Reference library
What is in my air?
13indoor air pollutants, each with India’s NAAQS limit alongside WHO’s health-based guideline, the particle size or molecular scale that decides whether filtration can touch it, what a consumer monitor is really telling you, and which technology genuinely helps.
Air purifiers are sold on a single number too — CADR, or coverage in square feet — and like the TDS meter it answers one narrow question well and every other question badly. It tells you how fast a machine can clean particles from a sealed chamber. It says nothing about whether particles are your problem.
This library separates the pollutants by what they physically are, because that is what decides everything downstream. Particles can be filtered mechanically; gases cannot. Molecules of carbon monoxide pass through every filter medium in a domestic purifier. Allergen from dust mites settles out of the air within minutes and is therefore almost untouched by filtration, while cat allergen stays suspended for hours and is genuinely reduced by it.
Each entry follows the same order: the limits and who set them, the scale of the thing, where it comes from, what you can and cannot detect, what removes it, what does not, and — for indoor pollutants especially — the source control that beats filtration.
What each technology actually does
This is the whole category in one table. Almost every purchasing mistake in air purification is a mismatch between a row and a column.
| Technology | Particles | Gases & odours | Carbon monoxide | Adds anything? |
|---|---|---|---|---|
| HEPA (H13+) | Excellent | None | None | No |
| Activated carbon (by mass) | None | Good, if enough carbon | Negligible | No |
| Chemisorbent (KMnO₄ / treated) | None | Good for formaldehyde | None | No |
| Pre-filter | Coarse only | None | None | No |
| UV-C inside a purifier | None | None | None | Ozone if 185 nm lamp |
| Ioniser | Deposits, not removes | Minimal | None | Ozone, varies by design |
| Ozone generator | None | Reacts, creates by-products | None | Ozone — a regulated pollutant |
| Ducted extraction at source | Excellent at source | Excellent at source | Excellent | No |
“Good, if enough carbon” is doing real work in that table. Gas removal scales with carbon mass and contact time, and the difference between a carbon-coated mesh weighing tens of grams and a granular bed weighing kilograms is the difference between a deodoriser and a filter. Ask manufacturers for the carbon mass in grams.
Particulate matter
2 entries
- PM2.5PM2.5 is airborne particulate matter with an aerodynamic diameter of 2.5 micrometres or less — small enough to stay suspended for hours and to reach deep into the lungs.≤ 2.5 µm aerodynamic diameter
- PM10PM10 is airborne particulate matter with an aerodynamic diameter of 10 micrometres or less, which includes the PM2.5 fraction and adds the coarser dust particles above it.≤ 10 µm
Chemical vapours
2 entries
- Volatile Organic Compounds (VOCs)Volatile organic compounds are carbon-based chemicals that evaporate at room temperature — a class of hundreds of substances rather than a single pollutant, ranging from harmless to genuinely hazardous.Individual gas molecules
- FormaldehydeCH₂OFormaldehyde is a colourless gas with a sharp odour, released principally by the urea-formaldehyde adhesives in plywood, MDF and particleboard, and one of the few indoor pollutants with a specific WHO guideline value.A single small gas molecule
Combustion products
4 entries
- Carbon MonoxideCOCarbon monoxide is a colourless, odourless gas produced by incomplete combustion, which binds haemoglobin far more strongly than oxygen does and can be fatal at concentrations that give no sensory warning.A single small gas molecule. No filter medium in a domestic purifier interacts with it.
- Nitrogen DioxideNO₂Nitrogen dioxide is a reddish-brown reactive gas produced by high-temperature combustion, with traffic dominating outdoors and gas cooking dominating indoors.A reactive gas molecule.
- Cooking EmissionsCooking emissions are the mixture of fine particulates, combustion gases and organic vapours generated by high-temperature cooking, which in most Indian homes produce the day's highest indoor pollutant concentrations.Predominantly PM2.5 and ultrafine particles below 0.1 µm, plus NO₂ and CO where gas is the fuel, and aldehydes and other VOCs from heated oil.
- SmokeSmoke is the aerosol produced by incomplete combustion of organic material, combining very high fine-particle concentrations with a substantial gas and vapour fraction that filtration alone does not address.Predominantly PM2.5 and ultrafine particles, plus VOCs, aldehydes, PAHs and carbon monoxide in the gas phase.
Reactive gases
1 entry
Biological allergens
4 entries
- PollenPollen grains are plant reproductive particles, typically 10 to 100 micrometres across, and among the easiest airborne allergens to filter because of their size.10–100 µm for intact grains; allergen-carrying fragments can be well under 2.5 µm.
- Dust Mite AllergenHouse dust mites are microscopic arachnids living in bedding, upholstery and carpets, whose faecal particles carry the allergen responsible for a large share of perennial allergic rhinitis and asthma.Allergen-carrying particles 10–40 µm
- Pet DanderPet dander is the shed skin, saliva proteins and sebaceous secretions of animals — with the principal cat allergen, Fel d 1, notable for being small, sticky and persistently airborne.Cat allergen: a large fraction below 5 µm and much of it under 2.5 µm. Dog allergen is generally somewhat larger. Both are far smaller than dust mite allergen.
- Mould SporesMould spores are the airborne reproductive units of fungi, always present indoors at low levels, and multiplying wherever surfaces stay damp for more than a couple of days.2–10 µm for most spores; fungal fragments and mycotoxin-bearing particles can be considerably smaller.
What to buy first, in order
If you are spending on indoor air quality in an Indian home, this ordering gets more per rupee than starting with a purifier. Three of the four cost less than a mid-range unit, and two address problems no purifier can touch.
- A carbon monoxide alarm, if you have any gas appliance — especially a bathroom geyser. ₹1,500–3,500, EN 50291 or UL 2034 certified.
- A kitchen chimney that ducts outdoors. Check yours; many Indian flat installations recirculate, which returns combustion gases to the room. This is usually the largest single exposure in the household.
- Draught-proofing the largest gaps — under-door gaps, unused exhaust openings, window frames. A few hundred rupees, and it raises the effectiveness of everything that comes after it.
- One correctly sized purifier for the bedroom, run with the door closed. Size for four to five air changes per hour, not two.
Frequently asked questions
What does an air purifier actually remove?
Do air purifiers actually work in Indian homes?
Are ionisers and ozone generators safe?
Is India's air quality standard the same as WHO's?
What should I buy first for indoor air quality?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
National Ambient Air Quality Standards (NAAQS), notification dated 18 November 2009
Central Pollution Control Board, Ministry of Environment, Forest and Climate Change · published 2009-11-18 · checked 2026-08-01
India's legally applicable ambient air standards. They are ambient (outdoor) standards — India has no equivalent statutory indoor air quality standard for homes.
World Health Organization · published 2021-09-22 · checked 2026-08-01
The 2021 revision tightened most values substantially. WHO guideline values are health-based targets, not regulatory limits, and are considerably stricter than India's NAAQS.
WHO Guidelines for Indoor Air Quality: Selected Pollutants
World Health Organization · published 2010-01-01 · checked 2026-08-01
Covers formaldehyde, benzene, carbon monoxide, naphthalene, nitrogen dioxide, PAHs, radon, trichloroethylene and tetrachloroethylene specifically as indoor pollutants.
NAAQS values are ambient standards. India has no statutory indoor air quality standard for homes, so on indoor pollutants the ambient limits and WHO’s indoor guidelines are the only reference points available — and both are stated on every page rather than blended into a single number.