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Read an air quality monitor by sorting the screen into three groups before reading any value. Particulate mass, carbon dioxide, temperature and humidity are measured and comparable with published guidance. Volatile organic compound and formaldehyde figures on a consumer instrument are relative indicators that respond to events rather than absolute concentrations. The index and the colour band are conversions a manufacturer applied to one of the other channels, and acting on them instead of on the underlying number is how people end up chasing readings that mean nothing.
Which numbers are measured and which are derived
Three questions applied to any value on the screen settle how much weight it deserves. Is there a sensor specific to it, or is it an inference from a general detector. Is it reported in a unit that published guidance also uses. And does it have a defined averaging period, or is it an instantaneous sample.
Carbon dioxide passes all three on an instrument with an infrared sensor. Particulate mass passes the first two. Temperature and humidity pass all three and are the most reliable values on most of these devices. A volatile organic compound index fails the first, because the element behind it responds to a family of gases rather than to one. An air quality index fails the first entirely, since there is no sensor for it at all. That ordering, rather than the position of the numbers on the screen, is the order to read them in.
Reading particulate numbers in the right units
Particulate is reported two completely different ways and the numbers are not interchangeable. Mass concentration is micrograms per cubic metre and is what health guidance uses. Count concentration is particles per litre or per cubic foot above a size threshold and is what cleanroom work uses. A monitor can show both, and the figures differ by orders of magnitude for the same air.
| Value on screen | Unit it is usually in | What a change actually indicates | What it does not tell you |
|---|---|---|---|
| PM2.5 | Micrograms per cubic metre | Combustion, cooking, smoke, outdoor air entering | What the particles are made of |
| PM10 | Micrograms per cubic metre | Dust, pollen, abrasion, activity in the room | Whether the coarse fraction is reaching your lungs |
| Particle count | Particles per litre, above a size | Filtration performance, cleanliness trends | Mass exposure, which guidance is written in |
| Carbon dioxide | Parts per million | How well ventilation matches occupancy | Anything about toxicity at indoor levels |
| Formaldehyde | Milligrams per cubic metre or parts per billion | An event, since the channel is cross sensitive | A reliable long term background figure |
| Volatile organic compounds | An equivalent concentration index | That something changed in the room | Which compound, or at what real concentration |
| Index | A bounded scale with bands | Nothing on its own, it is a conversion | The concentration that produced it |
The mass figures are the ones to anchor on, because guidance is published in them. The long term guideline concentration for fine particulate is in the single digits of micrograms per cubic metre and the short term figure is in the middle teens, which puts a kitchen reading of eighty during frying in perspective: high, short lived, and worth extracting rather than worth alarm.
A carbon dioxide trace is a ventilation reading
The carbon dioxide channel is the most useful thing on most of these instruments, and it is useful for a reason people often get wrong. Indoor concentrations are nowhere near harmful. What the number does is measure the balance between how much air people are breathing out and how much fresh air is arriving, which makes it a direct, continuous, cheap reading of ventilation.
Read it as a shape rather than a value. A bedroom that starts at 500 parts per million and plateaus at 2,400 by morning has told you its air change rate is low, and the height of the plateau is the measurement. A meeting room climbing 300 parts per million every twenty minutes is telling you the occupancy has overwhelmed the supply. A room sitting at outdoor concentration all day with people in it either has very good ventilation or a sensor with a stuck baseline, and the way to tell is to take it outside and read it there.
That is also why the automatic baseline behaviour matters for interpretation. A sensor that assumes its lowest weekly reading was outdoor air will slowly rewrite its own zero in a room that never ventilates fully, and the trace stays plausible while the absolute values drift. Where a unit exposes a manual calibration, it is worth using: the Lwyinp ten in one air quality tester states a calibration mode reached by pressing the power button three times, along with a display that tracks the average concentration of each gas rather than only the instantaneous value. An average over a stated window is a far better basis for a decision than whatever the screen happens to show as you walk past.
Colour bands hide the number you need
Colour grading is the feature that makes these instruments approachable and the feature most likely to mislead. The LifeBasis eleven in one detector states four grades, good, moderate pollution, poor and abnormal, with the icon colour changing by concentration and a ticking alert plus a flashing indicator once a level is exceeded. That is a clear and usable design. What the listing does not give is the concentration at which each colour changes, for any channel. Not stated in the listing bullets, which means the boundary between green and yellow is a decision somebody made and did not publish.
Three habits defuse the problem. Read the number, not the colour, whenever the number is available. Learn your own room baseline so that a change is visible without reference to a manufacturer colour scale. And treat an audible alert as a prompt to look at the figures rather than as a verdict, because an alert on a cross sensitive channel fires for a cleaning spray as readily as for a genuine problem. Other units draw the lines in other places, which the Bonoch monitor notes and the Sacnahe monitor notes record where the listings state them.
Reading the trend instead of the instant
Almost every real conclusion from one of these instruments comes from a record rather than from a glance. A single number cannot distinguish a background level from an event, cannot show whether a change is a trend or noise, and cannot be compared with anything if it was not written down.
- Let it warm up. Heated element channels mean nothing in the first minutes after power on, and the first readings after a cold start are often the highest of the day for no physical reason.
- Establish a baseline. Twenty four hours in one position with normal activity gives the curve everything else is compared against.
- Change one thing. Open the window, run the extract fan, change the filter, cook. A monitor is at its best as a before and after instrument.
- Export rather than squint. A week of data read on a screen is a week of guessing. The Temtop M2000C bundle states export of readings as comma separated values over a universal serial bus, which turns the question into one you can answer with a chart instead of a memory.
- Re-read the volatile channels in clean air. Take the unit outdoors or to a well aired room and see where those numbers sit. If they do not come down, the channel is drifting rather than detecting.
Turning a screenful of readings into one decision
In practice the whole display reduces to four actions, and knowing which reading triggers which is the entire skill. Rising carbon dioxide means ventilate, and nothing else on the screen changes that. Rising particulate means find the source and extract it at the source, since a cooker hood beats an air cleaner for cooking and an air cleaner beats nothing for smoke drifting in from outside. High humidity with a cold surface means a condensation and mould risk, which is a fabric problem rather than an air problem. And a volatile compound channel that has climbed and stayed up after a refurbishment means ventilate harder for longer, because materials off gas for weeks.
Everything else is context. If a channel on your instrument cannot trigger one of those four actions, it is informational, and reading it as a verdict on the air is reading more than the sensor can support. The physical basis for each channel, which is what sets those limits, is examined channel by channel in the Sculpriv monitor notes, the specific case of the volatile compound figure in what a volatile organic compound reading means, and a worked example of judging one instrument channel by channel in the Vetouch monitor notes.
Frequently Asked Questions
My monitor shows a high formaldehyde reading. Should I be worried?
Check what you did in the last hour first. Cleaning sprays, hand sanitiser, aerosols, alcohol, nail products and fresh paint all raise that channel on a consumer instrument because the sensing element responds to a broad family of compounds. A reading that falls back within an hour of ventilating was reporting an event, not a background level.
What carbon dioxide number should I ventilate at?
Outdoor air sits around 420 parts per million, a well ventilated room stays under about 800, and a room past roughly 1,500 is noticeably stale with the ventilation clearly losing to the occupancy. Those are ventilation judgements, not safety thresholds. The occupational limit over a working day is far higher, at 5,000 parts per million.
Why does the reading jump when I walk past?
Exhaled breath is around a hundred times more concentrated in carbon dioxide than room air, so leaning over a monitor spikes it for a minute or two. A fan driven instrument also responds to being picked up because the airflow through the optical chamber changes. Read it from a distance and leave it in one place.
Is a particulate reading of 35 bad?
It depends entirely on the unit. Thirty five micrograms per cubic metre is a meaningful short term exposure, several times the long term guideline. Thirty five thousand particles per litre is a different quantity measuring a different thing. Check which unit the screen is showing before comparing the number to anything.
Should I believe the index or the raw numbers?
The raw numbers, every time. An index is a conversion applied to one channel, usually particulate, and different authorities define the breakpoints differently. The underlying concentration is comparable with published guidance and with your own earlier readings, and the index is only comparable with itself.
How long should I watch before deciding anything?
A day for a ventilation question and a week for anything slower. A single reading describes a moment that may be the moment somebody opened a window or sprayed a surface. The useful quantities are the overnight carbon dioxide plateau, the daily particulate baseline, and whether either is trending.