisture. Preferred in cold, dry climates; ERVs suit humid climates.
HVAC: Heating, Ventilation, and Air Conditioning — a building's combined climate system. Many HVAC systems recirculate indoor air by default and introduce little fresh air unless configured to do so.
IAQ (Indoor Air Quality): The overall healthfulness of air inside buildings, encompassing CO2, particulate matter, VOCs, humidity, temperature, radon, and biological contaminants. IAQ beginner's guide →
IDLH (Immediately Dangerous to Life or Health): The NIOSH threshold above which a gas concentration poses an immediate threat. For CO2 it is 40,000 ppm (4%) — roughly 100 times normal indoor levels, encountered only in industrial accidents or sealed spaces.
NDIR (Non-Dispersive Infrared): The proven sensing technology behind accurate CO2 monitors: infrared light is shone through an air sample, and the amount absorbed at CO2's specific wavelength (4.26 µm) reveals the concentration. Look for this term on any monitor you buy. NDIR vs photoacoustic →
OSHA PEL (Permissible Exposure Limit): The US occupational limit for workplace exposure. For CO2 it is 5,000 ppm averaged over an 8-hour shift — an industrial safety ceiling, not a comfort target. Indoor air quality degrades far below this level.
Photoacoustic Sensor: A newer CO2 sensing technology that detects the pressure waves created when CO2 molecules absorb pulsed infrared light. Comparable accuracy to NDIR in a smaller, lower-power package; used in sensors like the Sensirion SCD40/41. How photoacoustic works →
PM2.5 / PM10 (Particulate Matter): Airborne particles smaller than 2.5 or 10 micrometres — smoke, dust, combustion byproducts. PM2.5 penetrates deep into the lungs and is the pollutant behind most AQI warnings. Separate from CO2 and requires its own sensor. PM2.5 vs CO2 →
PPM (Parts Per Million): The standard unit for gas concentrations: 1,000 ppm means 1,000 out of every million air molecules are the measured gas (0.1%). Outdoor CO2 is ~420 ppm; 1% concentration equals 10,000 ppm.
Radon: A naturally occurring radioactive gas that seeps from soil into basements and ground floors; the second-leading cause of lung cancer. Measured in pCi/L or Bq/m³ by specialised sensors found in monitors like the Airthings Wave Plus.
RH (Relative Humidity): The amount of moisture in air as a percentage of the maximum it could hold at that temperature. The comfort and health sweet spot is 40-60%; outside that range, viruses survive longer and mould risk rises. Humidity & IAQ guide →
VOC (Volatile Organic Compounds): A broad family of carbon-based gases emitted by paints, furniture, cleaning products, and cooking. Measured by monitors as a combined "TVOC" index. Distinct from CO2 — a VOC sensor cannot measure CO2 accurately. VOC monitoring explained →