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An indoor air quality monitor measures airborne particle concentration, carbon dioxide concentration, air temperature and relative humidity, and on most multi channel units one broad gas signal printed as TVOC or HCHO. Every other figure on the screen, including the AQI value and any health index, is arithmetic applied to those few measurements rather than a sensor of its own. That distinction matters because a monitor advertising ten or eleven readings almost never holds ten or eleven sensing elements, and readings that share an element also share its blind spots.
The five quantities behind one air quality number
Strip the packaging claims off a mainstream indoor monitor and a short list of sensing elements is left: an optical particle chamber, an infrared gas bench for carbon dioxide, a combined temperature and humidity die, and usually one reactive gas element. The LifeBasis 11-in-1 air quality detector is unusually open about this, naming an NDIR infrared carbon dioxide sensor, a laser particle sensor, a semiconductor sensor, a photoelectric sensor and a temperature and humidity sensor behind its eleven indexes. Five elements, eleven numbers. The particle sensor by itself accounts for PM1.0, PM2.5, PM10 and the raw particle count, because those are four ways of summarising one stream of scattering events.
| Reading | How it is sensed | What it genuinely tells you | What it cannot separate |
|---|---|---|---|
| PM1.0, PM2.5, PM10 | Laser scattering in a small pumped or fanned chamber | A mass concentration estimate per size band | Cooking smoke from dust from pollen from talc |
| Carbon dioxide | Infrared absorption across a fixed optical path | Real CO2 concentration, and ventilation by proxy | Very little, which is why it is the strongest channel |
| TVOC | Heated metal oxide film or an electrochemical cell | That the mix of reactive gases in the room changed | Which compound changed, and whether it matters |
| HCHO | Electrochemical or semiconductor cell | A formaldehyde estimate with known cross sensitivity | Formaldehyde from alcohol vapour and cleaning sprays |
| Temperature, humidity | Resistive or capacitive die, often on one chip | Conditions the other sensors depend on | Its own self heating from nearby electronics |
| AQI or health index | Calculated from the rows above | One colour for a dashboard | Which input drove the number |
What a particle sensor counts and what it cannot identify
An optical particle sensor shines a laser across a stream of air and watches a photodiode placed off axis. A particle crossing the beam throws a brief pulse of scattered light at the detector, and the height of that pulse scales with particle size. The firmware counts pulses, sorts them into size bins, then converts counts into the mass per cubic metre figures that get labelled PM2.5 and PM10. Two assumptions sit inside that conversion: an assumed particle density and an assumed shape. Both are reasonable for urban dust and combustion soot, and both are wrong for fibres, fog droplets and fine flour.
The practical consequence is that a particle channel is excellent at detecting change and poor at identifying cause. A spike while a pan is searing, a spike while a candle burns and a spike while someone shakes out a rug look the same on the trace. That is still useful, because it tells you a source exists and whether extraction clears it, which is exactly the judgement behind our notes on monitors used in kitchens. It is not useful for deciding whether the particles are hazardous, and no consumer sensor closes that gap.
Carbon dioxide is the one channel with a hard physical meaning
Carbon dioxide absorbs infrared light strongly near a wavelength of 4.26 micrometres. A non dispersive infrared bench exploits that directly: an emitter at one end, an optical filter and detector at the other, and a measurement of how much of that band survives the journey. More carbon dioxide in the path means less light arriving, and the relationship is governed by absorption physics rather than by a correlation someone fitted. That is why a carbon dioxide reading in parts per million can be compared between instruments and against guidance thresholds, while a TVOC index cannot.
It is also why carbon dioxide is the best single indicator of whether a room is ventilated for the number of people in it. Occupants exhale it continuously and nothing in a normal room removes it except air exchange, so the concentration climbs until ventilation balances the load. Outdoor air sits a little above four hundred parts per million, and the gap between that and your indoor reading is a direct measure of how diluted the air you are breathing is. Monitors built around this logic, including the Extech IAQ320 datalogger with its programmable sampling rate and high and low limits, are the ones worth putting in an occupied room rather than a cupboard. The same reasoning drives the picks in indoor air quality monitors.
The VOC and formaldehyde readings respond to a family of gases
A metal oxide gas element is a heated ceramic film whose electrical resistance falls when reducing gases adsorb onto its surface. It does not know what landed on it. Ethanol, acetone, limonene from citrus cleaner, hydrogen and dozens of other compounds all push the resistance the same direction, which is why the output is honestly labelled total volatile organic compounds rather than any single substance. Electrochemical cells, which the Lwyinp 10-in-1 air quality tester states it uses for HCHO and TVOC, are more selective because they rely on a specific electrode reaction, but they still show cross sensitivity, typically to alcohols in the formaldehyde case. The optical principle behind that unit CO2 channel is not stated in the listing bullets.
Read these channels as relative and local. A step change after painting, after unpacking new furniture or after using solvent cleaner is real information about that event. An absolute figure in micrograms per cubic metre from a heated film is an estimate derived from a calibration gas that is nothing like the mixture in your room, and it drifts as the film ages. Spraying alcohol based cleaner near the inlet to see the formaldehyde number jump is the clearest demonstration of why the label is a family name.
Things these monitors are bought for that no sensor reports
The honest list is longer than most buyers expect, and saying so is more useful than implying coverage.
- Mould. No consumer instrument detects spores or identifies them. Sustained high relative humidity is the proxy that actually predicts growth.
- Radon. This needs an alpha track or ionisation chamber device measuring over days. It shares nothing with a particle or gas sensor.
- Carbon monoxide. Requires a dedicated electrochemical cell in a certified alarm. A TVOC film may twitch, which is not protection.
- Allergens by type. Pollen, dust mite debris and pet dander are counted as particles, never named.
- Bacteria and viruses. Not measurable by any of these methods. Carbon dioxide is used as a ventilation proxy for airborne transmission risk, which is a different claim.
- Asbestos and fibres. Fibre shape defeats the size and density assumptions in optical counting entirely.
Where a monitor earns its place is in making invisible, slow changes visible, and in letting you check whether an intervention worked. The ability to pull readings off the device matters more than the number of channels for that purpose. The Temtop M2000C bundle states CSV export over USB for exactly this kind of long term tracking, along with a thermometer accurate to plus or minus 0.5 degrees Fahrenheit and humidity to plus or minus 3 percent RH.
Reading your own monitor without over-reading it
Work from the strongest channel outward. Trust the carbon dioxide figure as a number, because the physics behind it is specific and comparable, and use it to judge ventilation. Trust the particle channel as a change detector, and watch whether a spike clears in minutes or lingers for an hour, since the clearance time tells you more about the room than the peak value does. Treat TVOC and HCHO as event markers tied to something you did. Treat AQI and health index values as summaries you should look behind before acting.
Then decide what the room actually needs. A high carbon dioxide reading is answered by air exchange, not by a filter. A particle spike is answered by extraction at source and then by filtration. A humidity figure stuck high is a damp problem, not an air cleaning problem. Our broader comparisons in air quality monitors and the quieter requirements of nursery monitoring both start from that split, because the instrument only earns its keep when the reading points at a specific action.
Frequently Asked Questions
Can an air quality monitor detect mould?
No. No consumer monitor has a sensor for mould spores. A laser particle counter will register spores along with every other particle in the same size band, and it cannot say what they are. Persistent high relative humidity is the useful proxy, because mould needs moisture, so a humidity reading above the mid sixties in a cool room is the signal worth acting on.
Does a monitor with a VOC channel detect carbon monoxide?
Not usefully. A metal oxide VOC element responds to reducing gases and carbon monoxide is one of them, so a large release may nudge the reading, but there is no calibration behind it and no alarm threshold that means anything. Carbon monoxide needs a dedicated electrochemical cell in a certified alarm, kept separate from your air quality monitor.
Why do PM2.5 and PM10 move together on my display?
Because they come from the same sensor. One laser chamber counts scattering events and sorts them into size bins, then the firmware totals the bins into separate mass estimates. PM10 includes everything PM2.5 includes, so the two tracks are mathematically linked and will rise and fall together unless the particle size distribution in the room changes.
Is the AQI number on the screen the same as the public air quality index?
Rarely. Published indexes are defined for outdoor measurements over fixed averaging windows, usually one hour or twenty four hours, and they include pollutants a domestic monitor has no sensor for. A monitor showing an instantaneous indoor AQI is applying its own formula to its own two or three channels, so treat it as a traffic light rather than a comparable figure.
What does a health index reading actually combine?
Whatever the maker decided, which is why it is the least transferable number on the display. It is computed from the measured channels, so it carries all their limitations and hides which one moved. If the index goes amber it is worth reading the underlying particle, carbon dioxide and gas figures before you change anything in the room.