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.
The core principle: if you can see it or it makes you sneeze, a purifier
can probably remove it. If it is a gas, the purifier reduces it at best, and for carbon
monoxide, not at all.
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.
Undersizing is the most common reason air purifiers disappoint. Work out the CADR your room actually needs — the coverage area on the box assumes a lower air-change rate than an Indian winter requires.
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.