Your air purifier is not bringing PM2.5 down

Does this match your symptoms?

• On-board PM2.5 readout barely moves after an hour
• A separate monitor disagrees with the purifier's own display
• Air feels no different during high-pollution days
• It worked last winter and does not seem to now

Work through the causes in this order

Ordered by how often each turns out to be the real cause — not by what it costs to fix. 5 of these cost nothing to resolve.

1. Most commonYou can fix thisFree to fix

   1. Unit undersized for the room

   How to confirm it

   Compare your room area against the unit's rated coverage, then halve the rating. A 240 m³/h CADR unit is genuinely effective in a bedroom of roughly 120-150 sq ft in Indian winter conditions, not the 350 sq ft on the box. Open-plan spaces are effectively much larger than their nominal floor area.

   The fix

   Move the unit to a smaller room, or add a second unit. Buying one large purifier for a whole flat rarely works — air does not circulate between rooms well enough.

   Part: None, unless a second unit is needed

2. Most commonYou can fix thisFree to fix

   2. Room is not closed

   How to confirm it

   Doors and windows open, or a permanently open kitchen, means you are trying to filter the outdoors. Test with everything shut for 45 minutes and compare.

   The fix

   Close the room during purification. Where a chronic gap exists — an unsealed door, an unused exhaust vent — sealing it costs little and improves results more than upgrading the machine.

   Part: None

3. Most commonYou can fix thisFree to fix

   3. Running on sleep or auto at low fan speed

   How to confirm it

   Sleep mode typically delivers a small fraction of rated CADR. Auto mode on some units is also conservative. If it has been on sleep all night, it has been moving very little air.

   The fix

   Run on high for 30-45 minutes to pull the room down, then drop to a lower speed to hold it. This is the single most effective change most owners can make, and it is free.

   Part: None — free to fix

4. CommonYou can fix thisUp to ₹4,500

   4. Loaded HEPA or blocked pre-filter

   How to confirm it

   Remove the filter and look at it. A grey or black HEPA surface, or a pre-filter matted with dust and hair, restricts airflow well before the change indicator triggers — and the indicator is usually a timer rather than a sensor.

   The fix

   Vacuum or wash the pre-filter — in most Indian homes this is needed monthly, not annually. Replace the HEPA if it is visibly loaded. Never wash a HEPA filter unless the maker explicitly says it is washable; water destroys the media in most.

   Part: Pre-filter cleaning, or HEPA replacement

5. OccasionalYou can fix thisFree to fix

   5. Air bypassing the filter

   How to confirm it

   Check the filter is seated correctly, that any plastic wrapper was removed when new — this happens more often than you would think — and that the seal around the filter frame is intact. Air takes the path of least resistance, so a small gap defeats a good filter.

   The fix

   Reseat the filter properly and replace any perished gasket. Where the machine's own seal design is poor, there is no fix — only the buying lesson.

   Part: Gasket, where replaceable

6. OccasionalYou can fix thisFree to fix

   6. The purifier's own sensor is wrong

   How to confirm it

   Compare against an independent PM2.5 monitor. Cheap optical sensors drift, and they respond to humidity — many units read alarmingly high in monsoon simply because of moisture in the sample.

   The fix

   Clean the sensor's optical chamber and inlet, which is a documented maintenance task on many models. If it still disagrees with a reference monitor, trust the monitor.

   Part: None

When to stop and call a technician

• Airflow is obviously weak with a new filter fitted — this points at the fan or motor
• The unit is under warranty and genuinely underperforms its rating in a correctly sized, closed room

When replacement beats repair

Costs shown are indicative bands for India, published so you can sanity-check a quote — they are not quotations, and they vary by brand, city and whether the technician is brand-authorised. Last reviewed 2026-08-08.