The Purifier Lab · Air
What happens to an air purifier's performance as its filter loads?
How much does an air purifier's measured PM2.5 decay performance change between a new filter and the same filter after 3 and 6 months of continuous use in Indian conditions — and does the unit's own filter-life indicator track that change?
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
Run the standard PM2.5 decay protocol on the same unit with a new filter, at 3 months and at 6 months of continuous operation in a real Indian home, and record the pre-filter and HEPA mass gain at each point.
Instrument
Laser optical particle counter plus a 0.1 g precision balance for filter mass
Model: To be stated on first published run
Calibration: Particle counter co-located as per the decay protocol. Balance checked against a calibration mass before each weighing.
What is held constant
- Same physical unit throughout — not a second sample of the same model.
- Same room, same decay protocol, same starting concentration band.
- The unit runs continuously in a normal occupied home between test points, at a logged fan speed, so loading is real rather than accelerated.
- Operating hours recorded from the unit's own counter where it has one, and from a plug-in energy meter's runtime where it does not.
- Pre-filter cleaned on the manufacturer's stated schedule, and that schedule is recorded — cleaning it more or less often changes the HEPA's loading rate.
- HEPA and pre-filter weighed dry at each test point, at the same ambient humidity.
Steps
- 1
Baseline with a new filter
Weigh the new HEPA and pre-filter. Run the full decay protocol at both fan speeds. Record the unit's filter-life indicator reading.
- 2
Deploy for three months
Install the unit in a normal occupied home and run it continuously at a logged fan speed. Record operating hours, and clean the pre-filter on the manufacturer's schedule.
- 3
Three-month test point
Remove and weigh both filters. Return the unit to the test room and repeat the full decay protocol at both fan speeds. Record the filter-life indicator.
- 4
Deploy for a further three months
Reinstall the same filters and continue as before.
- 5
Six-month test point
Weigh, retest, and photograph both filters against a colour reference. Publish the images alongside the data as part of the autopsy record.
- 6
Publish the whole series
Decay curves at each test point, filter mass at each test point, operating hours, and what the unit's own indicator said at each point versus what the measurements showed.
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 unit, one home, one pollution environment. Loading rate in a Delhi winter and a Bengaluru monsoon are not comparable, and this measures one of them.
- Filter mass gain includes absorbed moisture, which varies with ambient humidity. Weighing at a consistent humidity reduces but does not eliminate this.
- Removing and reinstalling a filter disturbs the accumulated cake, which may slightly change performance at the next test point.
- The result describes this filter design. A pleated HEPA and a cylindrical composite filter load differently and are not interchangeable conclusions.
- Six months is not end of life. It is the interval at which most Indian households are being told to replace, which is the question being tested.
Note