Studies and research reports have underlined that it's the chemical composition of ultrafine particulates PM2.5 or PM10, and not their volume, which is more crucial in determining the toxicity of air.
The Environment Pollution (Prevention and Control) Authority (EPCA), a Supreme Court-appointed pollution watchdog, made a similar point in a report to the top court, that particles from coal and diesel are more harmful than wind-blown dust, as they can lead to an increase in heart disease-related deaths.
"Similarly, particles from diesel combustion are very toxic and have been classified by the World Health Organisation as a class I carcinogen for strong links with lung cancer, putting them in the same bracket as tobacco smoking and asbestos.
"This suggests that we must prioritise the more harmful particulates for action. Combustion sources- vehicles, power plants and industry- need more stringent and priority action," the report says.
The EPCA mentioned it separately under a section addressing number versus toxicity.
An Indian Institute of Technology-Kanpur report, which assessed the chemical composition of pollution from various sources in the city, also concluded that combustion- vehicular and industrial alike- was responsible for the formation of PM2.5 in greater quantity.
Among PM2.5 and PM10, the most dominant pollutants in Delhi's air, PM2.5 is deadlier owing to its tinier size, up to 30 times smaller than the width of a human hair, aiding it in lodging deep in the lungs and subsequently entering the bloodstream.
The IIT report found that PM2.5 Nitrate particles formed from nitrogen oxides and sulphate particles formed from sulphur dioxides can be 25 per cent of the total PM2.5 load in the city.
Both nitrates and sulphates are classified as 'secondary particles', which are formed due to the reaction of gases such as sulphur dioxide (SO2) and oxides of nitrogen (NOx) emitted from vehicles, thermal power plants and industries.
"In a broad sense, fractions of secondary particles of both PM 10 and PM2.5 in two seasons were consistent and need to be controlled for better air quality in Delhi and the National Capital Region," the report pointed out.
It estimates that of around 312 tons of NOx produced per day in the city, nearly 52 per cent come from industrial point sources such as power plants and 36 per cent from vehicular emissions, "probably making it the most important emission".