The pollutants, and what they do to you

Airborne particles under 2.5 micrometres across — roughly 1/30th the width of a human hair. They are small enough to pass through the lungs' defences, reach the alveoli, and cross into the bloodstream.

Inhalable particles under 10 micrometres, including everything counted as PM2.5 plus coarser material like road dust, pollen fragments and construction debris. Most is filtered by the nose and upper airway, but enough reaches the bronchi to inflame them.

A highly reactive gas that is not emitted directly. It forms when nitrogen oxides and volatile organic compounds cook in sunlight, which is why it peaks on hot, still, sunny afternoons and collapses overnight. Useful in the stratosphere, damaging at breathing height.

A reddish-brown, sharply acrid gas produced whenever fuel burns hot enough to oxidise the nitrogen in air. It is the best single chemical marker for traffic pollution, and concentrations fall off steeply within a few hundred metres of a busy road.

A pungent gas released when sulphur-bearing fuel or ore is burned. It constricts airways within minutes at high concentrations and converts in the atmosphere into sulphate particles, becoming part of the PM2.5 problem.

An odourless, colourless gas from incomplete combustion. It binds to haemoglobin roughly 240 times more readily than oxygen does, so it starves tissue of oxygen without any sensation of suffocation. Outdoor levels are rarely acutely dangerous; indoor levels can be lethal.

A sharp alkaline gas overwhelmingly agricultural in origin. Its main air quality significance is indirect: it neutralises atmospheric acids to form ammonium nitrate and sulphate, which are major components of secondary PM2.5. India's national index is the only major one that scores it directly.

Modelled mineral dust lifted from deserts and dry soils and carried thousands of kilometres. It is already counted inside PM10 and PM2.5, but is broken out separately here because a dust event behaves differently from combustion smog and calls for a different response.

How much sunlight the entire column of atmosphere above you scatters or absorbs, measured at 550 nm. Below 0.1 is a crystal-clear sky; above 0.5 is visibly hazy; above 1.0 usually means heavy smoke or dust aloft. It is the satellite proxy for haze, and unlike surface readings it sees pollution above the ground.

Strength of sunburning ultraviolet radiation reaching the ground, on an open-ended scale where 0–2 is low and 11+ is extreme. Not an air pollutant, but it is the same sky and the same outdoor decision — and heavy haze measurably suppresses it.

A potent greenhouse gas, harmless to breathe at ambient concentrations. It matters for air quality indirectly: methane is a precursor to background ground-level ozone, so it raises the ozone floor across whole hemispheres.