Delhi's air quality worsened within the severe category on Thursday as unfavourable meteorological conditions hindered the dispersion of pollutants.

Recent findings from a joint project by the Delhi government and IIT-Kanpur found that vehicular emissions accounted for about 38 per cent of the capital's air pollution on Wednesday. This reduced to 25 per cent on Thursday.

Secondary inorganic aerosols -- particles such as sulfate and nitrate that are formed in the atmosphere due to the interaction of gases and particulate pollutants from sources like power plants, refineries, and vehicles -- is the second major contributor to Delhi's foul air, accounting for 30 to 35 per cent of the air pollution in the city over the last few days.

Calm winds and low temperatures are allowing accumulation of pollutants and relief is unlikely over the next few days, an official at the India Meteorological Department (IMD) said.

An improvement in the wind speed November 21 onwards might bring air pollution levels down, he said.

Delhi's 24-hour average Air Quality Index, recorded at 4 pm every day, stood at 419 on Thursday. It was 401 on Wednesday, 397 on Tuesday, 358 on Monday and 218 on Sunday, 220 on Saturday and 279 on Friday.

Neighbouring Ghaziabad (376), Gurugram (363), Greater Noida (340), Noida (355) and Faridabad (424) also recorded very poor to severe air quality.

An AQI between zero and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, 401 and 450 severe and above 450 severe plus.

Delhi's air quality dropped over the last few days despite the state government implementing stringent measures, including a ban on construction work and the entry of diesel-guzzling trucks into the city, to control pollution.

According to IQAir, a Swiss company that specialises in air quality monitoring, Delhi was the most polluted city in the world on Thursday, followed by Baghdad and Lahore.

A system developed by the Pune-based Indian Institute of Tropical Meteorology to identify the contribution of different pollution sources showed stubble-burning accounted for 7.5 per cent of the air pollution in the capital on Thursday. It is likely to come down to 3.5 per cent on Friday and 3 per cent on Saturday.

An official of the Commission for Air Quality Management (CAQM) had earlier said stringent measures, including a ban on construction work and the entry of polluting trucks in the national capital, under the final stage of the central government's air pollution control plan called the Graded Response Action Plan (GRAP) will continue until further orders.

The Delhi government on Thursday set up a six-member special task force to ensure strict implementation of measures outlined in the GRAP in the capital.

Delhi's special secretary (environment) will head the STF whose members include senior officials from the departments of transport, traffic, revenue, Municipal Corporation of Delhi (MCD) and Public Works Department (PWD).

The STF will coordinate with all the departments involved in the enforcement of pollution control measures and submit a report to the government daily, Environment Minister Gopal Rai told reporters.

Earlier this week, Rai had said the city government may implement the odd-even car rationing scheme if the AQI crosses the 450 mark.

The government last week postponed the implementation of the scheme, which permits cars to operate on alternate days based on the odd or even last digit of their registration numbers, after a notable improvement in the city's air quality due to rain on Friday.

Doctors say breathing in the polluted air of Delhi is equivalent to the harmful effects of smoking approximately 10 cigarettes a day.

Prolonged exposure to high levels of pollution can cause or exacerbate respiratory problems such as asthma, bronchitis and chronic obstructive pulmonary disease (COPD) and dramatically raise the risk of cardiovascular disease, they said.

Unfavourable meteorological conditions, combined with vehicular emissions, paddy-straw burning, firecrackers and other local pollution sources, contribute to hazardous air quality levels in Delhi-NCR during winters.

According to a Delhi Pollution Control Committee (DPCC) analysis, the city experiences peak pollution from November 1 to 15, when the number of stubble-burning incidents in Punjab and Haryana increases.

Punjab on Thursday reported 1,271 stubble burning incidents, taking the total number of such fires to 31,932 since September 15.

According to a report compiled by the Energy Policy Institute at the University of Chicago (EPIC) in August, air pollution is shortening lives by almost 12 years in Delhi.

-With PTI Input