The Purifier Lab · Air
How fast does an air purifier actually clear a 200 sq ft room?
Starting from a known elevated PM2.5 concentration, how much does a given air purifier reduce it in 5, 15, 30 and 60 minutes in a sealed 200 sq ft room — and how does that compare with the reduction its CADR rating implies?
Results
No results yet. This page publishes the protocol only.
We have written and published the method below, and we have not yet run it. There are no measurements on this page, no charts and no percentage claims, because there is nothing to report.
The protocol is published first deliberately. It means we cannot quietly amend the method once we have seen a result we did not like, and it lets anyone — including manufacturers — tell us the method is wrong before we run it. If you can see a flaw in it, tell us.
The protocol
Elevate PM2.5 in a sealed 200 sq ft room to a controlled starting concentration, run the unit under test at a stated fan speed, and log PM2.5 at fixed intervals against a natural-decay control run with no purifier operating.
Instrument
Laser optical particle counter, logging at 1-minute intervals
Model: To be stated on first published run, with serial
Calibration: Co-located against a second identical unit before and after each session; any pair diverging by more than 10% at 100 µg/m³ invalidates the session.
What is held constant
- Same room for every run — 200 sq ft, 10 ft ceiling, approximately 57 m³.
- Doors and windows closed and taped at the same points for every run. Under-door gap sealed with the same draught excluder.
- Air conditioning off. Ceiling fan off. No occupant movement in the room during logging.
- Monitor at 1 m height, 2 m from the unit, never in the direct discharge stream.
- Starting concentration brought to 150–200 µg/m³ and allowed to stabilise for 5 minutes before logging begins.
- Temperature and relative humidity recorded at the start and end of every run.
- A natural-decay control run with no purifier operating is recorded in the same session, because a sealed room clears slowly by itself and that fraction must be subtracted.
Steps
- 1
Seal and stabilise the room
Close and tape the same points on every run. Seal the under-door gap. Leave the room undisturbed for 15 minutes so the air is still before anything else happens.
- 2
Elevate PM2.5 to the starting band
Introduce particulate from a consistent source until the monitor reads between 150 and 200 µg/m³, then stop and allow 5 minutes for mixing and stabilisation. The source is stated on every published run because different sources produce different particle size distributions.
- 3
Record the natural decay control
With no purifier running, log PM2.5 for 60 minutes. This is what the room does on its own through settling and infiltration, and it is subtracted from every subsequent result. Skipping this step is why most published purifier tests overstate performance.
- 4
Run the unit under test
Re-elevate to the same starting band, then start the unit at a stated fan speed. Log PM2.5 every minute for 60 minutes. Do not enter the room.
- 5
Repeat at a second fan speed
Repeat the full sequence at the unit's maximum speed and at its quietest usable speed, because the difference between them is what a household actually experiences at night.
- 6
Publish everything
Raw minute-by-minute readings, ambient conditions, fan speed, the control run, and any anomaly. A run that went wrong is published with a note explaining what went wrong, not discarded.
What this test cannot tell you
Published with the protocol rather than buried after the results. A test whose limits are not stated is a claim, not a measurement.
- One room. Results describe this room's volume, leakage and layout, and do not transfer directly to a differently shaped or leakier space.
- An optical particle counter infers mass concentration from light scattering, assuming a particle density and refractive index. Readings are repeatable and internally comparable, but they are not reference-grade gravimetric measurements.
- The particulate source is not identical to ambient urban PM2.5. Particle size distribution differs, and capture efficiency varies with size.
- A single session cannot separate filter condition from unit design. Units tested with a new filter will outperform the same unit at six months, and that is a separate test.
- We do not measure gas-phase pollutants in this protocol at all. A unit that clears particulate quickly may leave odour entirely untouched.
Note