Category: Air Purifier

The One Rule Everything Else Follows

An air purifier cleans one enclosed room. It does not treat a flat, it
cannot keep up with an open door, and it cannot outrun a window. Every placement question
below — fans, AC, windows, exhausts, heaters — comes back to the same test:
is this room closed enough for the purifier to get ahead of the inflow?

That is why the bedroom is almost always the right first room. It has a door that shuts,
it is where you spend eight continuous hours, and it is the easiest space in an Indian home
to seal. Start there, get it working, and only then think about the living room.

Where in the Room

• Not in a corner and not against a wall. Most units draw air from the
  sides or back and push it out of the top. Leave at least 30–50 cm clear around the
  intake so it is breathing the room, not the skirting board.
• Off the floor is fine; on the floor is fine. Height matters far less
  than clearance. A bedside table or the floor beside the bed both work.
• Near the source, if there is one. If the room’s main inflow is the door
  to a corridor or a road-facing window, place the purifier between that and where you
  sleep, so it intercepts rather than chases.
• Away from curtains and soft furnishings that can be drawn into the
  intake or block it.
• Where you can see the display from the bed. A number you can check is
  worth more than a light you have to trust.

Ceiling Fans and Air Conditioners

The instinct is that a fan and a purifier are fighting each other. Mostly they are not.
A ceiling fan on low is an ally: it mixes the room so the purifier is
drawing from all of it instead of the pocket immediately around the unit, and it carries
cleaned air to the bed instead of leaving it in a column above the machine.

Two cautions. Keep the purifier out of the fan’s direct downdraft, so its intake is not
competing with a stream of air being pushed down at it. And keep the fan slow — a fan
at full speed lifts settled dust off surfaces faster than the purifier removes it.

The same logic applies to the AC. Place the purifier a metre or two to one side of the AC,
not in its blast, and let the AC’s circulation distribute the cleaned air. One thing the AC
does not do: filter. The mesh in a split AC is a dust screen for the coil, not a HEPA, and
it does nothing for PM2.5.

Sliding Windows and Door Gaps

This is the question the international brands cannot answer, because their showrooms do not
have Indian aluminium sliding windows. A sliding window that is closed is still open
along its whole track — the overlap between the two panes and the channel they
run in are gaps the width of a coin, the full height of the frame. The purifier is running
against a continuous inflow it cannot see.

Fix it before you buy the purifier, not after, because it changes which purifier you need:

• Foam or rubber weather-strip along the overlap where the panes meet
  and in the channel where the pane seats when closed. A few hundred rupees at any hardware
  shop.
• A brush or rubber seal on the door bottom, especially if the door faces
  a corridor or the main door.
• Tape over the unused exhaust vent if the room has one with no fan in
  it. It is a pipe to outside.

Do the window-sill test to see if it worked: wipe a road-facing sill clean and see how long
it stays that way. Days means you are sealed enough. Hours means keep going.

Exhaust Fans, Open Windows and Oxygen

An exhaust fan empties the room of purified air. It pushes air out, the
room’s pressure drops, and outdoor air is pulled in through every gap to replace it. Run the
bathroom exhaust only while the bathroom is in use, keep its door shut, and keep the bedroom
door closed while the kitchen chimney runs. Cooking with the bedroom open undoes the
evening.

The window cracked open for oxygen is a related habit and it is mostly
unnecessary. An air purifier removes particulate; it does not consume oxygen or remove carbon
dioxide. In a closed bedroom overnight, CO₂ does rise, and a sealed room can feel
stuffy by morning. The answer is a short, deliberate ventilation in the afternoon when
outdoor air is at its cleanest, not a window left open through the night when it is at its
worst. If the room genuinely needs continuous fresh air, that is a case for a filtered inlet,
not for diluting the purifier.

Room Heaters and Humidifiers

A room heater and a purifier coexist without difficulty. Keep them apart so
the purifier is not drawing hot air across its sensor, and keep the purifier’s cable clear of
the heater. The dry throat people blame on running both is the heater’s doing — radiant
and fan heaters dry the air, the purifier does not.

Humidifiers are the harder case. Ultrasonic cool-mist units throw very fine droplets, and
with tap water those droplets carry dissolved minerals that dry into a fine white dust. Both
load the HEPA, and both register on the purifier’s sensor as pollution, so the machine ramps
up and wears its filter fighting the humidifier. Keep them at least two metres apart, run the
humidifier on distilled or RO water, and keep room humidity below about 50 percent —
above that, the filter stays damp and the sour smell starts.

The Semi-Open Living Room Problem

Most Indian living rooms are not rooms. They open onto a passage, a kitchen, a dining area
or a balcony, and the volume the purifier is trying to treat is effectively the whole flat
plus whatever the balcony door admits. A purifier in a semi-open living room reduces;
it does not clear.

Three honest options:

• Treat the bedroom instead. Eight hours in clean air beats sixteen in
  slightly-less-dirty air, and the bedroom is the room that closes.
• Close what can be closed — the balcony door, the kitchen door, a
  curtain across the passage — and size for the combined space that remains, using the
  calculator below rather than the box figure.
• Accept reduction and run it anyway, knowing what it is doing. That is a
  legitimate choice; it is only a mistake if you expected clearance.

Frequently Asked Questions

Can I keep an air purifier under a ceiling fan?

Yes, and it often helps. A ceiling fan on low mixes the room’s air so the purifier draws from the whole room rather than a pocket around itself. Keep the purifier out of the direct downdraft so its own intake is not fighting the fan, and keep the fan slow enough that it is not stirring settled dust off surfaces.

Should the air purifier be opposite the AC or near it?

Either works; what matters is that the AC’s airflow and the purifier’s are not cancelling each other. Placing the purifier a metre or two from the AC, off to one side, lets the AC’s circulation carry cleaned air around the room. Do not put the purifier directly in the AC’s blast, and do not rely on the AC’s own filter for particulate; it is a dust screen, not a HEPA.

Does an exhaust fan pull the purified air out of the room?

Yes. An exhaust fan lowers pressure in the room and outdoor air is drawn in through every gap to replace what it pushed out. Run bathroom and kitchen exhausts only while needed, and keep the bedroom door closed while they run. A kitchen chimney does the same thing on a larger scale, which is why cooking with the bedroom door open undoes an evening of filtration.

Can a humidifier and an air purifier run in the same bedroom?

They can, but not side by side. Ultrasonic cool-mist humidifiers throw fine droplets and, with tap water, mineral dust that loads the HEPA and can register on the sensor as pollution. Keep them at least two metres apart, use distilled or RO water in the humidifier, and keep room humidity below about 50 percent so the filter stays dry.

Is it worth running an air purifier in a semi-open living room?

Usually not on its own. A purifier cleans an enclosed volume; a living room that opens onto a passage, kitchen or balcony has an effectively unlimited volume and continuous inflow. Treat the bedroom first, where the door closes and you spend eight hours. If the living room matters, close what can be closed and size the purifier for the combined space, or accept that it reduces rather than clears.

Two Kinds of Feature

Every air purifier spec sheet mixes two kinds of feature. There are the ones that change
what the machine does — how much air it cleans, whether you can bear to
run it, whether it is telling you the truth. And there are the ones that change how it
feels to own — the app, the voice assistant, the remote, the finish. The second
kind is fine. The problem is that spec sheets present them as equals, and buyers routinely
trade a size up for a Wi-Fi chip.

This guide takes the features in order of how much they matter. The first two decide
whether the purifier works at all. The rest are worth having only once those are settled.

Noise — The Feature That Decides Whether You Use It

The most common failure mode of an air purifier in a bedroom is not filtration. It is that
it gets switched off at 11pm because it is too loud, and stays off. A purifier that is
off for the eight hours you are in the room is furniture.

What to read on the spec sheet:

• dB at the lowest speed, or sleep mode — not the maximum, which
  is the number brands lead with when it flatters them and hide when it does not. You will
  not run at maximum overnight.
• CADR at that speed, if published. A purifier that is quiet because it
  is barely moving air is not quiet; it is off with the light on.
• Whether the display dims in sleep mode. A bright ring in a dark bedroom
  is its own reason to unplug it.

The trade this creates is real: a larger unit running on low is quieter than a smaller unit
running on medium to move the same air. That is a second argument, after pollution, for
sizing up.

The Sensor — And Why It Shows Green

Almost every purifier now has a particulate sensor, and almost every owner has had the
experience of the machine glowing a contented green while the outdoor AQI is 350 and the
external monitor on the shelf reads red. Three things are going on.

• It measures its own exhaust neighbourhood. The sensor sits on the
  machine, which, once running, is surrounded by the cleanest air in the room. It is honestly
  reporting a pocket.
• The thresholds are generous. Brands choose where green ends, and many
  choose a level that a health-based scale would call moderate. Green means acceptable to the
  manufacturer, not clean by a standard you picked.
• The lens is dirty. Dust on the optical sensor reads as less scattering,
  which the machine interprets as cleaner air. It drops to low speed and does less, and the
  green light stays on.

Sensor type matters somewhat: laser-scattering sensors resolve finer
particles and react faster than the cheaper infrared kind, which drift more
and respond mainly to larger dust. But the feature that matters more than the sensor is the
display. A numerical PM2.5 readout lets you check the machine; a colour band asks you
to trust it. Given the choice, take the number.

Auto Mode, Sleep Mode and Timers

Auto mode lets the sensor set the speed. It is only as good as the sensor,
which is the point of the previous section: on a machine that reads green too readily, auto
mode is a way of running on low all winter. On a machine with a decent sensor and honest
thresholds, it is genuinely useful — it ramps up after cooking or a door opening and
settles when the room is clear.

Sleep mode is usually a fixed low speed with a dimmed display. Check what
it does on the specific model: on some it disables the sensor entirely, which means a smoky
evening goes unanswered until morning.

Timers are useful for one thing: pre-cleaning the bedroom an hour before
you go in, so you are not waiting for it to catch up. Schedule it for the evening; leave it
running overnight.

Apps, Voice Assistants and Remotes

These change how it feels to own, not what it does. In descending order of use:

• The app is worth having for history — seeing the PM2.5 trace over
  a week tells you when the room gets dirty and why — and for scheduling. It is also how
  most brands surface filter-life estimates. Do not pay a size class for it.
• Voice assistants let you turn it up without getting out of bed. Nice.
  Not a filtration feature.
• The remote is the app for people who do not want the app. If the model
  has one, fine; if it has an app, the remote is redundant.
• Multi-device automation — running the purifier when the AC comes
  on, or when the front door opens — is the one genuinely useful smart-home trick, and
  only if you already run that ecosystem.

Child Lock, Filter Indicators and Power Draw

Child lock is a real feature in a home with a toddler: a purifier at floor
level with a row of touch buttons on top is an invitation. Look for it if that describes your
house; ignore it otherwise.

Filter indicators on most units are timers. They count hours against an
assumed load and tell you to replace on schedule, which in an Indian winter is late. Treat the
indicator as a reminder to look, and go by airflow and the state of the filter face.

Power draw is the feature buyers over-weight. On the low and sleep speeds
you run overnight, a purifier draws on the order of a bedside lamp; at full speed, somewhat
more than a ceiling fan. Over a season, filter replacements cost several times the
electricity. Efficiency badges are welcome but they should not decide anything; the
running-cost guide has the arithmetic.

The Worksheet

Feature	Changes what it does?	What to check
CADR for your room	Yes — everything	Against 4–5 air changes an hour, not the box area
Noise at sleep speed	Yes — whether you run it	dB on low, and whether the display dims
Numerical display	Yes — whether you can verify it	A PM2.5 number, not a colour
Laser sensor	Somewhat	Better than infrared; still needs cleaning
Auto mode	Only if the sensor is honest	Run the one-night test
App / voice / remote	No	Convenience; do not pay a size class
Child lock	No	Yes if you have a toddler
Energy rating	Marginally	Filters cost more than power

Frequently Asked Questions

What is the most important feature in an air purifier?

Adequate CADR for the room, which is not a feature so much as the whole point. After that, low noise at the speed you will actually run overnight, because a purifier you switch off at night does nothing for eight hours. Everything else on the spec sheet comes after those two.

Why does my air purifier show green when the outdoor AQI is red?

Three reasons, often together. The sensor samples air at the machine’s own intake, which is the cleanest spot in the room once it has been running. Many brands set the green threshold generously, so a level an external monitor would call moderate shows as good. And the sensor lens collects dust and reads low. Clean the sensor, compare with an external monitor for a night, and use a model with a numerical readout.

Is a laser sensor better than an infrared one?

Generally yes. Laser-scattering sensors resolve smaller particles and respond faster; infrared sensors are cheaper, cruder and drift more. Either needs cleaning, and either can be set to show green too readily. The sensor type matters less than whether the machine shows you a number you can check.

Do I need an app-connected air purifier?

No. The app is convenient for scheduling and for seeing history, and useful if you want the purifier to run with other devices. It does not change what the filter does. If the choice is between a connected model and a larger unconnected one for the same money, the larger one is the better purifier.

How much electricity does an air purifier use?

Less than most people expect. On the low and sleep speeds you run overnight, typical units draw on the order of a bedside lamp; at maximum speed, somewhat more than a ceiling fan. Over a season the filter replacements cost far more than the power. The running-cost guide on this site sets out the arithmetic.

The Maintenance Schedule That Actually Works Here

The maintenance interval printed on an air purifier box was set somewhere with cleaner air
than Delhi in November. Follow it and the purifier spends half the season running at a
fraction of its capacity, looking clean on the outside and doing very little. In an
Indian pollution season, maintenance is a fortnightly job, not a quarterly one.

Component	Pollution season (Oct–Feb)	Rest of the year
Pre-filter	Wash or vacuum every 2–4 weeks	Monthly
HEPA	Inspect fortnightly; replace when airflow drops or the media is grey through	Inspect monthly
Carbon layer	Replace with the HEPA, or sooner if it smells	Replace with the HEPA
Sensor inlet	Wipe monthly	Every 2–3 months
Housing and grille	Damp cloth fortnightly	Monthly

The Pre-Filter — Wash It, and Wash It Often

The pre-filter is a coarse mesh or foam layer at the intake. Its job is to catch hair, lint,
large dust and the visible grime so that the HEPA behind it only has to deal with the fine
particulate it was designed for. A clogged pre-filter is the most common reason a
purifier stops performing, and it is the one problem that costs nothing to fix.

• Vacuum it in place if the housing allows, or remove it and vacuum the
  intake face.
• Wash it under a tap with mild detergent if the manual permits —
  most mesh pre-filters are washable, most foam ones are, some paper-backed ones are not.
• Dry it completely before refitting. A damp pre-filter in front of a
  HEPA is how microbial growth starts.
• Do it before the season, in September, so the machine enters October
  clean.

The HEPA — What You Can and Cannot Do

HEPA media traps particles by contact with a dense mat of very fine fibres. That structure
is the filter. Anything that disturbs it — water, detergent, hard brushing, high
suction — reduces what it can catch, and the damage is invisible. A washed HEPA looks
cleaner and filters worse.

Action	Verdict	Why
Wash with water	Never	Collapses the fibre structure; does not recover when dry
Gentle vacuum on the intake face	Occasionally	Removes surface dust and restores some airflow; does not clear embedded fines
Tap or shake out	Lightly, outdoors	Dislodges loose material; do not beat it against a surface
Sunlight to sanitise	No	Does not clean; UV degrades some media and frame adhesives
Compressed air	No	Drives particulate deeper and can tear the media
Replace	The only real fix	Capacity is finite; a saturated HEPA is a resistance, not a filter

The Carbon Layer and the Sour Smell

Most purifiers pair the HEPA with an activated-carbon layer for odours and gases. Carbon
works by adsorption — molecules stick to its surface — and it has a finite
capacity that Indian cooking, incense and traffic exhaust fill quickly. When it is
full, it stops adsorbing and starts releasing, which is the source of the sour,
vinegar-like or musty smell that appears after a few months.

The second cause is moisture. In a humid room, or one where a humidifier runs nearby,
microbial growth on the filter produces the same sourness. Either way the filter is finished.
Replace it, and if humidity is the cause, move the purifier away from the humidifier and the
damp wall.

Why the Filter Turns Black in Three Weeks

New owners in Delhi-NCR often panic when a filter fitted in October is black by November.
It is not a fault. It is the filter doing exactly what you bought it for,
at a load the manufacturer’s schedule never anticipated.

Two things to check:

• Which layer is black. If it is the pre-filter, wash it and carry on;
  that is its job. If the HEPA face is uniformly grey-black through the media, it is
  saturating and the replacement is due sooner than the box says.
• Where the load is coming from. A filter that blackens fast in a sealed
  bedroom points at an indoor source — a kitchen without extraction, incense, a gap
  under the door to a busy road. Fixing the source extends every filter after this one.

Cleaning the Sensor

The particulate sensor is a small optical chamber, usually behind a slotted inlet on the
side or back. Dust settles on its lens and the reading drifts — usually downward, so the
machine reads cleaner air than it has, drops to a low speed, and does less. A purifier
that always shows green is often a purifier with a dirty sensor.

• Switch off and unplug.
• Open the sensor cover if there is one; otherwise work through the inlet slots.
• Wipe the lens with a cotton bud lightly dampened with isopropyl alcohol. Do not spray
  anything into the chamber.
• Let it dry fully before powering on.
• Monthly in season. If the reading still never moves, cross-check against an external
  monitor before trusting auto mode.

When to Actually Replace

The replacement indicator on most purifiers is a timer, not a measurement. It counts hours
and assumes a load. Use these signs instead:

• Airflow has noticeably dropped at the same speed setting.
• The machine runs at high speed more often in auto mode without the air
  outside getting worse — it is working harder to move air through a clogged filter.
• The HEPA face is grey through, not just dusty on the surface.
• It smells, sour or musty, when running.
• Six months of pollution-season running have passed, whatever the
  indicator says.

Keep one replacement filter in the cupboard from October. Supply tightens in November when
everyone else discovers theirs is finished, and a purifier waiting for a filter is a fan.

Frequently Asked Questions

How often should I clean my air purifier in India?

In pollution season, wash or vacuum the pre-filter every two to four weeks, wipe the sensor inlet monthly, and inspect the HEPA at the same time. Outside the season, monthly for the pre-filter is enough. The interval on the box was set for cleaner air than most Indian cities have between October and February.

Can a HEPA filter be washed with water?

No. HEPA media is a dense mat of very fine fibres that trap particles by contact; water and detergent break that structure and it does not recover when dry. A washed HEPA looks clean and filters worse. The only washable component is the pre-filter, and on most units the carbon layer must not be washed either.

Can I vacuum a HEPA filter to extend its life?

Gently vacuuming the intake face removes loose surface dust and can restore some airflow, but it does not clear the fine particulate embedded in the media, which is what saturates the filter. Treat it as a way to reach the next replacement, not to postpone it indefinitely. Never vacuum aggressively; the fibres tear.

Does putting a HEPA filter in sunlight sanitise it?

Not meaningfully. Sunlight will not clean out trapped particulate, and prolonged UV exposure degrades some filter media and the adhesives holding it in the frame. If the concern is smell or microbial growth, the cause is moisture and a saturated carbon layer, and the fix is replacement, not sun.

Why does my air purifier smell sour or like vinegar?

Usually one of two things. Either the activated carbon layer is saturated and is off-gassing what it absorbed, or moisture in a humid room has allowed microbial growth on the filter. Both mean the filter is finished. Replace it, and if humidity is the cause, keep the purifier away from humidifiers and damp walls.

Why Smog Attacks the Throat and Eyes First

The first sign of the season for many people is not a cough. It is a scratchiness at the
back of the throat by mid-morning and eyes that sting by the evening — both of them
gone within a day of leaving the city, and back within a day of returning. That pattern is
the diagnosis: it is the air.

Three things in polluted winter air act on the surfaces they touch first:

• Fine particulate settles on the moist lining of the throat and the
  surface of the eye, where it irritates directly and carries whatever it is coated in.
• Ozone and nitrogen dioxide, both elevated in traffic-heavy winter air,
  are oxidising gases that inflame mucous membranes on contact.
• Dryness. Winter air holds less moisture; indoor heating removes more.
  The thin protective film on the throat and eyes that normally handles all of this is thinner
  than usual, and everything gets through.

The Heater Makes It Worse

The room heater arrives in the same week as the smog, and it gets blamed for the wrong
thing. People run a purifier and a heater together, wake with a dry throat, and conclude the
purifier is drying the air. It is not. A purifier moves air through a filter
and returns it at the same humidity. The heater — radiant, fan or oil-filled — is
what dries the room, and a dry room in polluted air is the worst combination for the throat.

• Run the heater at the lowest setting that keeps the room comfortable, and switch it off
  once the room is warm rather than leaving it cycling all night.
• A bowl of water near the heater, or a humidifier kept well away from the purifier, puts
  some moisture back. Aim for a room that feels neither dry nor damp; above about half
  humidity the purifier’s filter starts to suffer.
• Keep the two appliances apart, so the purifier’s sensor is not reading the heater’s hot,
  dry outflow.

What Actually Helps at Home

Measure	What it does	Worth it?
Filter the bedroom overnight, door closed	Eight hours of recovery in cleaner air. This is where the throat heals or does not	The single largest lever
Seal the bedroom	Weather-strip the window track and door bottom so the purifier gets ahead of the inflow	Yes, before the purifier
Saline rinse for eyes and nose	Physically clears deposited particulate; restores surface moisture	Yes, morning and evening
Drink more than usual	Keeps the mucous membranes hydrated from the inside	Yes
No indoor combustion	Incense, candles, mosquito coils and indoor smoking add directly to the load in the one room you are trying to keep clean	Yes, absolutely
Kitchen extraction on before the oil goes in	Cooking aerosol is a major indoor irritant; the chimney catches it only if it is already running	Yes
Fitted N95 outdoors on severe days	Protects the throat and lungs; does nothing for the eyes	On severe days
Menthol lozenges and warm drinks	Relieve the sensation without treating the cause	Comfort only

What a Purifier Does — and Does Not Do — for This

Set expectations exactly. A correctly sized HEPA purifier in a sealed bedroom removes the
particulate component of the problem from that room for the hours you are in it. Most people
notice the difference within a few nights: waking without the scratch, eyes that are not
gritty by morning. That is a real and worthwhile effect, and it is limited to the room
and the hours.

What it does not do:

• It does not humidify. If dryness is a large part of your symptom, the
  purifier alone will not fix it.
• It does not reliably remove ozone or nitrogen dioxide. A carbon layer
  reduces them somewhat; it is not designed for them and saturates quickly.
• It does not follow you. The commute, the office, the living room with
  the balcony door open — those hours are untreated, and they are most of the day.

Undersizing is the usual reason a purifier fails to deliver even the limited effect it can.
The calculator below works out what your bedroom actually needs.

When to See a Doctor

A pollution sore throat is uncomfortable, not dangerous, and it eases when the air does.
Some things are not that, and pollution season overlaps with viral season, asthma season and
the weeks when chronic bronchitis flares. See a doctor if:

• The symptoms last more than two weeks without improving.
• There is fever, wheezing, chest tightness or breathlessness.
• There is blood in what you cough up, or the eyes are painful rather than
  irritated, light-sensitive, or the vision changes.
• A child, an older adult, or anyone with asthma or heart disease is
  affected badly.
• You have a morning cough with phlegm every winter. That pattern has a
  name, and it is worth having assessed rather than attributed to the air each year.

Frequently Asked Questions

Why does my throat hurt when the AQI is high?

Fine particulate, ozone and nitrogen dioxide in polluted air irritate the mucous membranes lining the throat and eyes on contact. Winter air is also dry, and indoor heating dries it further, so the protective moisture on those surfaces is thinner than usual. The combination produces the scratchiness and stinging that arrive with the season and ease when the air clears.

Does an air purifier help with a sore throat from pollution?

It helps with the particulate part, in the room it runs in, with the door closed. It does not add moisture, and it does nothing for the hours you spend outside or in untreated rooms. Most people find the largest improvement comes from filtering the bedroom overnight, which is where the throat recovers or fails to.

Can a room heater and an air purifier run together?

Yes. The purifier removes particles; the heater dries the air. The dry throat people notice is the heater’s doing, not the purifier’s. Keep them apart in the room, use the heater at the lowest setting that works, and consider a bowl of water near it or a humidifier kept well away from the purifier.

What relieves burning eyes from smog?

Rinsing with clean water or sterile saline, keeping the bedroom air filtered overnight, avoiding rubbing, and reducing screen time in the evening when the eyes are already dry. Lubricating eye drops without preservatives help; anything that promises to whiten or clear redness should be avoided. If the eyes are painful, light-sensitive or the vision changes, see a doctor.

When is a pollution sore throat something more serious?

If it lasts more than two weeks, if there is fever, wheezing, chest pain, blood, difficulty swallowing, or breathlessness, or if a child or an older adult is affected badly, see a doctor. Pollution season coincides with viral season, and asthma and chronic bronchitis both flare in the same weeks. A persistent morning cough with phlegm every winter is worth having assessed rather than attributed to the air.

What It Is Built For

An air purifier is a fan that pushes air through a filter. The filter that does the work,
HEPA, stops particles — dust, smoke particulate, pollen, dander, the
fine PM2.5 that makes a Delhi winter dangerous. It does that very well. Most purifiers add a
layer of activated carbon that adsorbs some odours and gases, less well and
for a limited time.

That is the whole capability. Everything below is a case where a buyer, reasonably, expects
the machine to do something it cannot — and in one case, where the expectation could
hurt someone.

Carbon Monoxide — The Dangerous One

No consumer air purifier removes carbon monoxide. HEPA stops particles; CO
is a gas. Activated carbon adsorbs many gases; CO is one it barely touches. Some purifiers
advertise gas sensors and gas filtration. None of them is a CO safety device, and the product
pages are careful, in the small print, not to claim it.

This matters in Indian homes specifically because CO sources are common and indoors:

• A gas geyser in a bathroom with the door shut.
• A generator or inverter charging in a closed space.
• A chulha, angithi or coal heater used indoors in winter.
• A car idling in a closed garage under the flat.
• Heavy traffic air in a ground-floor room on a main road, which is what
  the Mumbai searches are about — the CO there is real, and a purifier does not touch
  it.

The response to suspected CO is ventilation now, and a CO alarm permanently. Headache,
dizziness, nausea and confusion in a closed room with a burning appliance is an emergency,
not an air-quality problem. Do not run the purifier and wait.

Sulphur Dioxide, Nitrogen Dioxide and Other Gases

Landfill fires, industrial plumes, refinery neighbourhoods and traffic all put gases into
the air alongside the particulate: sulphur dioxide, nitrogen dioxide, volatile organic
compounds, ozone. The purifier splits the problem in two. The particulate it
removes well. The gases fall to the carbon layer, which:

• Reduces them, sometimes substantially, when the carbon is fresh.
• Saturates — in weeks, in a bad neighbourhood — and then stops.
• Can begin releasing what it adsorbed once full, which is the sour smell owners notice
  after a few months.
• Is thin on most consumer units. A few hundred grams of carbon is not an industrial gas
  scrubber, whatever the marketing says.

If gas is your dominant problem — you can smell it, your eyes sting when the
particulate reading is low — the honest answer is sealing the room against the inflow,
ventilating when the plume has passed, and a purifier with a genuinely large carbon stage,
replaced on a schedule set by your nose rather than the indicator.

Carbon Dioxide and Oxygen

A purifier does not remove carbon dioxide and does not add oxygen. It returns air with
exactly the gas composition it took in. In a bedroom sealed well enough for the purifier to
work, CO₂ climbs overnight from breathing, and the room feels stuffy by morning
even though the particulate is low. People blame the purifier or open a window all
night to fix it, which lets the winter air back in.

The fix is to separate the two problems. Seal for particulate. Ventilate for CO₂,
briefly and deliberately, in the afternoon when the outdoor air is at its least bad, and then
close up again. Ten minutes of cross-ventilation resets the room; a window left cracked all
night is the worst of both.

Smoke Is Not One Thing

Source	Particulate	Gases and CO	What a purifier does
Stubble and crop burning	Very high	Some	Works well; it is mostly particles
Diwali crackers	Very high, briefly	Metals, sulphur compounds	Works for the haze; carbon takes some of the rest
Landfill or industrial fire	High	High — SO2, VOCs	Removes the visible smoke; only part of the sting
Chulha, wood, coal indoors	Very high	Carbon monoxide	Catches particles; does nothing for the CO. Vent it outside first
Incense and agarbatti	High	Some VOCs	Works; better still is not lighting it in the treated room
Cooking	High during and after	Some	Works; a chimney running before the oil goes in works better
Cigarette	High	Many	Removes the particulate; the gases and the residue on surfaces remain

Ionisers and the Dust on the Bedsheet

Some purifiers add an ioniser, and some cheap devices are nothing but one. An ioniser
charges airborne particles so they stick to the nearest surface. The air reads cleaner; the
particles are on the wall, the bedsheet and the floor, from where the next footstep or the
ceiling fan lifts them back up. It moves the dust; it does not remove it. Some
ionisers also generate ozone, which irritates exactly the throat and eyes the buyer was trying
to protect. If a purifier has an ioniser switch, leave it off. If it is only an ioniser, it is
not a purifier.

What to Do Instead

• For carbon monoxide: a CO alarm and ventilation. Never a purifier.
• For gases and smells: seal the inflow, ventilate when the plume has
  passed, and treat the carbon stage as a consumable with a short life.
• For CO₂ and stuffiness: a timed afternoon ventilation, not a
  window open all night.
• For indoor burning of any kind: extraction to outside first, purifier
  second.
• For particulate — which in an Indian winter is most of the harm
  — a HEPA purifier, sized for the room, in a room that closes. That is what it is for,
  and it is genuinely good at it.

Frequently Asked Questions

Does an air purifier remove carbon monoxide?

No. Carbon monoxide is a gas, HEPA filters only stop particles, and activated carbon does not capture CO in any useful amount. If you suspect CO — from a generator, a geyser burner, a chulha or a car in a closed garage — the answer is ventilation and a CO alarm, immediately. No consumer air purifier protects against it.

Can an air purifier remove sulphur dioxide or landfill fire smoke?

Partly. The particulate in landfill or industrial smoke is what HEPA is for and it removes it well. The gases in it — sulphur dioxide, nitrogen dioxide, volatile organics — are handled only by the carbon layer, which reduces them for a while and then saturates. Expect the purifier to remove the smoke you can see and only some of the smell and sting.

Does an air purifier remove CO2 or add oxygen?

Neither. A purifier passes air through a filter and returns it with the same gas composition. In a sealed bedroom overnight carbon dioxide rises regardless of the purifier, which is why a room can feel stuffy by morning even with clean air. The answer is a short, timed ventilation when outdoor air is at its cleanest, not a window left open all night.

Is an air purifier useful for chulha or wood smoke?

For the particulate, yes, and wood smoke is heavy in it. For the gases and the carbon monoxide that come with indoor biomass burning, no. If a chulha is used indoors, the priority is a chimney or a hood that vents outside, and ventilation while it burns. A purifier in the same room is a distant second and gives false reassurance about the CO.

Do ionisers just make dust settle on surfaces?

Largely, yes. An ioniser charges particles so they cling to the nearest surface — walls, bedsheets, the floor — rather than capturing them. The air reads cleaner and the room gets dustier, and the dust lifts again whenever anything disturbs it. Some ionisers also produce ozone, itself an irritant. A HEPA filter captures particles and holds them; that is the difference.