The air in India’s capital, Delhi, air is currently hazardous even for healthy people to breathe for 24 hours, say Indian guidelines and those of the World Health Organization. But Delhi’s citizens can expect to be breathing similar air for the next three months, as another winter begins with daily particulate matter counts exceeding the WHO safe limit by 10 times on a good day and by 30 times on a bad one.
Often called the world’s most polluted capital city, Delhi sits near the western end of a smoke cloud that stretches across northern India and is visible from space. A conurbation of 28 million people, partly surrounded by the Delhi ridge, it traps pollution each winter as cold air sinks.
Each October and November farmers in neighbouring states burn off the stubble on their fields. As the wind dies and the air cools in November the smoke gathers in Delhi. This year state schools have been closed since 3 November, when several monitoring stations reported scores of 480 on the city’s Indian Air Quality Index (AQI), which has a top “hazardous” score of 500.
IQAir, a Swiss air quality technology company, calculated that those data would correspond to …