What's in my air

PM2.5 — the pollutant every air purifier is actually sold against

PM2.5 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 diameterLast reviewed 2026-08-08

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.

Published limits for PM2.5
Standard	Limit	What it means
India NAAQS — annual average	40 µg/m³	—
India NAAQS — 24-hour average	60 µg/m³	May be exceeded on 2% of days in a year, but not on two consecutive days.
WHO 2021 guideline — annual	5 µg/m³	—
WHO 2021 guideline — 24-hour	15 µg/m³	WHO also publishes interim targets, recognising that many countries cannot reach the guideline quickly. India's NAAQS annual value sits near WHO Interim Target 1.

Published limits for PM2.5

Where it comes from

Combustion
Vehicles, industry, thermal power, brick kilns and biomass burning. The dominant outdoor source in Indian cities.
Crop residue burning
Seasonal and regionally huge — the north Indian October–November episode is the clearest example in the world of a source that dwarfs everything else for a few weeks.
Road and construction dust
Contributes more to PM10 than PM2.5, but resuspension of fine dust is a substantial and often underestimated PM2.5 source.
Indoor cooking
Frying, tadka and any high-temperature cooking generate PM2.5 directly in the room. Indoor peaks during cooking routinely exceed simultaneous outdoor levels.
Secondary formation
Sulphates, nitrates and organics forming in the atmosphere from gaseous precursors — a large share of urban PM2.5 that is not emitted as particles at all.

Why it matters

WHO's 2021 revision was driven by evidence of health effects at concentrations far below the previous guideline, which is why the annual value was cut from 10 to 5 µg/m³.
Particles this small deposit in the deepest parts of the lung rather than being cleared in the upper airway.
The exposure that matters is the annual average, not the worst day. A month of clean air does not offset eleven months at 90 µg/m³.
Indoor exposure dominates total exposure for most people simply because most people spend most of their time indoors — which is what makes indoor filtration worth doing at all.

Note

Health statements on this page describe what the standards bodies cited in the Sources section associate with exposure above their limits. They are not a diagnosis, and they are not a statement about your water or your air — only a test tells you that.

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.

How to detect PM2.5
Question	Answer
Can you sense it?	Visible as haze above roughly 100 µg/m³, and often smellable when the source is combustion. Below that, undetectable — and the 40–80 µg/m³ range that most Indian cities live in is largely invisible.
What a consumer monitor shows	Consumer monitors use optical particle counters: they shine a laser at the airstream and infer mass concentration from scattered light. That inference assumes a particle density and refractive index, so readings are indicative rather than reference-grade. They are genuinely useful for relative change — does the purifier help, does closing the window help — and should not be quoted as absolute measurements.
Field test	A ₹4,000–12,000 laser monitor is the most useful single purchase for anyone serious about indoor air. Place it away from windows and cooking, and compare readings over time rather than against a friend's different device.
Laboratory test	Gravimetric PM2.5 by reference method — Filter-based sampling and weighing, per CPCB reference methods (indicative ₹3,000–₹12,000)

How to detect PM2.5

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 filtration (H13 or better)Effective
≥99.95% at MPPS per EN 1822 H13
The core technology and it genuinely works. A correctly sized unit will drop room PM2.5 substantially within 20–40 minutes with the door closed.
Electrostatic precipitationPartial
Charges particles and collects them on plates. Efficiency falls sharply as plates load, and many designs produce ozone.
Caveat: Check for CARB certification or an equivalent ozone emission figure before buying.
Sealed HVAC filtration (MERV 13+)Effective
The best whole-home answer where a ducted system exists. Rare in Indian homes, standard in commercial buildings.
Mechanical ventilation with filtrationEffective
Fresh-air systems that filter incoming air address the root problem — infiltration — rather than continually re-cleaning the same room.
Closing windows during peak episodesPartial
Free, and among the highest-impact actions during a severe episode. Infiltration continues, but the rate drops several-fold.

What does not work

Each of these is commonly assumed — or actively marketed — to help with pm2.5. None of them do.

Ozone generators
They do not remove particles, and they add ozone — itself a NAAQS-regulated pollutant with its own health effects. There is no scenario in a home where this is the right device.
'Ionisers' used alone
Charged particles deposit on walls, floors and furniture rather than being removed from the room. Airborne measurements improve; the particles are still in your house, and many ionisers emit ozone.
Houseplants
The NASA study people cite was a sealed chamber experiment on gaseous VOCs, not particulates. The air-exchange rate needed to matter in a real room would require hundreds of plants.
Water-based 'air washers'
Water captures large particles reasonably and fine particles poorly. Against PM2.5 specifically, performance is far below HEPA.
Running a purifier with the door open
Not a technology failure but the most common usage failure. The unit's CADR is being asked to clean the whole house, and it cannot.

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 you cook, not only when something burns. Indoor cooking peaks are frequently the highest PM2.5 anyone in the household is exposed to all day.
Do not burn incense, mosquito coils or candles in a closed room — all are direct, potent PM2.5 sources.
Seal the largest infiltration gaps: under-door gaps, unused exhaust openings, window frames. This raises the effectiveness of everything else you do.
Check outdoor AQI before opening windows for ventilation. On a good morning, ventilation beats filtration; on a bad evening, it undoes it.

The 0.3 micron misunderstanding

HEPA filters are rated at 0.3 µm because that is approximately the Most Penetrating Particle Size — the hardest size to capture. It is the filter's worst case, not its limit.

Larger particles are captured by interception and impaction; they are too big to follow the airstream around the fibre. Smaller particles move erratically under Brownian motion and collide with fibres by diffusion. Around 0.3 µm, both mechanisms are at their weakest simultaneously.

So a filter rated 99.97% at 0.3 µm is typically better than 99.97% at 0.1 µm and better again at 0.05 µm. The widespread claim that HEPA 'cannot catch anything smaller than 0.3 microns' inverts the actual physics.

Note

This is worth knowing because it is used against HEPA to sell ionisers and other alternatives. The 0.3 µm figure is a worst-case test point chosen precisely because it is the hardest one to pass.

Sizing: what CADR actually has to be

Clean Air Delivery Rate is the volume of particle-free air a unit delivers per hour, measured under ANSI/AHAM AC-1. To size a purifier, work from air changes per hour.

Room volume in cubic metres × target air changes per hour = required CADR. For meaningful reduction during a pollution episode, aim for 4–5 air changes per hour rather than the 2 that many marketing claims assume.

A 200 sq ft room with a 10 ft ceiling is about 57 m³. At 5 air changes per hour that is roughly 285 m³/h of CADR — considerably more than the units usually recommended for that room size.

Room	Volume (approx.)	CADR for 3 ACH	CADR for 5 ACH
100 sq ft bedroom, 10 ft ceiling	28 m³	85 m³/h	142 m³/h
150 sq ft bedroom	42 m³	127 m³/h	212 m³/h
200 sq ft room	57 m³	170 m³/h	283 m³/h
350 sq ft living room	99 m³	297 m³/h	495 m³/h
500 sq ft open-plan area	142 m³	425 m³/h	708 m³/h

Frequently asked questions

What is a safe PM2.5 level?

WHO's 2021 health-based guideline is 5 µg/m³ annual and 15 µg/m³ over 24 hours. India's NAAQS limits are 40 µg/m³ annual and 60 µg/m³ over 24 hours. Meeting the Indian standard therefore still means air well above the health-based target — which is worth knowing when a city is described as 'within limits'.

Does a HEPA filter remove PM2.5?

Yes, very effectively. An H13-class filter removes at least 99.95% of particles at the Most Penetrating Particle Size, and PM2.5 is comfortably within the range it handles. Whether your room's PM2.5 actually falls depends on the unit's CADR relative to the room volume and on the door being closed.

Can HEPA filters catch particles smaller than 0.3 microns?

Yes, and generally better than it catches 0.3 micron particles. 0.3 µm is approximately the Most Penetrating Particle Size — the filter's worst case. Below it, Brownian diffusion causes particles to collide with fibres more readily, so capture efficiency rises again.

Do air purifiers actually work for PM2.5 in Indian homes?

Within one room, with the door closed, and correctly sized — yes, measurably and quickly. The common disappointments come from three things: a unit sized for 2 air changes per hour rather than 4 to 5, running it with doors open, and expecting one unit to cover a whole flat.

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.
WHO Global Air Quality Guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide
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.
EN 1822: High efficiency air filters (EPA, HEPA and ULPA)
European Committee for Standardization (CEN) · checked 2026-08-01
Defines HEPA classes by efficiency at the Most Penetrating Particle Size (MPPS), typically around 0.1–0.3 µm. H13 requires ≥99.95% at MPPS; H14 requires ≥99.995%.
ANSI/AHAM AC-1: Method for Measuring Performance of Portable Household Electric Room Air Cleaners
Association of Home Appliance Manufacturers · checked 2026-08-01
The standard that defines Clean Air Delivery Rate (CADR) and the test chamber conditions it is measured in. A CADR figure not measured to AC-1 or an equivalent standard is not comparable to one that was.

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.