As Delhi government prepares to unveil its plan to restrict vehicles with an aim to reduce pollution in the capital, air quality in Delhi is once again in the category of ‘very poor’ and is likely to stay that way over the weekend.

In Delhi, the average levels of particulate matters (PM 2.5 and PM 10) – fine particles that can enter into the respiratory system and cause damage – were, respectively, 218 and 359 micrograms per cubic metre as opposed to the safe limits of 60 and 100, an IMD official said. The World Health Organisation’s recommended maximum is 25.

WHO also considers the quantity of PM 2.5 as the best indicator of the level of health risks from air pollution.

“Air quality will fluctuate between poor and very poor. It is not as bad as last year due to fairly strong wind movement, essentially westerlies, which is aiding in dispersal of the suspended particulate matters,” said SK Peshin, deputy director general, Meteorology.

An IMD official said that the situation is expected to persist over the weekend.

However, there is no possibility of heavy smog in the next few days although the days to watch out are on December 20 and 21.

 The scheme of ‘odd-even’ restrictions on the movement of vehicles and other details that will come into force from January 1 are being worked out. The government will publish these measures next week.

Meanwhile, three studies have found that the actual culprit in winters is not vehicular emissions, reports Economic Times.

Two reports — one by former faculty members of Indian Institute of Technology, Delhi, and the other by US, Indian and University of Birmingham researchers — said wood smoke is the prime contributor to the presence of PM 2.5 — fine particles with a width of 2.5 microns or less — in the air in winters. According to the third study, power plants are the main source of this pollutant.

The first two studies also point out that the total number of cars on the road may be less than the 2.8 million that are registered in New Delhi. The actual number of private cars being run is only half of the vehicles that get registered, said former IIT-Delhi professor Dinesh Mohan. This is because vehicles are often not de-registered when they are trashed or sold, he said.

Mohan was part of the team that produced a report titled "Benchmarking Vehicle and Passenger Travel Characteristics in Delhi for On-road Emissions Analysis" after conducting the study over a three-year period that ended in 2014.

According to the report, wood smoke accounted for 23.3% of the PM 2.5 in winters and vehicular traffic 16.2%. "A number of brick kilns around Delhi bring in a lot of PM 2.5 mass to the city and are one of the leading sources for pollution," said Mohan.

Further, IIT-Delhi claims that so far, all studies on pollution in the national capital have been restricted to the city or parts of Delhi and have not taken areas in the National Capital Region into account. "There is a need for an integrated study on the air quality of Delhi to be conducted, which would also include NCR besides Delhi. We (IIT-Delhi) are in the process to conduct this," said a professor at IIT-Delhi, who along with other faculty members made a proposal to the Central Pollution Control Board and the Delhi Pollution Control Committee last week to conduct an integrated study in the region.

Meanwhile, a yet-to-be-released study by IIT-Kanpur holds power plants to be the main source of pollutants, contributing 30% of the total PM 2.5 mass in the city's air during winters. According to people working closely on the IIT-Kanpur study, the other sources are crop/bio-mass burning (26%) and diesel vehicular pollution (26%). Wood smoke is included in crop/bio-mass burning.

"In the summers, the fly ash (from construction and charcoal burning) is the main contributor to the high PM 2.5 levels in Delhi," said a person familiar with the IIT-Kanpur study.