Air
What is PM2.5?
PM2.5 is airborne particulate matter with a diameter of 2.5 micrometres or less — roughly a thirtieth the width of a human hair. It matters more than any other pollutant Delhi measures because particles that small are not filtered out by the nose and throat: they reach the deepest part of the lung and cross into the bloodstream. India's 24-hour standard is 60 µg/m³.
Why the size is the whole story
PM2.5 is not a substance. It is a size class, and everything in it gets there by being small.
The 2.5 refers to aerodynamic diameter in micrometres, not to what the particle is made of. Soot from a diesel engine, ash from a stubble fire, sulphate formed in the air from power-station emissions and dust ground fine by traffic are all PM2.5 if they are small enough. This is why the number can be high with no single visible cause.
Size decides where a particle stops. Anything above about 10 µm is caught in the nose and upper airway. Between 2.5 and 10 µm it reaches the bronchi. Below 2.5 µm it reaches the alveoli, where gas exchange happens, and the smallest fraction passes into the blood — which is how an air pollutant ends up implicated in heart attacks and strokes rather than only in coughs.
A large share of Delhi's PM2.5 is secondary: it was not emitted as a particle at all. Sulphur dioxide, nitrogen oxides and ammonia react in the atmosphere over hours to days and form sulphate, nitrate and ammonium particles far from where the gases were released. Secondary formation is the reason a city can cut its own tailpipe emissions and still have bad air.
- µg/m³
- Micrograms per cubic metre — mass of particles in a given volume of air. The unit every Indian air standard is written in.
- Primary particle
- Emitted as a particle: soot, ash, road dust.
- Secondary particle
- Formed in the atmosphere from gases. Not emitted by anything, and not reduced by filtering a chimney.
What India permits
Two figures, both notified under the National Ambient Air Quality Standards of 2009, and both routinely broken in Delhi in winter.
PM2.5, 24-hour mean
60 µg/m³
CPCB, NAAQS 2009
PM2.5, annual mean
40 µg/m³
CPCB, NAAQS 2009
PM10, 24-hour mean, for comparison
100 µg/m³
CPCB, NAAQS 2009
The World Health Organization's 2021 guideline is 15 µg/m³ over 24 hours and 5 µg/m³ annually. It is a guideline, not a law, and it is not what Indian regulation is measured against. Both numbers are worth knowing and they should never be quoted as though they were the same kind of thing: one is enforceable in India and one is a health target.
How it is measured, and how the number moves
The reading you see is a mass concentration averaged over a window, at one point in a city.
Continuous monitors measure PM2.5 either by beta attenuation — passing a beam through collected particles and reading the loss — or by an oscillating microbalance that weighs them directly. Both report mass per cubic metre. CPCB's live feed publishes the resulting sub-index rather than the raw concentration.
PM2.5 in Delhi is strongly seasonal and strongly meteorological. The same emissions produce a very different reading in November, when a shallow inversion layer traps air near the ground, than in July, when rain scavenges particles out of it. A fall in the number is not by itself evidence that anything was emitted less.
This site reads the worst reporting monitor in the city, named, rather than a city average — see what Delhi's AQI is for why the two differ.
What this number cannot tell you
- It cannot tell you what the particles are. Mass concentration treats soot and sea salt identically.
- It cannot tell you who emitted them. That requires source apportionment, which is a separate and contested exercise.
- It cannot be compared across averaging periods. A 24-hour mean and an hourly reading are different quantities and only one of them has a legal limit.
- It says nothing about the ultrafine fraction below 0.1 µm, which is not separately regulated in India and is not measured by the network.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- National Ambient Air Quality Standards, 2009 Central Pollution Control Board · The 60 and 40 µg/m³ figures above
- WHO global air quality guidelines, 2021 World Health Organization · The 15 and 5 µg/m³ health guidelines — not Indian law
- National Air Quality Index Central Pollution Control Board · How the concentration becomes an index value
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
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