To arrive at this conclusion, Gargava, who presented the findings at a workshop organised by the PHD Chamber of Commerce, deducted the emission load of vehicles from that coming out of all sources, further deducting the load from cars and then writing off the exempted cars. By the emission load measure, even solid waste burning contributes to only 3% of the load but is considered a major health risk.

A similar model of assessing the impact of vehicles was presented by some parties in the MC Mehta case against air pollution in the Supreme Court, according to activists. But several scientists are now questioning the focus on emission load alone in prioritising sources of air pollution.

Another participant, UmeshKulshrestha, professor at the School of Environmental Sciences at JNU, told TOI, "We shouldn't go by the load alone. Dust, for instance, has been a natural component for centuries now. We need to segregate the toxic content-metals, pesticides and carbon. The strategies cannot be based on emission load. The agencies should assess toxicity and exposure."

Bhargav Krishna, research fellow at the Public Health Foundation of India (PHFI), said Gargava's study was purely from a PM2.5 perspective. "Vehicles, both petrol and diesel, contribute to the sulphur oxide (SOx) and nitrogen oxide (NOx) loads as well. Both add to the particulate load as secondary particles, the second most stable source of air pollution load across seasons as per the IIT Kanpur study."

Looking at the impact of such a strategy through an emission load lens is irrelevant if the impact on health is the ultimate goal, he added. "We know that the kerb side exposure to PM2.5 is often 1.5-2 times as much as that of ambient concentration. This is a significant source of exposure when we consider that an average Delhiite commutes between 1.5-2 hours a day, sometimes more. Exposure, and not ambient concentration, should form the basis and their sources should be tackled on a priority basis."

Gargava said he had conducted the analysis much before the odd-even scheme was implemented. "I have made some assumptions, like not everyone will follow the scheme, they may use their second vehicle etc. The error margin could be high. Plus, this is not CPCB's stand on the subject." He clarified that his analysis was only to say that a single measure won't be sufficient.

Dr T K Joshi, visiting professor, Drexel University School of Public Health, Philadelphia highlighted the massive health burden from air pollution exposure. Some of the "emissions are mixtures of many volatile organic compounds making it difficult to identify all individual compounds in the mixture", he said, adding that these could lead to increase in hospital admissions, cardiovascular disorder and lung cancer.

Anumita Roy Chowdhury, head of Centre for Science and Environment's clean air campaign, said, "The data presented Gargava actually shows that in the best case scenario the odd-and-even scheme has reduced emission loads equivalent to 30% of PM from diesel generator sets or 20% of municipal solid waste burning or 6% of industrial pollution during the fortnight."