What's in my air
Smoke — episodic, extreme, and the hardest test of a purifier
Smoke 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.
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 — PM2.5 24-hour | 60 µg/m³ | North Indian smoke episodes regularly produce 24-hour averages several times this figure, and hourly peaks far higher. |
| WHO 2021 guideline — PM2.5 24-hour | 15 µg/m³ | — |
Where it comes from
Crop residue burning
The October–November episode across Punjab, Haryana and western Uttar Pradesh, transported into the Delhi NCR airshed and beyond.
Firecrackers
Sharp, short and extreme. Diwali night produces some of the highest hourly concentrations recorded in Indian cities each year, with a distinctive metal-salt composition.
Waste burning
Municipal and garden waste burned locally — often the dominant source at neighbourhood scale, and the one most amenable to local action.
Biomass cooking and heating fuel
A year-round source in many households and a major contributor to winter smoke in smaller towns.
Forest fires
Seasonal in the Himalayan states, central Indian forests and the Western Ghats.
Why it matters
- Smoke episodes are when indoor filtration matters most, and they are also when it is most likely to be inadequate.
- The combination of very high particle load and a strong gas-phase component means a HEPA-only unit will clear the haze and leave the smell — which reads to the household as failure.
- Infiltration dominates during an episode. A leaky flat re-fills with smoke as fast as the purifier cleans it, and no amount of CADR fixes an open window.
- The exposure is short but intense, and it disproportionately affects people with asthma and other respiratory conditions.
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? | Highly detectable — visible haze, strong smell, throat and eye irritation. Smoke is the one episode type that people reliably notice without a monitor. |
| What a consumer monitor shows | Optical monitors track smoke particles well and the readings during an episode are dramatic. Useful for judging whether your sealing and filtration are working. |
| Field test | Compare indoor and outdoor readings during an episode. A well-sealed, filtered room should sit at a small fraction of outdoor; if it tracks outdoor closely, infiltration is your problem rather than filtration. |
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.
- HEPA at high CADREffective
Handles the particle fraction well. During an episode, size for 5+ air changes per hour rather than the usual 3.
- Substantial activated carbonEffective
Required for the smell, which is gas-phase. This is where thin carbon meshes are exposed as inadequate — the haze clears and the smoke smell remains.
- Sealing and infiltration reductionEffective
Draught excluders under doors, closing unused exhaust vents, taping obvious window gaps. Cheap, and during an episode it can matter more than the purifier.
- Creating one clean roomEffective
Concentrating filtration in a single closed bedroom rather than spreading it across the flat. Concentrating capacity in the room where you sleep is the highest-value use of a fixed budget.
- N95 masks outdoorsEffective
Not indoor air quality, but the correct complement during a severe episode. A well-fitted N95 handles smoke particles; a cloth or surgical mask does not.
What does not work
Each of these is commonly assumed — or actively marketed — to help with smoke. None of them do.
HEPA alone against smoke smell
The odour compounds are gases. The room gets clearer and continues to smell of smoke, which is the most common complaint during episodes.
Ionisers
Overwhelmed by episode-level particle loads, and they deposit smoke residue on surfaces rather than removing it.
Ozone generators
Ozone reacting with smoke VOCs produces further irritants including formaldehyde. Actively worse during a smoke episode.
Undersized units in open-plan space
Not a technology failure but the standard usage failure, and it is most visible during episodes.
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.
- Buy filters before the season starts. During an episode, stock disappears and prices rise.
- Seal the largest infiltration paths — under-door gaps and unused exhaust openings — before the season rather than during it.
- Designate one clean room, usually the bedroom, and concentrate capacity there.
- Do not burn waste, and challenge burning in your neighbourhood — at local scale it is often the dominant source.
- Check AQI before ventilating. During an episode, keep windows closed and rely on filtration.
Frequently asked questions
Does an air purifier help with crop burning smoke?
Why does my room still smell of smoke after running the purifier?
How do I prepare for Diwali or crop-burning season?
Do N95 masks work against smoke?
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.
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.