Category: Air Purifier

Read This First

This article is about environmental triggers, and it is worth being clear about its limits
before anything else.

A cough lasting more than three weeks should be assessed by a doctor. There
are many causes — infection including tuberculosis, asthma, reflux, post-nasal drip, medication
side effects, and others — and several of them need treatment that no change to your home will
provide.

What follows is for the situation where the cough is clearly tied to being at home:
better at work, better on holiday, worse at night, worse in a particular room. That pattern is
worth investigating — alongside seeing a doctor, not instead of it.

What the Pattern Tells You

Keeping a rough note for a week — where you were, what you were doing, how bad it was — turns
a vague complaint into something a doctor can work with, and frequently identifies the trigger
on its own.

Why Coughs Are Worse at Night

Almost universally reported, and there are concrete reasons for it.

• Lying flat changes the mechanics. Nasal secretions drain backward into
  the throat rather than forward, and reflux is easier against gravity.
• The bedroom is the longest continuous exposure of the day. Seven to nine
  hours in one closed room, in close contact with bedding.
• Airways narrow overnight as a normal part of the daily rhythm — which is
  why asthma characteristically worsens at night.
• Outdoor pollution peaks overnight, when cool still air holds it near the
  ground, and infiltrates through the night.

Environmental Triggers Worth Checking

• Damp and mould. Look behind furniture, along external walls, around
  window reveals and inside cupboards. A musty smell counts as evidence even with nothing
  visible.
• Bedding allergen. Dust mite reservoir in mattress and pillows — the
  likeliest suspect if symptoms are worst on waking.
• Cooking smoke. High-heat oil generates significant fine particulate. In
  an open-plan flat it reaches every room. Chimney used every time, on before the oil.
• Indoor combustion. Incense, mosquito coils, candles, indoor smoking. A
  coil burning nightly in a closed bedroom is a substantial and continuous source.
• Renovation residue. New paint, new furniture and construction dust
  release VOCs and fine particles for weeks to months.
• Outdoor infiltration. If the cough tracks the AQI, the gaps around
  windows and doors are the pathway.

The Every-Winter Sore Throat

A sore throat that recurs each winter and eases in spring is usually about conditions rather
than repeated infection.

Three things change together: indoor air gets drier, which dries the throat lining and makes
it more easily irritated; people spend more time in closed rooms, concentrating whatever is in
them; and across northern India airborne particulate rises steeply from October.

If it recurs on the same schedule every year, and improves when you travel somewhere with
cleaner air, that is worth mentioning to a doctor as a pattern rather than presenting each
episode as a fresh illness.

What to Change at Home

In order of likely return:

1. Deal with any damp. Filtering spores does not address the moisture
   producing them.
2. Remove indoor combustion. Stop the coils and incense in bedrooms — this
   is free and frequently decisive.
3. Extract cooking at source. Chimney on before the oil, off ten minutes
   after.
4. Encase mattress and pillows; hot-wash bedding weekly. If symptoms are
   worst on waking, start here.
5. Seal the bedroom window and door. Cuts overnight infiltration during the
   hours you are most exposed.
6. Filter the bedroom, door closed, sized for the room.

Change one thing at a time and give it a week or two. Changing five things at once may make
you feel better without telling you which one mattered — and you will not know what to maintain.

When to See a Doctor

Go promptly if any of these apply:

• The cough has lasted more than three weeks
• You are coughing blood
• There is breathlessness, chest pain, or wheeze
• There is fever, night sweats, or unexplained weight loss — tuberculosis
  remains common in India and is entirely treatable when diagnosed
• A child coughs most nights, or coughs after exertion
• An existing inhaler is needed more often than before

Improving your indoor environment is worth doing regardless. It is not a substitute for
finding out what is actually wrong.

Frequently Asked Questions

Why do I have a constant cough at home that won’t go away?

A cough persisting beyond three weeks needs medical assessment rather than home troubleshooting. That said, when a cough is clearly tied to being at home — better at work, better on holiday, worse at night — an environmental trigger is worth investigating alongside seeing a doctor: indoor allergens, damp and mould, unextracted cooking smoke, or outdoor particulate infiltrating an unsealed home.

Why does my child cough at night but not during the day?

Night-time cough in children commonly reflects three things acting together: lying flat, which changes airway mechanics and allows post-nasal drip; the bedroom being where allergen exposure is longest and most concentrated; and asthma, which characteristically worsens at night. Persistent night cough in a child should be assessed by a doctor — it is one of the more common ways childhood asthma first presents.

Can air pollution cause a chronic cough?

Exposure to fine particulate irritates and inflames the airways and can both cause and sustain a cough, particularly during high-pollution months. But pollution is not a diagnosis — a persistent cough has many possible causes, some needing treatment, so it should not be assumed to be the air without a doctor ruling out the alternatives.

Why do I get a sore throat every winter?

A recurring seasonal pattern usually reflects the season’s conditions rather than repeated infection: dry indoor air, more time in closed rooms with higher allergen concentration, and — across northern India — a large seasonal rise in airborne particulate. If it recurs every year at the same time and eases when you travel, the environment is a reasonable place to look.

Does an air purifier help with a persistent cough?

If particulate or airborne allergen is a genuine trigger, reducing exposure in the bedroom can help — and improvement over a week or two is itself informative. It is not treatment. A cough lasting more than three weeks, or any cough with breathlessness, fever, weight loss or blood, needs a doctor first.

What Happens on Diwali Night

Air quality across Indian cities deteriorates sharply on Diwali, and the pattern is
consistent enough to be predictable: levels begin climbing through the evening, peak late at
night, and stay elevated into the following day.

Two things make the spike larger than the fireworks alone would suggest.

The timing. Diwali falls within the season when cool, still air already
traps pollution close to the ground. The same quantity of smoke released in June would disperse
far more readily.

The baseline. In the northern plains, Diwali usually lands during or near
the stubble-burning weeks. The festival smoke is added to an already elevated load rather than
to clean air.

Why It Ends Up Indoors

Fine particulate does not stop at the wall. It enters through the same gaps that admit
ordinary outdoor air — around window frames, under doors, through vents — and on Diwali it gets
considerable help.

Doors open constantly: guests arrive, people go out to the balcony or the street to watch,
children come and go. Every opening admits a volume of the outdoor air at its worst moment.

The result is that indoor levels track the outdoor spike with a lag. A home that has done
nothing in particular will see its own air deteriorate through the evening and stay poor
overnight.

How Long It Lasts

Outdoors, the peak is generally overnight and into the following morning, with elevated
levels persisting for a day or more depending on wind.

Indoors is the part people underestimate. Particulate that entered during the night does not
simply leave. Without ventilation when outdoor air improves, or filtration to remove it, it
stays suspended and settles slowly — which is why the morning after can feel worse indoors than
the night itself, particularly for anyone with asthma.

The Sources Inside Your Own Home

Not all of it comes from outside, and this part is entirely within your control.

• Diyas. Oil lamps are combustion. Individually modest, and a room with
  many of them burning for hours accumulates a real load. Oil generally produces less
  particulate than paraffin candles.
• Incense. A concentrated source in a small space — a single stick in a
  closed room produces a surprisingly high local concentration.
• Cooking. Festival cooking means more frying, for longer, than an ordinary
  day. High-heat oil is one of the sharpest particulate events in any home kitchen.
• Indoor fireworks. Sparklers and anything else lit on a balcony next to an
  open door put smoke directly into the house.

Celebrating Without Breathing the Worst of It

If Someone at Home Is Vulnerable

For a child with asthma, an elderly parent with a respiratory or cardiac condition, or a
newborn, the calculation is different — and worth planning rather than improvising at ten
o’clock at night.

• Prepare the clean room in advance. Closed, sealed, filtration running from
  the afternoon, so it starts the evening clean rather than trying to catch up.
• Keep reliever medication accessible and know where it is. Emergency
  services are stretched on Diwali night.
• Keep them out of the immediate area where fireworks are being lit —
  exposure at close range is far higher than the ambient figure.
• Watch the following morning, not just the night. Symptoms often show up
  with a lag.

None of this requires anyone to celebrate less. It requires one prepared room and a few
decisions made in advance.

Frequently Asked Questions

How bad does air quality get during Diwali?

Sharply worse, and quickly. Particulate levels in Indian cities typically rise steeply through Diwali evening and peak late at night, often reaching several times the pre-festival level. The spike is amplified because Diwali falls in the season when cool, still air already traps pollution near the ground.

Does firecracker smoke get inside the house?

Yes, readily. Fine particulate passes through the same gaps around windows and doors that admit ordinary outdoor pollution, and Diwali adds the complication that doors open frequently for guests and for going out to watch. Indoor levels follow the outdoor spike with a lag rather than staying clean.

How long does Diwali pollution last?

The outdoor peak is usually overnight and into the following morning, but elevated levels commonly persist for a day or more depending on wind. Indoors, without ventilation or filtration, particulate that entered during the night can remain elevated well into the next day.

Are diyas and incense bad for indoor air?

Both are combustion, and both release fine particulate directly into the room. Oil diyas produce less than candles, and incense is a concentrated source in a small space. None of this argues against them — it argues for ventilating the room afterwards rather than leaving the smoke to settle overnight.

Should I run an air purifier during Diwali?

It helps in a closed room, and the sensible pattern is to designate one room — usually a bedroom — kept shut with filtration running, so there is somewhere with better air to sleep. It cannot keep up with a door that keeps opening, and it does nothing for the time spent outdoors.

What Dust Mites Actually Are

Dust mites are microscopic arthropods, invisible without a microscope, that live in bedding,
mattresses, upholstery and carpets. They feed on the skin cells people and pets shed
continuously.

Two things are worth establishing immediately, because both are widely misunderstood.

Their presence is not a hygiene failure. They are in essentially every home
in the world. Their numbers respond to humidity and to the amount of soft material available,
not to how often you clean.

The mite is not what you react to. The trigger is a set of proteins in mite
waste, which become airborne when bedding is disturbed and settle again within minutes. That
detail determines which interventions work.

Where They Thrive in India

Mites have no way to drink. They absorb moisture from the air, which makes humidity the
single factor that governs their population.

• Humid coastal cities — Mumbai, Chennai, Kolkata, coastal Kerala — sustain
  high populations year-round.
• Monsoon raises them everywhere, including in cities that are otherwise
  dry.
• A dry Delhi winter suppresses them — which is diagnostically useful,
  because if your symptoms are worse in a dry Delhi winter, the cause is more likely
  outdoor particulate than mites.

That seasonal contrast is one of the more reliable ways to tell the two apart without
testing.

What the Symptoms Look Like

Sneezing, blocked or runny nose, itchy or watery eyes, sometimes cough or wheeze. Not
distinctive in themselves. The pattern is what distinguishes them:

• Worst on waking, easing through the morning
• Worse in bed, and when making the bed or vacuuming
• Better away from home — often noticeably so on holiday
• Worse in humid weather
• Year-round rather than seasonal, unlike pollen allergy

What Works, Ranked

The first two account for most of the achievable benefit. If you do nothing else, do those,
and give them a fortnight before judging.

What Doesn’t Work

• Dry dusting and sweeping. Both relaunch settled allergen into the air —
  frequently making symptoms worse for several hours afterward.
• Sunning the mattress. Traditional and pleasant, but Indian outdoor
  humidity is often high enough that it does not reliably dry a mattress to the point of killing
  mites.
• “Anti-allergy” sprays, in most cases. Evidence is weak and any effect is
  short-lived compared with a physical barrier.
• An air purifier alone. It cannot reach the reservoir inside your
  mattress.
• Cleaning more often at the same temperature. Frequency without heat
  leaves the population intact.

If There’s Asthma in the House

Dust mite allergen is one of the more common indoor asthma triggers, and this raises the
stakes from discomfort to something worth being systematic about.

Reducing trigger exposure is a recognised part of asthma management — but it works
alongside prescribed treatment, never instead of it. If someone in the house has
asthma:

• Do the encasings and hot washing properly rather than partially
• Keep pets out of the bedroom regardless of whether they seem to be a trigger
• Treat increasing reliever use as a signal to see a doctor, not to buy more equipment
• Mention the home measures at the next appointment — it is useful clinical information

Getting It Confirmed

Everything above assumes dust mites are the trigger. That is a reasonable guess given the
pattern, and a guess is a poor basis for rebuilding a bedroom.

A doctor can confirm with a skin-prick test or a specific IgE blood test, both routine. A
confirmed trigger changes what you do: it tells you whether to invest in encasings and humidity
control, or to look instead at pets, mould, cockroach allergen or outdoor particulate — which
need entirely different responses.

If symptoms are significant enough to affect sleep or daily function, that is reason enough
to get tested rather than continuing to experiment.

Frequently Asked Questions

How do I get rid of dust mites in India?

You reduce them rather than eliminate them — no home is mite-free. The measures with the best evidence are allergen-proof encasings on mattress and pillows, weekly washing of bedding at around 60°C, and keeping indoor humidity down, since mites depend on moisture. Reducing soft furnishings in the bedroom and using a sealed HEPA vacuum help further.

What are the symptoms of dust mite allergy?

Sneezing, a blocked or runny nose, itchy or watery eyes, and sometimes cough or wheeze. The distinguishing feature is the pattern rather than the symptoms: worst on waking and in bed, better after leaving the room, better when away from home, and often worse in humid weather.

Do dust mites die in hot water?

Yes — washing at around 60°C kills mites and removes allergen. A cool wash removes some allergen but leaves the mites alive, so weekly hot washing of sheets and pillowcases is materially more effective than washing more often at a lower temperature.

Does an air purifier help with dust mite allergy?

Partly, and it should not be the first purchase. Mite allergen is relatively heavy and spends most of its time settled in bedding and mattresses rather than airborne, so encasings and hot washing address the reservoir while a filter only handles what has been stirred into the air.

Are dust mites worse in humid cities?

Considerably. Mites absorb moisture from the air and struggle when humidity is persistently low, which is why they are a bigger problem in Mumbai, Chennai, Kolkata and coastal Kerala than in Delhi during a dry winter — and why symptoms often worsen everywhere during monsoon.

What GRAP Is

GRAP — the Graded Response Action Plan — is the emergency framework used in
Delhi-NCR when air quality deteriorates. Rather than a single set of rules, it is a ladder:
as air worsens, progressively stricter measures come into force, and as it improves, they
relax.

It is administered by the Commission for Air Quality Management (CAQM), and
the specific measures at each stage have been revised between seasons. That matters for a
practical reason: lists published in previous years are frequently out of date,
and the version circulating on social media in November is often last year’s.

The Four Stages

What Triggers a Stage

Broadly, the stages follow the CPCB air quality index categories — Poor, Very Poor, Severe
and Severe Plus — with each successive band invoking the next stage.

The mechanism has evolved in one significant way worth knowing: CAQM has moved toward
invoking stages on forecast rather than waiting for measured air to cross a
threshold. The reasoning is straightforward — measures like halting construction take time to
have any effect, so acting after the air is already severe is acting too late.

In practice this means a stage can be announced on a day that does not yet feel that bad.
That is the system working as intended, not an error.

What It Means for Your Household

GRAP is aimed at emission sources, not at households. But the announcements are a useful
signal, because they tell you what the coming days are expected to look like.

• Stage 1 announced — the season has started in earnest. If sealing and
  filter replacement are not done, do them now.
• Stage 2 — drop the routine afternoon ventilation to a shorter window.
  Move outdoor exercise indoors.
• Stage 3 — treat it as a stay-indoors period. Expect school disruption at
  short notice and have a plan for children at home. Filtration running in occupied rooms.
• Stage 4 — the severe-day protocol: everything shut, no indoor combustion,
  fitted N95 outdoors, minimum time outside.

What GRAP Doesn’t Do

Two honest limitations, both worth understanding.

It is a response, not a standard. GRAP manages severe episodes. Air well
below the level that triggers Stage 1 remains substantially above WHO guideline values. When
restrictions lift, the emergency has passed — the air is not good.

It does not address indoor air at all. Every measure targets outdoor
emission sources. Nothing in GRAP touches cooking emissions, incense, infiltration through
unsealed windows, or whether the room you sleep in is filtered. That part remains entirely a
household matter, and it is the part you actually control.

Frequently Asked Questions

What is GRAP in air pollution?

GRAP stands for the Graded Response Action Plan — a set of emergency measures for Delhi-NCR that escalate in stages as air quality worsens. It is administered by the Commission for Air Quality Management, and each stage adds restrictions on activities that contribute to particulate pollution.

What is banned under GRAP Stage 3?

Stage 3 typically halts non-essential construction and demolition, closes stone crushers and mining, restricts certain categories of older and more polluting vehicles, and often moves younger school years online. The exact measures are revised by CAQM between seasons, so check the current order rather than relying on last year’s list.

What AQI level triggers each GRAP stage?

Broadly, the stages follow the CPCB air quality categories: Stage 1 with ‘Poor’ air, Stage 2 with ‘Very Poor’, Stage 3 with ‘Severe’ and Stage 4 with ‘Severe Plus’. CAQM has revised both the thresholds and the trigger mechanism over time — including moving toward invoking stages on forecast rather than only on measured air — so treat the bands as the framework rather than a fixed rulebook.

Does GRAP mean the air is safe once restrictions lift?

No. GRAP is a response to severe episodes, not a standard for clean air. Air quality well below the level that triggers Stage 1 is still substantially above WHO guideline values. Restrictions lifting means the emergency has passed, not that the air is good.

Do schools close under GRAP?

Often at the higher stages, and it is usually a state or district decision taken alongside GRAP rather than an automatic consequence of it. Younger year groups are typically moved online first. Announcements tend to come at short notice, which is why having a plan for children at home is worth arranging in advance.

Why Outdoor AQI Isn’t Your Answer

Almost everyone checking air quality opens an app showing a city AQI figure. Useful for
deciding whether to go running. Close to useless for deciding anything about your home.

That number comes from a monitoring station somewhere in the city, and it describes outdoor
air at that location. Your bedroom differs from it in both directions:

• Better, if the room is sealed and filtered — indoor levels can sit well
  below outdoor.
• Worse, if you are cooking, burning incense or a mosquito coil, or have
  just finished a renovation. Indoor sources can push a single room far above the city
  figure.

What a Home Monitor Measures

Most consumer devices report some subset of these:

If a device measures only one thing, PM2.5 is the one to have. If it measures two, PM2.5 and
CO2 together cover the two genuinely distinct problems: what is in the air, and
whether the room is ventilated. A purifier addresses the first and does nothing
for the second.

How Accurate Are Cheap Monitors?

Worth being straightforward about this, because both the marketing and the backlash overstate
their case.

Consumer monitors almost all use laser light-scattering: air is drawn past a
laser, particles scatter the light, and the device infers particle count and estimates mass. It
is an inference, not a direct measurement. Reference-grade instruments used at regulatory
stations work differently and cost orders of magnitude more.

Practical consequences:

• Absolute values can be off, sometimes substantially, and they drift with
  humidity — high humidity tends to inflate readings.
• Relative changes are reliable. If it reads 40 and jumps to 180 when you
  start frying, that jump is real even if neither figure is exact.
• Consistency beats accuracy for household use. The same device in the same
  place, watched over time, tells you what you need.

Reading the Numbers

India’s AQI bands — Good, Satisfactory, Moderate, Poor, Very Poor, Severe — are designed for
outdoor reporting. Applied indoors they are a reasonable rough guide, with two caveats worth
holding on to.

First, lower is always better. There is no threshold below which fine
particulate becomes harmless; WHO guideline values are considerably stricter than most national
standards, including India’s.

Second, compare against outdoors rather than against an ideal. Most Indian
homes will not achieve single-digit PM2.5. A realistic and genuinely valuable goal is a bedroom
running well below the outdoor figure during the bad months — and knowing that it is.

Checking Without Buying Anything

Two free diagnostics that tell you a surprising amount:

The window-sill test. Wipe a road-facing sill completely clean and note the
date. Wipe an interior surface far from any window equally clean. Compare after a week. If the
sill is dramatically dirtier — especially with black deposit — outdoor infiltration dominates,
and sealing matters more than filtering.

The return test. Leave the house for a few hours and pay attention to the
first breath on returning. You acclimatise to your own home’s air within minutes, so that first
impression — musty, smoky, stale — is the most honest reading you will get without a device.

Using It to Actually Change Something

A monitor that produces numbers nobody acts on is an expensive ornament. The value is in
running specific tests, once:

1. Baseline. Leave it in the bedroom for two days. Note the overnight
   pattern.
2. Cooking test. Watch it through a full cooking session with the extractor
   off, then repeat with it on. Most people find this the most persuasive thing the device ever
   does.
3. Sealing test. Read the bedroom before and after weather-stripping the
   window. This tells you whether infiltration is your problem.
4. Purifier test. Closed room, purifier on, watch how long it takes to fall
   and how far. If it barely moves, the unit is undersized or the filter is spent.
5. Filter check. Repeat the purifier test every couple of months. When
   clearing takes noticeably longer than it used to, the filter is done — regardless of what the
   indicator light says.

Frequently Asked Questions

How can I check the air quality inside my house?

A consumer PM2.5 monitor is the practical option — it measures particulate directly in the room rather than reporting a city-wide outdoor figure. Placed sensibly and used to watch changes rather than to obtain a precise absolute number, it tells you what outdoor AQI apps cannot.

Is indoor air worse than outdoor air?

Often, and that surprises people. Homes add their own sources — cooking, incense, candles, cleaning products, off-gassing furniture — on top of outdoor particulate that infiltrates continuously. During and after cooking, a kitchen can be the worst air in the building even on a moderate outdoor day.

Are cheap air quality monitors accurate?

For absolute precision, no. Most consumer devices use laser light-scattering, which estimates particle mass rather than measuring it directly, and readings drift with humidity. For the purpose that matters — spotting changes, seeing what cooking does, confirming a purifier works — they are accurate enough, provided you treat trends as the signal rather than any single figure.

What is a good indoor PM2.5 level?

Lower is better and there is no threshold below which particulate is harmless. As a practical benchmark, WHO guidance puts the recommended annual average well into single digits in micrograms per cubic metre. Most Indian homes will not hold that year-round; a realistic goal is keeping the room you sleep in substantially below the outdoor figure.

Do I need an air quality monitor if I have an air purifier?

Not strictly, but it is the only way to know whether the purifier is doing anything in your particular room. Built-in indicators on purifiers are often coarse and self-reported. An independent monitor is what tells you whether the unit is correctly sized and whether the filter still works.

Do the Cheap Things First

Before the appliance, three things cost far less and matter at least as much. A purifier
added on top of an unsealed room with incense burning next door is doing remedial work it should
not have to.

• Seal the room. Weather-strip the window frame; fit a seal at the door
  bottom. A few hundred rupees, and it reduces the inflow the purifier would otherwise fight
  continuously.
• Remove combustion from the home. Incense, mosquito coils, candles,
  indoor smoking. A mosquito coil in a small closed room is a substantial particulate source,
  and it is often burning in exactly the room you are trying to protect.
• Extract cooking properly. In a flat, kitchen emissions reach the nursery.
  Chimney on before the oil goes in, and for ten minutes after.

Sizing for a Nursery

Sizing is where most purchases go wrong, and nurseries are small enough that getting it right
is genuinely easy.

Work from CADR — Clean Air Delivery Rate, in cubic metres per hour — against
your room’s volume. For an infant’s room, aim for roughly five air changes per
hour rather than the two or three used in comfort-oriented marketing claims.

Volumes assume a 10 ft ceiling. Then apply the correction that matters: manufacturers
calculate coverage area at lower air-change rates, so treat the printed coverage as
roughly double the room the unit genuinely handles in polluted conditions.

Noise — The Spec Parents Underrate

This is the specification that decides whether the purifier gets used, and it is almost
always assessed wrongly.

Most units publish a noise figure at their lowest speed, where clean-air delivery is
a fraction of the headline CADR. The relevant question is not “how quiet is it?” but “how
quiet is it at the speed that actually delivers the air this room needs?”

A purifier too loud to run overnight gets switched off at bedtime — which is precisely the
nine to eleven hours it was bought for. Buying a slightly larger unit and running it at a lower
speed is usually the better answer: more delivery, less noise, longer filter life.

What to Avoid Outright

• Ozone generators. Sometimes sold as “activated oxygen”. Ozone is a
  respiratory irritant. Not in any occupied room, and emphatically not in a nursery.
• Ionisers as the purification method. They charge particles so they settle
  onto surfaces rather than removing them, and some produce ozone as a by-product. As a
  switchable extra on a good HEPA unit, simply leave it off.
• “HEPA-type” and “HEPA-style”. Not the standard. Look for a stated
  H13 or H14 grade.
• Fragrance or “aroma” functions. Adding scent compounds to the air of a
  room you are filtering is working against yourself.

Placement and Running Pattern

• Give it clearance. Roughly a metre of clear space around the intake and
  outlet. Pushed against a wall or behind the cot, its effective delivery drops.
• Not next to the cot. Cables, stability, and direct airflow onto an infant
  are all reasons to keep some distance.
• Run it continuously during the season, rather than switching it on at
  bedtime. A room takes time to clear, and starting at bedtime means the first hours of sleep
  happen in unfiltered air.
• Door closed. The whole design assumes a closed room.

Filters and Running Cost

Check the replacement filter’s price and rated life before buying the unit, and
multiply across three years. Some inexpensive purifiers are sold on the printer-cartridge model,
where filters cost more over three years than the machine did.

In a polluted winter, a filter rated for a year of “normal” use can saturate in a single
season. A washable pre-filter, rinsed monthly, meaningfully extends the life of the expensive
HEPA behind it.

What a Purifier Will Not Fix

Worth being clear, because these are common and expensive misunderstandings:

• Stuffiness. A purifier does not add oxygen or remove carbon dioxide. A
  sealed nursery still needs ventilating — briefly, in the afternoon.
• Damp and mould. Filtering spores does not address the moisture producing
  them. Fix the damp.
• Dust already on surfaces. Only airborne particles pass through the
  filter.
• A baby who is unwell. Persistent cough, any breathing difficulty, or
  fast or laboured breathing needs a doctor — promptly, and not an appliance.

Frequently Asked Questions

Do babies need an air purifier?

Need is a strong word. An infant spends nearly all their time in one room, breathes faster than an older child, and has developing lungs — so filtering that room is a reasonable and proportionate step in a polluted city. It sits behind sealing the room and removing indoor combustion sources, both of which cost far less.

What size air purifier do I need for a baby’s room?

Work from the room’s volume rather than the box’s coverage claim. For a typical 10 x 12 ft nursery with a 10 ft ceiling — about 34 cubic metres — aim for a CADR around 170 m³/h to achieve roughly five air changes an hour. Manufacturers quote coverage at lower air-change rates, so treat the printed area as about double the room it truly handles.

Are ionisers and UV safe in a baby’s room?

Avoid ionisers and anything marketed as an ozone or ‘activated oxygen’ generator. Ozone is a respiratory irritant and an infant’s airways are the last place to introduce one. A UV lamp inside a domestic purifier is generally harmless but contributes very little, because air passes it far too quickly. A plain HEPA and carbon unit is the right choice.

How loud should an air purifier be for a nursery?

Quiet enough to run continuously on a setting that actually delivers clean air. Many units are only quiet on their lowest speed, where the effective delivery is a fraction of the headline figure. Check the noise figure at the speed you would genuinely use overnight, not at the lowest one.

Where should I place an air purifier in a baby’s room?

Away from the wall with clear space around the intake and outlet, and not tucked behind furniture or the cot. Keep it out of reach of the cot for cable and stability reasons. Central placement with a metre or so of clearance works better than a corner.

A Third of Your Life, One Room

Most air quality advice is about the outdoors, or about the home in general. The bedroom
deserves separating out for a simple reason: it is where you spend the longest continuous,
entirely predictable block of time, in a small closed space, with your breathing rate steady and
your face inches from a large textile reservoir.

That combination makes it the highest-leverage room in the house. It is also the easiest to
control — one door, one window, one bed.

What Happens to the Air Overnight

Three processes run through the night, and they pull in different directions.

• Outdoor pollution peaks. Cool, still night air traps particulate near the
  ground. Outdoor levels are typically at their worst overnight and in the early morning —
  exactly when the window is likeliest to be open.
• Allergen gets stirred up. Every turn disturbs bedding and lifts settled
  dust mite allergen into the air immediately around your face.
• Carbon dioxide accumulates. In a closed room with the door shut, CO2 rises
  steadily through the night.

The awkward part is that the fix for the first pulls against the fix for the third.

The CO2 Problem Nobody Mentions

Air purifier marketing does not mention carbon dioxide, and there is a reason: a
purifier does nothing about it whatsoever. It recirculates and filters the air already
in the room. It does not add oxygen and it does not remove CO2.

In a closed bedroom with one or two people breathing, CO2 climbs through the night. Studies
have associated elevated bedroom CO2 with poorer sleep quality and reduced next-day alertness.
It is not dangerous at household levels — but “I slept badly in a stuffy room” has a measurable
basis.

This is why the sensible pattern is not simply “seal the room and filter it”:

• Ventilate properly before bed — ten to fifteen minutes with the window
  open, when outdoor air is at its least bad, then close it.
• Leave the door slightly ajar if the rest of the home is filtered, giving
  a larger air volume to draw on.
• Ventilate again in the afternoon, when outdoor levels are at their daily
  best.

Why Your Nose Blocks at Night

Extremely common, and it has two layers.

The baseline is physiology. Lying flat increases blood flow to the nasal
lining, which swells slightly and narrows the airway. This happens to everyone and is not a
problem in itself.

The addition is often allergen. If you are reacting to something in the
bedroom, that inflammation adds to the baseline and tips a mild narrowing into a genuinely
blocked nose.

The way to tell them apart is the travel test: if your nose blocks equally in a hotel room,
it is physiology or a nasal issue worth taking to a doctor. If it is markedly better away from
your own bed, the bed is the variable.

Setting the Room Up

1. Encase the mattress and pillows. If dust mite allergen is a factor, this
   is the highest-return single change — a barrier between you and the largest reservoir in the
   room.
2. Hot-wash bedding weekly. Around 60°C. Pillowcases matter most; your face
   is on them all night.
3. Seal the window frame and door bottom. Cuts overnight infiltration during
   the eight hours you are most exposed.
4. Nothing that burns. No mosquito coils, no incense, no candles. A coil
   burning in a closed bedroom is a significant continuous particulate source in exactly the wrong
   place.
5. Pets out. If anyone in the room has allergies, this is not negotiable and
   it is the most effective single measure.
6. Filter the room, sized for its volume, door closed, running before you go
   to bed rather than started at bedtime.

The Ventilation Trade-off

Anyone who reads carefully will notice the tension: seal the room against outdoor pollution,
but ventilate it so CO2 does not accumulate. Both are true, and the resolution is timing rather
than choosing a side.

What to Realistically Expect

Worth being honest, because sleep is a category where claims outrun evidence.

If your sleep is disrupted by a blocked nose, a night cough or allergy symptoms,
reducing bedroom triggers can make a genuine, noticeable difference — and encasings plus hot
washing plus filtration is a reasonable fortnight-long experiment.

If your sleep problem is insomnia, stress, screens, caffeine, an irregular schedule
or a sleep disorder, air quality is not the variable, and no purifier will change it.
Loud snoring with daytime sleepiness in particular should go to a doctor — sleep apnoea is
common, under-diagnosed, and unrelated to what is in the air.

The bedroom is worth optimising. It is not a treatment for every kind of bad night.

Frequently Asked Questions

Does air quality affect sleep?

There is reasonable evidence that it does. Poor air is associated with more disrupted sleep, and for anyone with allergies or asthma the mechanism is obvious — a blocked nose or a night-time cough fragments sleep directly. Carbon dioxide build-up in a closed room has also been linked with poorer sleep quality in controlled studies.

Should I sleep with the window open or closed?

In a high-pollution city, generally closed — outdoor particulate peaks overnight when cool still air holds it near the ground, so an open window admits the worst air of the day for eight hours. The trade-off is carbon dioxide build-up, which is why ventilating the room well before bed and again in the afternoon matters.

Why does my nose block up as soon as I lie down?

Partly normal physiology — lying flat increases blood flow to the nasal lining and narrows the airway on its own. If it happens every night in the same bed and clears when you are elsewhere, a bedroom allergen such as dust mite is likely adding inflammation on top of that baseline.

Will an air purifier help me sleep better?

If your sleep is disrupted by allergies, a blocked nose or a night cough, reducing airborne triggers can help — and it is worth trying for a fortnight before deciding. It will not fix insomnia, poor sleep hygiene, or a sleep disorder, and a purifier too loud to run overnight makes things worse rather than better.

Is CO2 build-up in a bedroom dangerous?

Not dangerous at the levels reached in a normal bedroom, but not irrelevant either. Carbon dioxide rises through the night in a closed occupied room, and studies have linked elevated levels with poorer sleep quality and next-day alertness. An air purifier does not help — only fresh air does.

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

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.

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.

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.