PM2.5, AQI and what the numbers actually mean
In this guide
PM2.5 — the measurement
PM2.5 means particulate matter with a diameter of 2.5 micrometres or less. The unit is micrograms per cubic metre (µg/m³). It is a direct physical measurement: how much of this size of particle is floating in one cubic metre of air.
These particles are small enough to reach deep into the lungs and cross into the bloodstream. That is why the size threshold matters more than the total dust count.
AQI — the index
AQI (Air Quality Index) is not a measurement. It is a scale that converts one or more pollutant readings into a single number between 0 and 500, so different pollutants can be compared on one axis.
The catch: AQI has no single international definition. The US EPA, the UK, China, India and Thailand each publish their own breakpoints. The same air can be AQI 120 under one system and AQI 160 under another.
Why the two numbers disagree
If you see "AQI 155" and "PM2.5 55 µg/m³" side by side and they seem inconsistent, they probably are not — they are using different scales. Common causes of confusion:
- Different breakpoint tables — a PM2.5 of 55 µg/m³ is "unhealthy" on the US scale but "very unhealthy" on some others
- AQI may reflect a different pollutant — if ozone or PM10 is higher, the AQI shown is the worst of them, not the PM2.5 figure
- Different averaging windows — a 1-hour peak and a 24-hour average are not the same number
- Indoor versus outdoor — outdoor stations do not measure your living room
What to do at each level
Using the US EPA scale for PM2.5 as the reference, because it is the most widely published:
| PM2.5 (µg/m³) | US AQI | Practical action |
|---|---|---|
| 0–12 | 0–50 | Normal. Open windows if you want. |
| 12–35 | 51–100 | Sensitive groups should limit prolonged outdoor exertion. Close windows if you have a purifier. |
| 35–55 | 101–150 | Everyone reduces outdoor exertion. Keep windows shut, run the purifier. |
| 55–150 | 151–200 | Avoid outdoor exercise. Purifier on high, seal the room. |
| 150–250 | 201–300 | Stay indoors. Run purifier continuously. Consider a mask outdoors. |
| 250+ | 301+ | Emergency conditions. Sealed room, purifier running, minimal outdoor time. |
What this means for your purifier
Once you know your actual indoor PM2.5, sizing a purifier becomes straightforward: match the CADR to your room volume, then confirm it can hold the level you want on a fan speed you can tolerate.
If indoor PM2.5 stays high even with the purifier running on high, the problem is usually air exchange — outside air is leaking in faster than the unit can clean it. Fix the gaps before buying a bigger purifier.
- Calculate the CADR your room needs →
- Work out the long-term filter cost →
- What to check before buying a purifier →