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 almost hit 500 – the highest the measurement will go and 100 times the limit deemed to be healthy by the World Health Organization.

Air quality in the city had deteriorated 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.

On Sunday, 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.

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.

Doctors in Delhi said they had begun to see the damaging impacts of pollution on the city’s residents. “The number of patients with breathing problems has increased, with more people having coughs, colds, watery and irritated eyes, and breathing problems. People of all ages are affected by this. It is time for us to wear masks and go out only when needed,” said Nikhil Modi, a doctor at Apollo hospital in Delhi.

According to the central pollution control board, pollution levels in Delhi in October were at their worst since 2020.

Despite the Delhi government, run by the Aam Admi party (AAP), insisting it has a pollution action plan, it appears there has been little impact on the sharp decline in air quality that blights the lives of Delhi residents every year, usually between November and January.

Methods deployed by the AAP government to tackle pollution have included the sprinkling of water on roads to reduce dust and the building of two 80ft high “smog towers”, costing more than $2m each, that are supposed to clean the air but have been deemed by scientists to be largely ineffective.