Two different problems: particles and gases
When a device claims to measure air quality, it is usually doing one or more of two different jobs. One is counting or sizing tiny solid and liquid particles floating in the air. The other is detecting particular gases, or groups of gases, at very low concentrations. The sensing principles, strengths and weaknesses differ, so it helps to treat them separately.
How particle sensors work
Particulate matter is described by size. PM2.5 refers to particles with a diameter of 2.5 micrometers or smaller, and PM10 to particles up to 10 micrometers. These include dust, smoke, pollen fragments and fine droplets.
Most small consumer sensors are optical. A tiny fan or heating element draws air through a chamber. A light source, usually a laser or LED, shines across the airflow, and a photodetector measures light scattered by particles passing through. More or brighter pulses suggest more particles. Firmware then converts pulse counts into an estimated mass concentration, typically expressed in micrograms per cubic meter.
That last conversion is where much of the uncertainty comes from. The sensor does not weigh anything. It assumes particles have a typical size and density, and different sources, such as cooking smoke, wildfire smoke and road dust, scatter light differently. Very small particles may scatter too little light to be seen at all. High humidity can also make particles absorb water and swell, which can inflate readings.
How gas sensors work
Metal-oxide sensors
A heated metal-oxide surface changes its electrical resistance when certain gases touch it. These sensors are cheap, small and responsive, so they are common in low-cost monitors. Their weakness is selectivity: they react to many gases at once. A device reporting a volatile organic compound (VOC) index is usually giving a relative, combined signal rather than the concentration of a specific chemical. Readings also depend on temperature and humidity, and the sensor often needs time to warm up and stabilize.
Electrochemical sensors
These work like a miniature battery cell. A target gas reacts at an electrode and creates a tiny current. They can be quite selective for gases such as carbon monoxide, and they are commonly used where more careful measurement is needed. They age, because the chemistry is consumed or degraded, so their lifetime is limited and often measured in years.
Non-dispersive infrared sensors
Carbon dioxide absorbs a specific wavelength of infrared light. A non-dispersive infrared (NDIR) sensor measures how much of that light is absorbed across a short path. This directly targets CO2 and is generally more reliable than guessing from a general-purpose gas sensor. Some cheaper devices instead estimate “equivalent” CO2 from VOC signals, which is not the same thing and can be misleading.
Other approaches
Photoionization detectors, catalytic bead sensors and laser spectroscopy exist for particular industrial and scientific needs. They are generally more specialized and expensive.
Why cheap units differ from reference instruments
Official air monitoring stations use reference or equivalent methods, which are standardized, regularly calibrated and operated by trained staff. A low-cost sensor differs in several ways.
- Method. Reference instruments often conditioned the air sample, for example by drying it, to reduce the effect of humidity. Small sensors typically do not.
- Calibration. Reference equipment is checked against known standards on a schedule. Consumer sensors are usually calibrated once at the factory, if at all, and then drift. The general idea is covered in the discussion of sensor calibration.
- Interference. Cross-sensitivity to temperature, humidity and other gases is more of an issue in inexpensive parts.
- Lifetime. Fans clog, lasers dim, and chemical elements are used up.
- Siting. A station is placed according to strict rules. A home monitor sits wherever there is a power outlet, perhaps next to a kitchen.
None of this makes cheap sensors useless. They are often good at showing relative change: smoke from a candle, a spike while cooking, or CO2 rising in a closed room. They are less suited to telling you whether the absolute value passes a health threshold.
Reading the numbers sensibly
- Look at trends and patterns more than single readings. A sudden jump after cooking is informative even if the exact figure is uncertain.
- Compare two units placed together, if you have them. Large disagreements point to a problem.
- Give new sensors time to stabilize, and keep them away from vents, windows, heat sources and direct sprays.
- Check whether a number is measured or estimated, particularly for CO2 and VOC values.
- Treat claims of health diagnosis cautiously. A monitor can suggest that ventilation might help, but it is not a medical tool.
A note on safety devices
Smoke alarms and carbon monoxide alarms are certified safety products built to specific standards. A general air quality monitor is not a substitute for them. Devices connected to a network add useful notifications, but the core alarm should work independently of Wi-Fi.
Practical tips for deployments
Air sensors that run all the time can be power hungry because of fans and heaters, so battery-powered designs often sample on a schedule, trading detail for longer battery life. Where results matter, such as a school or workplace, it makes sense to compare a few units against a better-characterized device and to expect periodic replacement of the sensing element. Combined with sensible placement, even a modest monitor can help a person notice patterns in their surroundings, as long as they remember what it is actually measuring.