What is PM2.5, and what else is in Delhi's air?

Delhi's air problem is mostly a particle problem: specks of solid and liquid so small that a cubic metre of winter air can hold hundreds of micrograms of them without looking like anything more than haze.

India's air quality index tracks eight pollutants: PM10, PM2.5, nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), ozone (O₃), ammonia (NH₃) and lead (Pb).¹ Six of them come up again and again in Delhi's daily numbers, and they are not the same kind of thing. Two are sizes of dust and droplets; the rest are gases.

What do PM10 and PM2.5 actually mean?

“PM” stands for particulate matter, and the number is a size cut-off in micrometres (µm), a thousandth of a millimetre. The US Environmental Protection Agency defines PM10 as inhalable particles generally 10 µm and smaller, and PM2.5 as fine inhalable particles generally 2.5 µm and smaller.² So PM2.5 is a subset of PM10: every fine particle is also counted in the coarser bucket.

The names describe size, not substance. A PM2.5 reading tells you how much mass of small particles is in the air, not what they are made of. In Delhi that mass is a changing mix of road and construction dust, soot from engines and fires, ash, and particles that form in the air itself when gases such as sulphur dioxide, nitrogen oxides and ammonia react. Studies that measure Delhi's particles chemically find that these “secondary” particles are a large part of the winter load.³⁴ That matters later: a particle formed over Delhi may have started as a gas emitted far away.

How small is a PM2.5 particle?

A human hair is the usual yardstick. The EPA puts an average hair at about 70 µm across, roughly 30 times wider than the largest fine particle.² Hair varies a great deal between people, so treat the comparison as an order of magnitude, not a measurement. The scale below is drawn to it.

What are the gases in the index?

Nitrogen dioxide (NO₂) gets into the air mainly from burning fuel, in vehicles, power plants and off-road machinery.⁵

Ozone (O₃) is not emitted directly. It forms when nitrogen oxides and volatile organic compounds react in sunlight, so it tends to peak on hot, bright days.⁶ In Delhi's 2024 bulletins, ozone was named among the prominent pollutants on days between February and December, but never on a Very Poor or Severe day; the worst day it shared was Poor.⁷

Sulphur dioxide (SO₂) comes mostly from power plants and other facilities that burn fossil fuels.⁸ Carbon monoxide (CO) is released whenever something burns, and outdoors mostly from engines.⁹ In Delhi these two rarely set the day's index. CO was named alongside other pollutants on a scattering of 2024 days, mostly in the clean monsoon weeks, and alone only once, on 13 September, when the index was 52.⁷

Why does PM2.5 get most of the attention?

Two reasons, one physical and one practical.

The physical reason is depth. The EPA notes that some particles under 10 µm can get deep into the lungs, some may even reach the bloodstream, and that PM2.5 poses the greatest risk to health.² The World Health Organization's 2021 guidelines set PM2.5 levels of 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours.¹⁰¹¹ The health chapter, What does air pollution do to health?, explains how those studies are done and how uncertain their numbers are.

The practical reason is that PM2.5 is what drives Delhi's worst days. On all 17 Severe days of 2024, and on 65 of the 70 Very Poor days, the CPCB bulletin named PM2.5, alone or with another pollutant, as the prominent pollutant. The other five Very Poor days were driven by PM10.⁷ When people in Delhi say “the AQI is 400”, they are almost always describing fine particles.

The next chapter, How do you read India's AQI?, shows how these concentrations become a single number, and why that number hides as much as it shows.