New Delhi: The air quality in Delhi has been fluctuating from poor to severe since the beginning of this year when Delhi began implementing its trial odd-even scheme to reduce the number of cars plying the roads.

But in the past two days, PM 2.5 levels (24 hour average) have reduced from 280 μg/m3, to 267 μg/m3 and finally to 248 μg/m3 on Thursday. Whether this is because of the odd-even scheme cannot be determined at this point, according to scientists. That is because Delhi’s air pollution is caused by a combination of factors—sources of pollution, meteorological conditions and geographical position.

Air pollution is caused by a number of pollutants such as ozone, carbon monoxide, nitrogen dioxide, methane, sulphur dioxide, benzene and mercury. But the key pollutants in Delhi—and probably the deadliest—are particulate matter PM 2.5 and PM 10.

PM 2.5 is particulate matter that is as big as 2.5 microns and PM 10 is particulate matter that is 10 microns. To understand how small these particles are, there are 25,000 microns in one inch. These fine particles can settle deep in our lungs or can even be absorbed in our bloodstream, which can lead to increased respiratory and cardiovascular diseases among people.

These fine particles come from vehicles including diesel and petrol cars and trucks which also emit carbon monoxide, oxides of nitrogen and un-burnt hydrocarbons. Dust from unpaved roads, burning of fuels, tobacco burning and gases from power plants that react with the atmosphere also make up PM 2.5. In the national capital region, we also have emissions from coal-based power stations. Small-scale industries in various cities and towns in the NCR like Ghaziabad, Faridabad, Gurgaon and Panipat also contribute to air pollution in Delhi, along with burning of crops in neighbouring states.

“But another important factor is that Delhi is a landlocked city so that the air does not get refreshed by sea breeze, and also has a few meteorological conditions that cannot be controlled. Due to small inversion conditions during night, we get less volume of air for dilution and pollutants are trapped,” explains B. Sengupta, former member-secretary of the Central Pollution Control Board. Another thing about Delhi is high ‘calm periods’ characterised by wind speeds of less than 0.3km per hour. These conditions also contribute to air pollution.

According to a study by the System of Air Quality and Weather Forecasting and Research (SAFAR), the biggest contributor to PM 10 is windblown dust and unpaved roads, while the biggest source of PM 2.5 is the transport sector closely followed by biomass burning.