How Delhi’s location and geography influence its chronic pollution levels

Delhi is perhaps the most polluted of the world’s megacities. Every winter, the city’s 30 million residents breathe air so toxic that visibility drops to mere metres. If you stand on top of one of Delhi’s monuments you can barely make out buildings across the street as the thick, acrid smog burns your eyes and scratches your throat.

But conditions can and do change rapidly. January 2025 offered a dramatic demonstration of how weather patterns can rapidly transform the city’s air quality.

On January 5, favourable winds improved air quality enough to lift some restrictions. Yet by January 15, as winds calmed and temperatures dropped, pollution levels soared dramatically, forcing the city to implement its maximum “severe +” interventions. These include banning trucks from entering the city, restricting private vehicles and moving schools to online classes.

Delhi didn’t suddenly have more cars, factories, power plants or construction sites from one week to the next. Those things are consistent sources of pollution. There are some events that add to air pollution in the shorter term, such as fireworks during Diwali, or the mass burning of unwanted crop debris (known as stubble), both of which take place in October or November.

But that wasn’t what happened in January. Instead, the sudden reversal revealed how...

Read more