Most consumer "air quality monitors" run on one or two sensors — typically PM2.5 plus a derived VOC index — and roll them into a single colour. They tell you the room is "bad" without telling you what is wrong.
The diagnostic problem.
"Air quality is bad" is a symptom. The causes are specific: people are breathing (CO₂), someone cooked (PM2.5), a new sofa is off-gassing (VOC), it is too humid (RH), it is too cold (T), a gas appliance is leaking (CO). Each cause has a different fix.
A real example: 2 PM, December.
A coloured-light monitor in the lounge turns amber. You do not know why. Open the window? Run the purifier? Move out?
The same room on a multi-channel monitor shows:
- CO₂: 1,400 ppm (high — five people in the room)
- PM2.5: 8 µg/m³ (fine — kitchen was OK)
- VOC: 250 ppb (fine)
- RH: 38% (fine)
- CO: 0 ppm (fine)
- Temperature: 21 °C (fine)
The diagnosis is unambiguous: too many people, not enough ventilation. Open a window for ten minutes; CO₂ drops to 700 ppm; everyone is comfortable again.
The CO question.
Carbon monoxide is the one channel where missing it has serious consequences. Many consumer "air quality monitors" do not include a CO sensor at all — and a CO leak from a gas appliance produces a coloured-light value of "good" because the PM and VOC sensors do not respond to it. A dedicated electrochemical CO sensor is the single most important safety channel after smoke detection.
The cost argument is weaker than it sounds.
Eleven calibrated sensors in one device cost less than buying five single-channel monitors and a CO detector. And they share one app, one dashboard, one MQTT topic.
The case for one device, eleven channels.
The same instrument that watches your overnight bedroom CO₂ watches PM2.5 from cooking, watches VOCs from fresh paint, watches humidity for mould risk, watches CO for safety, and watches temperature and pressure for completeness. One investment, one app, one record.