New Delhi, Nov 6 (IANS): Thick smog continued to shroud Delhi on Sunday as PM 2.5 shot up to the maximum level of 500 across the national capital, and reduced the visibility to only 200 metres in the early hours.

In the 401-500 bracket, pollutants affect even the healthy people and seriously impact those suffering from diseases, according to the national Air Quality Index.

Other pollutants like sulphur dioxide, nitrogen dioxide and ground-level ozone were in the "minimal impact" category.

The average PM 2.5 level was the maximum of 500 micrograms per cubic metre or "severe" at all measuring centres, including at Dwarka in Delhi's south-west, R.K. Puram in south, Punjabi Bagh in west, Madir Marg in central Delhi and Dilshad Garden and Anand Vihar in east.

The PM 2.5 was marginally lower, yet in the category of "very poor", at 326 in Shadipur in west Delhi.

The PM 10 reading at Mandir Marg, R.K. Puram, Punjabi Bagh, and Anand Vihar monitoring stations was in the "severe" category with the maximum touching 500.

Particulate matter less than 10 micrometres in diameter (PM 10) are so small that they can get into lungs, likely causing serious health problems.

Particulate matter less than 2.5 micrometers in diameter (PM 2.5) are more refined particles produced by burning fuels.

With the air quality in Delhi dipping to "severe" level for the past few day, experts said the smog in the national capital on Wednesday was the worst in the past 17 years.

Other pollutants were between "moderate" to "good" as per the Air Quality Index, except for a few recordings which showed nitrogen dioxide reaching "very poor" zone in parts of the city.

"Visibility at 8.30 a.m. was 200 metres, caused by the heavy fog and smoke. This situation is going to deteriorate if not controlled," an official of the India Meteorological Department told IANS.

On Sunday, the city recorded a minimum temperature of 13.5 degrees Celsius, one notch above the season's average.

As citizens protested at Jantar Mantar, Delhi Chief Minister Arvind Kejriwal announced schools in the city will be shut for next three days, and urged citizens to stay indoors as much as possible.