As winter arrived, air pollution got worse, covering the entire Indian capital of New Delhi in a thick layer of smog that was three times higher than what was considered to be acceptable.

Every winter, the capital of the country with the worst air pollution finds it difficult to breathe because the low temperatures and calm winds keep pollutants closer to the ground.

“As the minimum temperature is dropping, gradual fog occurrence during early morning hours is likely to increase, leading to deterioration of air quality index (AQI),” said the federal government’s System of Air Quality and Weather Forecasting and Research (SAFAR) in a daily bulletin.

The AQI in parts of the city shot up above 400 on Tuesday, which is classified as the ‘severe’ category of air pollution, according to the Central Pollution Control Board (CPCB).

The level of fine particles measuring 2.5 micrograms or PM2.5 was 180 micrograms per cubic metre of air as of 10am in the Delhi National Capital Region, CPCB data showed, three times above the 24 hour acceptable limit of 60 micrograms per cubic metre of air.

Authorities have brought in several measures over the years to improve the city’s air quality, including switching Delhi’s fleet of public transport to cleaner fuel, spraying water from on top of towers and on roads and controlling burning of firewood and waste during cold weather.

But experts have said these measures need to be applied across northern India and in cities and towns around New Delhi that form the wider National Capital Region, which also suffer from poor air quality, to effectively control pollution.