Air quality in Delhi hit severe levels on Friday and a thick toxic smog cloaked the city, marking the beginning of a pollution season that has become an annual catastrophe for India’s capital.

Schools were shut and non-essential construction was banned around Delhi as the air quality index in the city hit 500 – the highest the measurement will go and 100 times the limit deemed to be healthy by the World Health Organisation (WHO).

Air quality in the city had declined over the past week, attributed to a sharp rise in farmers in the neighbouring states of Haryana and Punjab burning their fields during the crop planting season, compounded by winds that carried the pollutants into Delhi and a drop in temperatures trapping the particles.

In recent days, the state of Punjab saw a 740% increase in farm fires, with more than a thousand recorded in a single day. Other causes of pollution in the city are car emissions, construction and the burning of rubbish at waste plants.

However, the pollution early warning systems in Delhi reportedly failed to predict the further rapid deterioration in conditions that took place late on Thursday.

Delhi, home to about 33 million people, is regularly ranked the most polluted city in the world. According to this year’s air quality life index, compiled by the University of Chicago’s energy policy institute, the people of Delhi could have their lives shortened by 11.9 years due to the poor air they breathe.

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