What's in my air
Nitrogen dioxide — the reason your kitchen needs an exhaust, not a purifier
Nitrogen dioxide is a reddish-brown reactive gas produced by high-temperature combustion, with traffic dominating outdoors and gas cooking dominating indoors.
Limits and standards
India's NAAQS are ambient (outdoor) standards, and there is no statutory Indian indoor air quality standard for homes. WHO's guideline values are health-based targets rather than regulatory limits, and are consistently stricter.
| Standard | Limit | What it means |
|---|---|---|
| India NAAQS — annual average | 40 µg/m³ | — |
| India NAAQS — 24-hour average | 80 µg/m³ | — |
| WHO 2021 guideline — annual | 10 µg/m³ | — |
| WHO 2021 guideline — 24-hour | 25 µg/m³ | — |
| WHO indoor guideline — 1-hour | 200 µg/m³ | — |
Where it comes from
Gas cooking
The dominant indoor source. An LPG or piped-gas hob produces NO₂ directly at the burner, and in a kitchen without effective extraction it accumulates rapidly.
Traffic
The dominant outdoor source, and it infiltrates indoors. Road-facing flats on busy streets show substantially higher indoor NO₂ than those facing away.
Generators
Diesel generators produce NO₂ alongside particulates and CO — relevant in buildings with generators in basements or courtyards.
Unflued heaters
Kerosene and LPG room heaters produce NO₂ directly into the room.
Why it matters
- WHO's 2021 revision cut the annual guideline from 40 to 10 µg/m³ on the strength of evidence linking NO₂ to respiratory effects, particularly in children and people with asthma.
- Indoor exposure during cooking can be the highest NO₂ exposure a household experiences, and it happens in the room where people also spend time.
- It matters most for households with an asthmatic member, where the association with symptom exacerbation is best established.
- It is also a marker: where NO₂ is high indoors from combustion, PM2.5 and CO are usually elevated too, from the same source.
Note
Can you detect it yourself?
Consumer air quality monitors are genuinely useful for tracking change and poor at absolute measurement. Knowing which of the two you are looking at prevents most of the disappointment in this category.
| Question | Answer |
|---|---|
| Can you sense it? | A sharp, acrid smell at higher concentrations, and reddish-brown in very high concentration. At typical indoor concentrations, undetectable. |
| What a consumer monitor shows | NO₂ sensors in consumer monitors are electrochemical and cross-respond to ozone in particular. Treat readings as indicative. Many general monitors do not measure NO₂ at all. |
| Field test | Passive diffusion tubes analysed by a laboratory give reliable weekly averages and are used in air quality studies. Available in India through some environmental laboratories. |
What removes it
Effectiveness ratings below describe the technology working correctly, within its service life, on water or air that suits it. “Depends” means there is a condition in the caveat that decides the outcome — read it before buying.
- Ducted kitchen extraction vented outdoorsEffective
The correct intervention. Capturing combustion products at the burner before they enter room air beats every downstream approach.
Caveat: It must actually vent outside. A recirculating chimney passes air through a carbon filter and returns it to the kitchen, doing almost nothing for NO₂.
- VentilationEffective
Opening a kitchen window while cooking is free and materially effective.
Caveat: Trades against outdoor particulates on high-AQI days — which is a real dilemma in a north Indian winter, and the honest answer is to extract mechanically rather than choose.
- Cooking on the back burnersPartial
Rear burners sit further under the extraction hood, and capture efficiency is substantially higher there than at the front.
- Activated carbon with impregnated mediaMinimal
Some impregnated carbons adsorb NO₂ to a degree.
Caveat: Capacity is limited and this is not what domestic purifier carbon is optimised for. Do not buy a purifier to fix a gas-hob NO₂ problem.
- Induction cookingEffective
Eliminates the source entirely. The most effective single change available, though rarely made for air quality reasons alone.
What does not work
Each of these is commonly assumed — or actively marketed — to help with nitrogen dioxide. None of them do.
HEPA filtration
A gas. HEPA has no effect.
Recirculating kitchen chimneys
They filter grease and some odour and return the air to the kitchen. Combustion gases go straight back into the room. This is the most consequential misunderstanding about kitchen ventilation in Indian flats.
Ionisers and ozone devices
No useful removal, and ozone chemistry with NOx produces further pollutants.
Source control that beats filtration
For most indoor pollutants the highest-leverage intervention is not a device. These are the changes worth making before, or instead of, buying anything.
- Use the kitchen exhaust every time the hob is on — from the moment you light it, not once smoke appears.
- Verify your chimney actually ducts outside. Many Indian flat installations are recirculating, and the difference is invisible from the kitchen.
- Cook on rear burners where possible; extraction capture is much better there.
- Never use an unflued gas or kerosene heater in a closed room.
- For a road-facing flat, keep street-side windows closed during peak traffic and ventilate from the other side.
Frequently asked questions
Does a gas stove pollute indoor air?
Does an air purifier remove nitrogen dioxide?
Is a recirculating kitchen chimney good enough?
What is a safe NO2 level indoors?
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.
This page states what published standards say and what the physics of each technology allows. It does not state what is in your supply — no page can. Where the two questions meet is a test result, and every entry in this library says which test to ask for.