Air
What is PM10?
PM10 is airborne particulate matter with a diameter of 10 micrometres or less. It includes everything counted as PM2.5 and adds the coarse fraction between 2.5 and 10 µm — road dust, construction dust, soil, pollen, material ground rather than burned. India's 24-hour standard is 100 µg/m³.
The coarse fraction, and why it is a different problem
PM10 and PM2.5 are not two pollutants. One contains the other, which is the first thing people get wrong about them.
Because PM10 is defined as everything at or below 10 µm, every PM2.5 particle is also a PM10 particle. A city cannot have more PM2.5 than PM10, and the difference between the two readings is the coarse fraction: the part between 2.5 and 10 µm.
That coarse fraction comes overwhelmingly from mechanical processes rather than combustion. Vehicles grinding road surface, tyres and brakes, unpaved shoulders, construction and demolition, and windblown soil from open ground. It is heavier, it settles faster, and it travels less far — which makes it a much more local problem than PM2.5.
It also responds to different action. Water-spraying a road, paving a shoulder or sheeting a construction site moves PM10 and barely touches PM2.5. Restricting diesel moves PM2.5 and barely touches road dust. Treating the two as one pollutant is how a control measure gets credited with an effect it cannot have had.
- Coarse fraction
- PM10 minus PM2.5 — particles between 2.5 and 10 µm. Not separately regulated, but it is what the difference between the two readings is.
- Fugitive dust
- Dust released without a stack or a pipe: from roads, sites and open ground. The dominant coarse source in Delhi.
What India permits
PM10 has a higher permitted concentration than PM2.5 because the health evidence for the coarse fraction is weaker, not because more dust is better.
PM10, 24-hour mean
100 µg/m³
CPCB, NAAQS 2009
PM10, annual mean
60 µg/m³
CPCB, NAAQS 2009
PM2.5, 24-hour mean, for comparison
60 µg/m³
CPCB, NAAQS 2009
Both standards are notified under the National Ambient Air Quality Standards of 2009 and both are enforceable. The WHO 2021 guideline for PM10 is 45 µg/m³ over 24 hours, which is under half the Indian limit.
How to read it
Read PM10 against PM2.5, never on its own.
The instruments are the same class as those used for PM2.5, with a different size-selective inlet ahead of the detector. What changes is the cut point, not the physics.
The useful reading is the ratio. When PM10 is high and PM2.5 is comparatively low, the problem is dust: a nearby site, an unpaved stretch, a windy dry day. When the two rise together and the gap narrows, combustion is driving it, and the source is likely to be regional rather than at the end of the street.
Coarse particles fall out of the air within hours and within kilometres. That makes a PM10 reading a much better description of the immediate neighbourhood of a monitor than a PM2.5 reading is.
What this number cannot tell you
- It cannot be subtracted from PM2.5 to get a second pollutant. The coarse fraction is a difference, not a measured quantity in India's network.
- It cannot be used to judge combustion sources. A day with heavy construction and no traffic can produce a high PM10 and an unremarkable PM2.5.
- It cannot be compared with older Indian data without care: the 2009 standards replaced an earlier framework with different limits and different classes of area.
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 100 and 60 µg/m³ figures above
- WHO global air quality guidelines, 2021 World Health Organization · The 45 µg/m³ guideline
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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