Air Pollution Doesn't Stop at the City Border

Pollution doesn't read maps. It drifts with the wind, mixes and settles wherever the weather carries it. Planners, though, work in administrative units, such as a municipality here and a state there. That mismatch is why the idea of an airshed matters.

An airshed is an area that shares a common flow of air, and it can extend hundreds of kilometres beyond city and state limits. Think of a river basin: whatever enters upstream affects everyone downstream.

Delhi sits inside the Indo-Gangetic Plain, which the World Bank treats as a transboundary airshed covering Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh, Bihar, Jharkhand and West Bengal. A 2019 study found that about half of Delhi's population-weighted PM2.5 came from outside the territory, roughly half of that from Haryana and Uttar Pradesh.

Chemistry adds to the problem. Over half of India's PM2.5 forms in the atmosphere from gases such as ammonia, sulphur dioxide and nitrogen oxides, which can travel across state lines. Pollution formed this way has no single chimney to point at.

This also isn't only a big-city problem. PM2.5 in villages and small towns on the plain is often similar to or higher than in the nearest large city. A town with few industries can still breathe a neighbour's emissions.

That is why the World Bank advocates an airshed approach. It stresses airshed-wide coordination, since substantial reductions can't come from tackling local emissions alone. India has made a start: the 2021 law creating the Commission for Air Quality Management gave it a statutory role in coordinated planning and enforcement across the NCR and adjoining areas.

Delhi still has to clean up its own roads, construction sites and fuels. But it can't reach clean air alone, because the air it breathes is shared with its neighbours. Fixing it will take the same cooperation the pollution already shows.