An air quality monitor turns something invisible into a number you can act on. Most measure three separate things: carbon dioxide in parts per million, which tracks how stale the air is and how well the room is ventilated; fine particulate matter in micrograms per cubic metre, which comes from cooking, candles, wood burners and traffic; and volatile organic compounds, which come from paint, cleaning products, adhesives and new furniture.
All of the monitors here also read temperature and relative humidity, because those two govern comfort, condensation and mould risk. Everything in this range is designed for indoor use in homes, offices, classrooms and workshops.
How to monitor air quality at home
Put the monitor where people actually spend time, at roughly seated head height, away from windows, doors, extractor fans and radiators. Anything within a metre of a window will read the outdoor air, and anything on a windowsill in sunlight will read its own casing. Then leave it alone for a week before you change anything, because what you are looking for is the daily pattern rather than any single reading.
The pattern is what tells you the story. Carbon dioxide climbing steadily through the evening and peaking overnight in a closed bedroom is normal and easily fixed. A sharp particulate spike every time you cook points at extraction rather than at the cooker. Humidity that sits above 65 per cent in one room but not others usually means a cold surface, not a damp house.
How carbon dioxide is measured
Almost all serious CO2 monitors use non-dispersive infrared sensing, usually shortened to NDIR. Carbon dioxide absorbs infrared light at a specific wavelength, so the sensor shines an infrared beam down a small chamber, measures how much arrives at the far end, and calculates the concentration from the shortfall. It is a genuine measurement of the gas itself.
The alternative you will see on cheap devices is an estimated or equivalent CO2 figure, written eCO2, which is inferred from a VOC sensor reading rather than measured. It responds to the right things in the right direction but it is not a CO2 measurement and should not be used where the number matters.
What the CO2 numbers mean
Outdoor air currently sits at roughly 420 parts per million, and that is the floor any indoor reading is working up from. Below about 800 ppm indicates good ventilation. Between 800 and 1,000 ppm is acceptable but worth watching. Above 1,000 ppm is the point at which UK guidance for schools and workplaces treats ventilation as inadequate, and above about 1,500 ppm most people notice drowsiness, headaches and difficulty concentrating.
How particulates are measured
Particulate sensors use laser light scattering. A small fan draws room air through a chamber, a laser is shone across it, and a photodetector counts the flashes of scattered light as individual particles pass through. The size of each flash indicates the size of the particle, which is how one sensor reports both PM2.5 and PM10.
PM2.5 means particles smaller than 2.5 micrometres, small enough to reach deep into the lungs, and it is the figure worth watching. The World Health Organization guideline is an annual average of 5 micrograms per cubic metre with a 24-hour average not exceeding 15. Cooking a stir-fry with the extractor off will comfortably exceed that for an hour.
Volatile organic compounds
VOC sensors are metal-oxide semiconductors: a heated film changes electrical resistance when reactive gases land on it. They cannot identify which compound they are detecting, so they report a total VOC level or a simple index rather than a concentration. Treat them as a trend indicator. A jump after painting, gluing, cleaning or unwrapping new furniture is exactly what they are for.
What we recommend
For a dedicated CO2 instrument with a proper NDIR sensor and a clear traffic-light display, the CO2 monitor AIRCO2NTROL OBSERVER 31-5002 is the one we point ventilation and classroom enquiries at. If you want air quality alongside full outdoor weather rather than as a separate device, the Bresser 7-in-1 Weather Station Air-Mon combines an outdoor sensor suite with indoor air quality readings on one console.
Related measurements
Air quality overlaps heavily with two other elements. Damp, mould and condensation problems are really Humidity problems, and the dedicated instruments are in Hygrometers. Cold surfaces and unheated rooms belong to Temperature. If you want to log conditions over weeks rather than watch them live, see Data Loggers, and for outdoor conditions alongside indoor air see Home Weather Stations or the full Elements index.