According to Dr Beig, air pollution in Delhi is creating localised temperature hot spots and is one of the main reasons behind the sweltering heat in the city. “This happens due to the heat island effect,” he added.

SAFAR, located in Pune is an expert body run under the Ministry of Earth Sciences, Government of India. “Our data shows that Palam also recorded the highest levels of air pollution in Delhi at around 85 ppb (parts per billion),” said Dr Beig. “The permissible limit is 50 ppb,” he added.

A study was done by SAFAR on the air quality of Delhi before the Common Wealth Games in 2010 and in 2014. “There is an increase of 30 per cent in vehicular emissions during this period while industrial emissions increased by 3-5 per cent,” said Dr Beig. “Our research tells us that the biggest reason for the massive increase in the air pollutants in Delhi is the widespread usage of diesel and adulterated fuel. The smoke emission by a truck running on diesel is equivalent to the same emission by five trucks running on petrol,” said Dr Beig.

The city had a vehicular population of 31.64 lakh in 1999-2000, while it touched 74.53 lakh in 2011-12, a rise of almost 136%, Delhi Economic Survey said, adding that the urban transport scenario in the city is "fast warming up".

Because of the increase in per capita income, the number of vehicles now grows at 7.2 per cent annually as compared to 4.72% in the last decade. The Centre for Science and Environment (CSE) has compared data from the Delhi Pollution Control Board from 2000 to 2013; in Delhi, PM 10 and PM 2.5 levels rose 50 per cent during this period. At 10 am on December 16, 2013 the monitoring station at R K Puram recorded PM 2.5 levels at 985 micrograms per cubic meter, Delhi Pollution Control Board officials concede. The standard set by the Indian government is 60.

Air quality is represented by annual mean concentration of fine particulate matter (PM10 and PM2.5,– particles smaller than 10 or 2.5 microns). Delhi is the most polluted city in the world, said a study released by World Health Organisation (WHO) report which came in May 2014. “Many factors contribute to this increase, including reliance on fossil fuels such as coal fired power plants, dependence on private transport motor vehicles, inefficient use of energy in buildings, and the use of biomass for cooking and heating,” the report said.

The previous UPA government had decided in January 2013 to increase diesel prices in small monthly doses until the difference between the retail price and the cost of production is bridged. The price of diesel in Delhi has gone up to 57.28 Rs/Ltr (June 01, 2014) from 50.25 Rs/Ltr (June 01, 2013). The latest petrol price in Delhi is 71.41 Rs/Ltr (April 16, 2014) while that of CNG is 38.15 Rs/Kg (May 03, 2014).

Although the gap between the price of diesel and petrol is slowly reducing yet the gap between number of diesel and petrol vehicles is not coming down.

In 2000, diesel cars accounted for only 4% of all car sales. Today, half the cars sold run on diesel, according to CSE report. Also, the price of a diesel vehicle normally costs much more than that of petrol. For example, the cost of Swift VXi-petrol is Rs. 6.34 lakhs on road while the cost of Swift VDi-diesel is Rs. 7.71 lakhs on road. Maruti which traditionally made petrol cars is now having 17 diesel car models in its showrooms.

There was a small advantage in Delhi’s air quality after CNG was introduced but the rate of increase in diesel cars was just too fast for any significant change to occur. The monthly diesel price hikes of 40-50 paise a litre are likely to continue as the Narendra Modi government is keen to cut the subsidy bill, a top Oil Ministry official had said in an official statement. This is part of the fuel price rationalisation policy of the previous government

This year also saw one of the coldest winters.