Lethal toxicity vs quantum of pollution in Delhi’s air

Not just quantum of pollution but also toxicity of emissions must be assessed to prioritise action on pollution sources

By Anumita Roychowdhury
Published: Monday 25 May 2020
Not just quantum of pollution but also toxicity of emissions in Delhi's air must be assessed to prioritise action on pollution sources. Photo: Down To Earth

As we brace ourselves to choke on dirty air once again post-lockedown, new evidences show that taking decisions based only on the quantum of pollution is not adequate to protect public health. It is equally important to include toxicity indicators of emissions to prioritise pollution sources for mitigation. Even with insignificant mass, emissions from some sources can be more toxic and harmful. 

Among 12 pollution sources thus assessed in Delhi, road dust that accumulates toxic material from a variety of combustion sources and vehicles, especially diesel emissions, have been found to be the most toxic. They contain the maximum traces of highly toxic and cancer-causing substances. In fact, along with road dust and vehicles, coal, coal-fired power plants, dung and wood have shown the highest toxicity.

This has emerged from new research at the Indian Institute of Technology (IIT) Kanpur, conducted by Ashutosh K Pathak, Mukesh Sharma, Pavan K Nagar. While highlighting the importance of this investigation, Sharma says: “Source-wise toxicity measures provide an opportunity to plan better emission control strategy both for mass emission and health benefits.” 

“The regulatory agencies should bring this dimension (of PM 2.5 toxicity) into regulation and control of emissions at source. More such studies can greatly help with new regulations and control strategies,” he adds. 

The new enquiry  

This new study has applied toxicity-based framework, which is different from the conventional practice of conducting only mass-based emissions inventory of PM 2.5. This is not adequate as all particles are not equally toxic. It is more important to look deeper into the chemical constituents of the particle to understand toxicity and harm level.

The IIT-K scientists have applied toxicity criteria to 12 pollution sources in Delhi that were earlier assessed in a conventional mass-based emissions inventory and source apportionment study by IIT Kanpur in 2015. They put under microscope 89 constituents of particles by referring to the USEPA’s Comptox database to estimate the threshold concentration of the constituents of PM2.5 for cancer causing chronic and acute health effects. 

What have they found? 

Their finding underscores that toxicity is not linearly correlated with mass emissions. Even at lesser quantum, some emissions can be more toxic and harmful.

Among all the 12 sources scanned — (coal power plants, vehicles, industry, hotels, domestic fuels, diesel generator sets, waste burning, construction and demolition, crematorium, and concreting), road dust that accumulates combusted material from other sources, vehicles (mainly diesel emissions), coal and coal plant, wood and dung, have been to found to have high level of toxicity.

Heavy metals, including chromium, cobalt, cadmium and arsenic, among others, present in these emissions make them disproportionately more toxic. Scientists conclude that substitution of these fuels with cleaner fuels and elimination of waste burning can reduce chronic toxicity effectively by about 40 per cent. 

This study reconfirms the highly toxic and carcinogenic potential of diesel emissions. There are many toxic compounds like metals, organics like polycyclic aromatic hydrocarbons (PAHs) that get adsorbed on the large surface area of diesel emissions that make it very toxic. PAHs that include highly toxic compounds like Dibenz (ah) anthracene, have insignificant mass but have very strong cancer causing potential even at trace amount. These make diesel emissions highly carcinogenic. 

Road dust becomes the worst offender mainly because it accumulates toxic compounds from vehicles, coal power plants, industry, household pollution or any other burning activities. Road dust is contaminated with atmospheric deposition from these sources. 

Consistent with global findings 

These new findings in Delhi are consistent with several other global studies including some in the journal Nature and from Health Effects Institute, Boston, that have concluded all PM2.5 particles are not equally toxic. The harmfulness of PM2.5 greatly depends on its chemical constituents. 

These studies have already indicated that mass of pollutants alone may underestimate health risk from PM 2.5 exposure. Coal combustion can lead to higher mortality. Overall, carbonaceous particle are more toxic than crustal material. 

Inform regulations 

“Therefore we need an approach which is a blend of toxicity and the mass emission of PM 2.5 so that we effectively control the toxicity and mass emissions”, emphasises Sharma. The scientists recommend that from health standpoint pollution assessment needs to be mass based, chemical constituent based and source toxicity based. This study provides the framework for such an assessment to inform policy. 

The source-wise toxicity measures needs to be taken forward to plan better emission control strategy both for mass emission and health benefits. Since the completion of this study, new regulatory action in Delhi has already closed down all coal power plants and introduced approved fuel lists that bans all dirty fuel sin Delhi. Its stringent implementation and replication in other regions is needed to reduce health risk and emissions. 

“The regulatory agencies should bring this dimension (of PM 2.5 toxicity) into regulation and control of emission at source. More studies into these new findings can greatly help in new regulation and control strategies, adds Sharma. 

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