During smog season, run your air purifier whenever someone is in the room, which for a bedroom means all night and for a living room means most of the day. Pollution keeps leaking in, so a purifier switched off for a few hours lets the room drift back towards the outdoor level. The easy way to do this without wasting power is Auto mode, which raises and lowers the fan as the room’s air changes.
The Short Answer
| Situation | How long to run it |
|---|---|
| Outdoor AQI Poor, Very Poor or Severe (common October to January in north India) | Continuously in occupied rooms, on Auto. All night in bedrooms. |
| Outdoor AQI Moderate | Auto mode, mostly in the evening and overnight when pollution tends to build. |
| Outdoor AQI Good or Satisfactory | Ventilate instead. Run the purifier after cooking, incense or cleaning, or on Auto so it wakes up only when needed. |
| Someone with asthma or allergies in the room | Lean towards continuous running in their bedroom through the season. |
The reason to keep it running is simple: a purifier does not clean a room once and leave it clean. Fine particles keep arriving through gaps around doors and windows, and from cooking, incense and people moving about. When the purifier stops, the room starts heading back towards the outdoor level.
How Long It Takes to Clean a Room
The clean-up time depends on air changes per hour (ACH): how many times the purifier can pass a volume of air equal to the whole room through its filter in an hour. You get it by dividing the purifier’s CADR (in m³/h) by the room’s volume (floor area times ceiling height, in m³).
- At 3 ACH, a volume equal to the room’s air passes through the filter about three times an hour.
- At 6 ACH, twice as often, so the room clears roughly twice as fast.
- A larger room, a higher ceiling, or an open door to the rest of the house all lower the effective ACH.
One air change does not mean the room is fully clean. Clean air from the purifier mixes with the dirty air already in the room, so each pass removes a share of the particles, not all of them. The level falls quickly at first and then more slowly, which is why a purifier needs a few air changes, not one, to bring a polluted room down, and why it has to keep running to hold it there.
For example, Boon Airvana delivers a CADR of 380 m³/h. In a typical 12 × 12 ft bedroom that is more than nine air changes an hour. Across about 484 sq ft with a 3 m ceiling, it is about 2.8 air changes an hour. Our air purifier size guide and the room size calculator work this out for your room.
Rule of thumb: the higher the air changes per hour, the faster the room clears and the more easily it stays clean. A correctly sized purifier on a lower fan speed often does more than an undersized one on maximum.
Doors and Windows: Closed or Open?
Close them while the purifier is working, if outdoor air is polluted. An open window on a smoggy day keeps bringing in fresh particles, and an open door turns a bedroom into a bedroom plus the hallway plus the living room, which spreads the purifier’s work over a much larger volume.
You do not need to seal the room airtight. Normal gaps are fine; the purifier is sized to cope with them. Two caveats:
- Carbon dioxide builds up in a closed room with people in it. A purifier does not remove CO₂ or add oxygen. Air the room briefly when outdoor AQI is at its lowest point in the day, then close up again and let the purifier recover the room.
- Never close up a room with a gas heater, angithi, coal or charcoal burning. A purifier does nothing for carbon monoxide. See what air purifiers cannot remove.
Where you put the purifier matters too: away from walls and curtains, with clear space around the intake. Our guide on where to place an air purifier covers Indian rooms, fans and ACs.
Boon Airvana’s Auto mode follows the room’s real-time AQI, Sleep mode runs quietly through the night, and Eco mode cycles off and on as the air settles or worsens.
See Boon Airvana →Running It While You Sleep
Yes, run it overnight. You spend more continuous hours in the bedroom than anywhere else, the door is usually shut, and in many north Indian cities pollution tends to build up in the evening and night. A bedroom purifier that runs from bedtime to morning does more for most people than one that runs in the living room during the day.
Three tips for nights:
- Start it before bedtime, so the room is already clean when you lie down.
- Use a sleep mode with a dimmed display and a quiet fan, so nobody is tempted to switch it off.
- Keep the door shut. An open door roughly undoes the sizing you did.
Auto and Eco Modes: the Practical Answer
Most people do not want to decide every hour whether the purifier should be on. That is what sensor-led modes are for:
- Auto mode reads the room’s particle level and raises the fan when pollution rises (cooking, incense, a door opened) and lowers it when the air is clean. It runs continuously but only works hard when needed.
- Eco mode (on Airvana) goes a step further: it cycles the purifier off and on as the AQI settles or rises, to save electricity.
- Scheduling and timers help where routines are fixed, for example starting the bedroom purifier an hour before bedtime. Airvana’s app also has an Auto-learn mode that learns your routine and starts purifying before you get home.
Only rely on Auto if the purifier has a real particle sensor. A purifier that guesses, or one with a vague dust sensor, may lower the fan while the fine particles are still high.
Electricity and Filter Life
Running a purifier 24/7 does use more electricity than running it for a few hours, but at lower fan speeds most purifiers draw far less than at full speed, which is why Auto and Eco modes matter. The actual cost depends on the model’s power draw, the speed it spends most time at and your tariff; our air purifier running cost guide shows how to work it out.
The bigger running cost is usually the filter. A filter that cleans more air catches more particles and fills faster, so running a purifier through a Delhi winter uses up a filter faster than running it in a clean month. Three habits help:
- Wash or vacuum the pre-filter regularly (Airvana’s pre-filter is washable). It catches hair and dust that would otherwise clog the main filter.
- Change the main filter when the reminder says it is due, not on a fixed calendar. On Airvana the reminder depends on the air it has actually cleaned.
- Do not switch the purifier off to “save the filter” on smoggy days. The filter is there to be used; an unused filter in a polluted room protects nobody.
See air purifier maintenance and filter care for the full routine.
Frequently Asked Questions
Should I run my air purifier 24/7?
In smog season, running it continuously in occupied rooms makes sense, because outdoor particles keep leaking in and the room drifts back to the outdoor level once the purifier stops. Use Auto mode so the fan only works hard when pollution rises. When outdoor AQI is Good or Satisfactory, ventilating the home is often enough.
How long does an air purifier take to clean a room?
It depends on air changes per hour: the purifier’s CADR divided by the room’s volume. At 3 air changes an hour, a volume equal to the room’s air passes through the filter about three times an hour; at 6, twice as often. Each pass removes a share of the particles, so a few air changes are needed to bring a polluted room down.
Should doors and windows be closed when using an air purifier?
Yes, when outdoor air is polluted. An open window brings in fresh particles and an open door spreads the purifier over a larger space. You do not need to seal the room airtight. Air the room briefly when outdoor AQI is lowest to clear carbon dioxide, and never close up a room with a gas heater, angithi or charcoal burning.
Is it safe to run an air purifier all night while sleeping?
Yes, a HEPA air purifier is designed to run for long periods, and the bedroom at night is where it does the most good. Start it before bedtime, keep the door shut and use a sleep mode so the noise and display do not disturb you. Avoid purifiers that produce ozone, especially in bedrooms.
How much electricity does an air purifier use if run 24 hours?
It depends on the model’s power draw at each fan speed, how long it spends at each speed, and your electricity tariff. Purifiers draw much less on low speeds than at full speed, so Auto and Eco modes keep 24-hour running costs down. Check the model’s rated power and use our running cost guide to work out your monthly figure.
Does running an air purifier all the time wear out the filter faster?
Yes, a filter that cleans more air catches more particles and fills sooner, especially in a smoggy winter. That is the filter doing its job. Wash the pre-filter regularly to protect the main filter, and replace the main filter when the purifier’s reminder says it is due rather than on a fixed calendar.
Set it once and let the room decide. Boon Airvana’s Auto mode follows the live AQI, and Eco mode cycles off and on to save electricity when the air is clean.
See Boon Airvana →