Air Quality Monitors Explained: Sensors, Readings and What They Can Miss
Air quality monitors explained: how laser particle, NDIR CO2, VOC and radon sensors work, what each reading means, the eCO2 trap and what monitors miss.
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A home air quality monitor is a small box of separate sensors: usually a laser particle counter for PM2.5, an NDIR or other sensor for CO2, a metal-oxide sensor for VOCs, and a temperature and humidity chip, with radon in some models. Each sensor has its own strengths. Particle and NDIR CO2 readings are real measurements you can compare with EPA and ASHRAE reference points; VOC readings are relative trends; and an "eCO2" figure is an estimate calculated from VOC signals, not CO2 at all.
This guide explains how each sensor works, what its number means and what no consumer monitor can detect, so you can pick one that measures what you actually care about.
The sensors inside
| Reading | Sensor type | What it does | Example |
|---|---|---|---|
| PM2.5, PM10, PM1 | Laser (optical) particle counter | A fan draws air past a laser; the sensor counts scattered light and estimates particle mass | Airthings View Plus, IQAir AirVisual Pro, Qingping Air Monitor Lite |
| CO2 | NDIR (non-dispersive infrared) | Measures infrared light absorbed by CO2 molecules | Aranet4, Airthings Wave Enhance, IQAir AirVisual Pro |
| CO2 | Thermal conductivity | Infers CO2 from how the air conducts heat compared with fresh air | Atmotube PRO 2 (Sensirion STCC4) |
| TVOC, eCO2 | Metal oxide (MOX) | A heated surface changes resistance when gases react with it; software converts the signal | Airthings View Plus and Wave Enhance (VOC) |
| Radon | Alpha spectrometry | Counts alpha particles from radon decay in a small chamber | Airthings View Plus (per Airthings) |
| Temperature, humidity, pressure | Environmental chip | Direct physical measurement | Nearly every monitor |
Particles: real, but estimated
Laser sensors count particles and convert the count to µg/m³ using assumptions about particle density and shape. That works well for typical household and smoke particles, and the best consumer monitors track reference instruments closely in South Coast AQMD's AQ-SPEC tests (field R-squared of 0.85 to 0.93 for the Qingping Air Monitor Lite). High humidity can make particles swell and read high. Compare the PM2.5 figure with EPA's annual standard of 9.0 µg/m³ and the AQI bands; see PM2.5 explained.
CO2: check the sensor type
NDIR sensors measure CO2 directly by how much infrared light it absorbs. Thermal-conductivity sensors are smaller and lower-power but looser: Atmotube states ±(100 ppm + 10%) for the STCC4 in its PRO 2, against ±50 ppm ±5% that Airthings states for the NDIR sensor in the Wave Enhance. ASHRAE describes indoor CO2 as a ventilation indicator rather than an overall measure of air quality. See why CO2 matters at home.
The eCO2 trap
Some cheap monitors and smart home sensors show "eCO2" or "CO2eq". Sensirion's SGP30 datasheet explains that its TVOC and CO2eq signals are calculated from ethanol and hydrogen measurements using internal algorithms. That number rises when you cook or clean and does not track ventilation reliably. If the spec sheet does not say NDIR (or name a real CO2 sensor), assume it is an estimate.
VOCs: a trend, not a concentration
Metal-oxide sensors respond to a broad mix of gases. They cannot identify a particular chemical, and the TVOC number from one brand is not comparable with another's. EPA notes that no federally enforceable standards exist for VOCs in non-industrial settings, so there is no official threshold to compare against. Use the reading to spot events (painting, cleaning, new furniture) and to confirm that ventilation clears them. See VOC monitoring in a new home.
Radon: slow by design
Radon sensors count rare decay events, so they need days to give a stable figure. Airthings, for example, states ±10% after 7 days for the View Plus. EPA lists continuous monitors among short-term test devices and says to follow up any result of 4 pCi/L or more with a second test. See the radon detector guide.
What monitors cannot detect
- Carbon monoxide, unless the device is a listed CO alarm. CPSC advises dedicated CO alarms on every level and outside sleeping areas.
- Mold. Humidity readings show the conditions mold likes, but EPA says sampling is usually unnecessary when growth is visible, and no monitor detects spores reliably.
- Specific chemicals such as formaldehyde, benzene or asbestos fibers, unless the device has a dedicated, documented sensor.
- Allergens. Pollen and dander show up only as part of the PM10 or PM2.5 mass, not as allergens.
Choosing the right monitor
| Your question | Readings you need | Where to look |
|---|---|---|
| Is my bedroom stuffy? | NDIR CO2 | Best CO2 monitors |
| Is smoke or cooking getting into my air? | PM2.5 | Best air quality monitors |
| Does my basement have radon? | Radon | Best radon detectors |
| Is my purifier working? | PM2.5 before and after | Qingping Lite review |
| What is the air like on my commute? | Portable PM and CO2 | Atmotube PRO 2 review |
Apps, history and smart home links
The sensor is half the product; the app is the other half. Look for history graphs of at least a week, data export, and alerts on individual readings. Bluetooth-only monitors need your phone nearby to sync; Wi-Fi models upload continuously. Smart home links vary: Awair lists IFTTT, Google Assistant and Amazon Alexa for the Element, and the Qingping Air Monitor Lite works with Apple HomeKit.
Accuracy and calibration
Makers' accuracy claims cover ideal conditions. Independent tests from AQ-SPEC are the best check for particles; see our guide to monitor accuracy. Many NDIR sensors self-calibrate by assuming they see outdoor-level air regularly, so a monitor kept in a room that never airs out can drift. VOC sensors need a burn-in period; Awair says 24 to 48 hours for the Element, and Airthings says up to 7 days for the View Plus's VOC and CO2 readings.
Frequently asked questions
How do air quality monitors work?
They combine small sensors: a laser particle counter for PM2.5, an NDIR or thermal-conductivity sensor for CO2, a metal-oxide sensor for VOCs, and chips for temperature and humidity. Some add a radon chamber.
What is the difference between CO2 and eCO2 on a monitor?
CO2 from an NDIR sensor is a direct measurement. eCO2 is calculated from VOC and hydrogen signals, as Sensirion's SGP30 datasheet describes, and does not reliably track ventilation.
Can an air quality monitor detect mold?
No consumer monitor detects mold reliably. It can show humidity above 60%, the condition EPA warns against, which helps you prevent mold.
Do air quality monitors detect carbon monoxide?
Most do not, and none should replace a CO alarm. CPSC advises CO alarms on every level and outside sleeping areas.
What does TVOC mean on a monitor?
Total volatile organic compounds: a combined signal from a metal-oxide sensor. It shows relative change, not a concentration of any specific chemical, and readings differ between brands.
Which reading on an air quality monitor matters most?
It depends on your home. PM2.5 for smoke and cooking, CO2 for stuffy rooms, humidity for mold risk, and radon on the lowest lived-in level.
Sources
- AQ-SPEC Sensor Evaluations: Summary of PM Sensors, South Coast Air Quality Management District
- Datasheet SGP30: Indoor Air Quality Sensor for TVOC and CO2eq Measurements, Sensirion
- Atmotube PRO 2 Technical Specifications, Atmotech
- ASHRAE Position Document on Indoor Carbon Dioxide, ASHRAE
- Volatile Organic Compounds' Impact on Indoor Air Quality, US Environmental Protection Agency
- A Citizen's Guide to Radon, US Environmental Protection Agency
- Carbon Monoxide Information Center, US Consumer Product Safety Commission
- Mold Testing or Sampling, US Environmental Protection Agency
- Awair Element: Air Quality Monitor For Your House, Awair
- View Plus product sheet, Airthings
Related guides
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Airthings View Plus Review: Radon, PM2.5 and NDIR CO2 in One Monitor
The View Plus is the most complete home air monitor Airthings makes: radon plus particles, CO2 and VOCs, on batteries or USB-C. It is expensive, and worth it mainly if you want radon too.
Airthings Wave Enhance Review: The Small Wave, Now With NDIR CO2
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