The Indian capital of New Delhi was covered in thick smog Tuesday as air pollution took a turn for the worse with the advent of winter.

The smog shot up concentrations of fine particles in the air three times above the acceptable limits.

The world's most polluted capital city struggles to breathe easy every winter as cold temperatures and calm winds trap pollutants closer to the ground.

Smoke from farmers burning crop stubble, vehicle exhaust and factory emissions combine every winter to blanket the capital in a deadly grey haze.

"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 meter of air as of 10 a.m. in the Delhi National Capital Region, CPCB data showed, three times above the 24-hour acceptable limit of 60 micrograms per cubic meter of air.

Earlier in November, Delhi shut primary schools to protect children from the toxic smog choking the megacity of 20 million people.

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 the 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 suffers from poor air quality, to effectively control pollution.