Delhi residents woke to a thick, grey smog on Thursday as air pollution in the Indian capital surged to emergency levels.

The state-run Central Pollution Control Board’s air quality index, which shows the concentration of poisonous particulate matter known as PM 2.5, stood at 440, up from 430 on Wednesday and about 12 times the US government-recommended level of 35.

Pollution levels hit 500 in some parts of Delhi, and there was very poor visibility in some areas. Last year the high was 450 on 23 December.

A sharp drop in temperatures and wind speed, combined with vehicle and industrial emissions, dust from building sites and smoke from the burning of waste, has increased pollution over much of north India including Delhi, which is home to more than 20 million people.

The cold weather has forced some people, especially those spending the night in the open, to burn small fires to keep warm, adding to the smog.

In October and November as pollution spiked, the government adopted measures ranging from a temporary ban on construction activities and waste burning to a clampdown on polluting industrial plants such as coal-fired power stations.

Critics say the measures were inadequate and poorly implemented, largely because of a lack of resources and political will.

“Climatic conditions have contributed to the crisis, but let’s admit that most steps announced in 2018 have failed miserably,” said Sunil Dahiya, a senior campaigner with Greenpeace.

India is home to the world’s 14 most polluted cities, with Delhi the sixth worst, the World Health Organization has said. Toxic air caused 1.24 million deaths in India in 2017, or 12.5% of the total, according to a study published in Lancet Planetary Health.