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Indoor air quality sensors, and what they measure
What the readings actually mean, how much to trust them, and the one thing these devices are emphatically not.
A monitor reports numbers. Knowing which of those numbers mean something, and which are close to noise, is what makes one worth owning.
On this page
Before anything else
An air monitor is not an alarm
These devices are not life-safety equipment. An air quality monitor that reports carbon monoxide is not a substitute for a carbon monoxide alarm built and tested to the standard that applies where you live. Nor is one a smoke alarm. Keep the proper alarms, whatever else you buy.
That distinction is the single most important thing on this page, which is why it sits at the top rather than buried in a footnote.
The difference is not marketing. A tested alarm is built to sound at defined concentrations within defined times, is independently verified against that standard, and is designed to keep working when the power and the internet do not. A monitor reports a number to an application.
Both are useful. They are useful for different reasons, and one will not save anybody's life.
The readings
What these sensors actually measure
Most consumer monitors cover some combination of the following. Few cover all of it, and the ones claiming to often measure less than the box implies.
| Reading | What it is | Where it comes from indoors |
|---|---|---|
| PM2.5 | Fine particulate matter, small enough to reach deep into the lungs. | Cooking, especially frying. Candles, wood burners, cigarettes. Traffic drifting in through a window. |
| PM10 | Coarser particles. | Dust, pollen, disturbed soft furnishings. |
| VOCs | Volatile organic compounds. A broad family of gases rather than one thing. | Paint, adhesives, cleaning products, new furniture, air fresheners. |
| CO2 | Carbon dioxide. Not toxic at indoor levels, but a good proxy for how stale a room is. | People breathing. It rises in a closed room and falls when a window opens. |
| Humidity | How much moisture the air is holding. | Bathing, drying laundry indoors, cooking, and the weather. |
| Temperature | Included on nearly everything. | Obvious, and useful mainly as context for the humidity reading. |
Why CO2 is more useful than it looks
It is not dangerous at the levels a home reaches. What it does is tell you how well a room is ventilating, because it accumulates when air is not being exchanged and drops promptly when it is. A bedroom reading that climbs overnight and falls the moment a door opens is describing the ventilation, not a hazard.
Expectations
How much to trust the numbers
Consumer sensors are good at showing change and considerably less good at absolute values. That distinction decides what they are worth.
What they do well
- Showing a spike. Frying something and watching PM2.5 climb, then fall as the extractor runs, is genuinely informative and hard to argue with.
- Comparing one room with another using the same device moved around over a few days.
- Tracking a trend across weeks, which is where a connected device earns its place over a standalone one.
Where they are weaker
- Absolute accuracy. A reading of 12 on a consumer monitor is not a laboratory 12. Treat the numbers as a relative guide.
- VOC readings especially. Most consumer VOC sensors report a single composite index rather than identifying anything. A high reading tells you something changed, not what.
- Drift. Sensors wander over time. Some self-calibrate against a rolling baseline, which works provided the room genuinely gets fresh air sometimes.
- Low concentrations. Accuracy is generally worst near the bottom of the range, which is where a well-ventilated home spends most of its time.
Interpretation
Making sense of what you see
Published thresholds vary by country and by which body set them, so a number that reads as alarming on one application is unremarkable on another. Rather than chasing a target, it is more useful to learn what your own home normally does.
A more practical approach
- Establish a baseline. Leave the device somewhere for a week and watch what ordinary looks like in your house.
- Pay attention to shape, not value. A sustained climb matters more than a brief spike from the grill.
- Connect readings to events. Cooking, cleaning, closing a door, drying laundry. The pattern explains far more than the figure.
- Watch humidity as carefully as anything. Persistently high humidity is what leads to damp and mould, and it is both easy to measure and often fixable.
Where readings are consistently poor and the cause is not obvious, that is a matter for a qualified professional rather than a device. Persistent damp, suspected combustion problems, or anything affecting somebody's health belongs with someone who can inspect the building.
Position
Where a monitor belongs
Placement affects readings more than most people expect, and a badly positioned monitor produces numbers that describe the position rather than the room.
- Away from windows and doors, which produce readings about the draught rather than the room.
- Not directly above a hob. Nothing useful comes from a sensor sitting in the plume, and cooking will dominate everything.
- At roughly breathing height, since that is the air people are actually in.
- Away from radiators, which distort both temperature and humidity.
- Somewhere with a little air movement, rather than a sealed cupboard or a shelf behind books.
One device moved deliberately around a house over a fortnight tends to teach more than three left permanently in convenient spots.
What follows
What to do with the information
A monitor changes nothing by itself. Its value is entirely in what you do differently afterwards.
Ventilation comes first
It is free, and it addresses CO2, humidity and most VOC accumulation at once. Opening a window for a few minutes does more for a stale room than any appliance. Extractor fans matter more than people think, particularly the one over the hob and the one in the bathroom.
Then filtration, where it is warranted
An air purifier addresses particulates and, with the right filter, some gases. It does nothing whatever for CO2, which is worth knowing before buying one to fix a stuffy bedroom.
- Match the unit to the room size. An undersized purifier in a large room accomplishes very little.
- Filters are a running cost. Establish what replacements cost before committing, since some are a substantial fraction of the purchase price each year.
- Note the noise rating. A purifier that has to run on its loudest setting to keep up will end up switched off.
Humidity is its own problem
Persistently high readings point at ventilation, drying laundry indoors, or something structural. Persistently low readings are common in winter with heating running. Both are worth addressing, and neither is solved by a purifier.
The smart bit
What being connected actually adds
A standalone monitor shows a number now. A connected one adds three things, and whether they justify the difference depends on what you want.
- History. Seeing a week of readings is where the patterns appear. This is the strongest argument for a connected device.
- Alerts. Useful for humidity in particular, where a slow climb is easy to miss and expensive to ignore.
- Automation. A reading can trigger something else, most usefully a fan or a purifier. This is where the category becomes genuinely practical rather than merely interesting.
What to check before buying
- Whether history is kept locally or on a service, and what happens to it if the service closes.
- Whether readings are available without the manufacturer's cloud, which decides whether automations survive an outage.
- Whether it works with whatever else you own, since a monitor that cannot trigger your fan is only a display.
Frequently encountered
Common situations, and what usually causes them
- Readings spike every evening. Cooking, nearly always. Watch whether the extractor is being used and whether it vents outside or merely recirculates.
- CO2 climbs overnight in a bedroom. Expected in a closed room with people in it. It is a ventilation observation rather than a hazard.
- VOC readings jump for no visible reason. Cleaning products, aerosols, a new piece of furniture, or something drying. Composite VOC sensors respond to a great many everyday things.
- Two monitors in the same room disagree. Common, and not necessarily a fault. Different sensors, different positions, different calibration. Use one consistently rather than comparing.
- Humidity reads high all winter. Often drying laundry indoors combined with reduced ventilation. Worth addressing before it becomes damp.
- The device reports carbon monoxide. Treat any reading as a reason to check your proper CO alarm and, if there is any doubt at all, to leave and call the relevant emergency number. Do not rely on a monitor for this.
Where a situation persists beyond these, the manufacturer is the right place to go next. Anything suggesting a combustion problem, persistent damp, or an effect on somebody's health belongs with a qualified professional rather than a device or a website.
Keep reading
These are not life-safety devices
An air quality monitor and a carbon monoxide alarm do different jobs. The alarm guide explains why one cannot stand in for the other.
Read the alarm guideHow we checked this
Where this comes from
- The literature each manufacturer issues for its own equipment, which is where intended behaviour is set down
- What Apple and Google publish for people building software, since that is where handset permissions and radio handling are described
- The wireless specifications themselves, which govern how each band and every low-power protocol actually conducts itself
- Opening the applications and observing them in whatever state they are in today, with every check dated
Nothing stated here rests on a forum thread or an article with no name attached to it. Where a point simply cannot be pinned down, the page admits as much instead of papering over it.
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