New study reveals increased mortality risk from short-term air pollution exposure in India; calls for more stringent air quality limits

A new study published in the Lancet Planetary Health finds that even levels of air pollution below current Indian air quality standards lead to increased daily mortality rates in India. It finds that across 10 cities in the country (Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Kolkata, Mumbai, Pune, Shimla and Varanasi), ~33,000 deaths per year can be attributed to air pollution levels that are above the WHO guidelines. Significant number of deaths were observed even in cities not considered to have  high air pollution, such as Mumbai, Bengaluru, Kolkata and Chennai. To come to these conclusions, the authors used data on PM_2.5 exposure in the 10 cities as well as daily counts of mortality between 2008 and 2019. 

This study breaks new ground in our understanding of air pollution and health in India:

1. It is the first multi-city study to assess the relationship between short-term air pollution exposure and death in India, with the cities included spanning a wide range of air pollution concentrations and situated in different agro-climatological zones
2. We utilise novel causal modelling techniques that isolate the heightened impact of local sources of air pollution such as waste burning and vehicular emissions among others
3. We generate estimates of mortality attributable to air pollution for cities (such as Mumbai, Bengaluru and Kolkata) and at lower concentrations previously unstudied in India

The results indicate that the national air quality standards should be made more stringent and efforts to control air pollution redoubled. Conducted by researchers from academic institutions in India (Sustainable Futures Collaborative, Ashoka University, Centre for Chronic Disease Control), Sweden (Karolinska Institutet), USA (Harvard University, Boston University), and elsewhere, many of whom are members of the CHAIR-India consortium, the study’s key findings were as follows:

1. During the study period, 7.2% of all deaths (~33,000 each year) across all 10 of these cities could be linked to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³
2. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 1.42% increase in daily deaths. This estimate nearly doubled to 3.57% when we used a causal modelling approach that isolates the impact of local sources of air pollution
3. Increases in risk of death were steep at lower concentrations of PM_2.5 and tapering off at higher concentrations, with significant effects observed below the current National Ambient Air Quality Standard of 60 μg/m³ for 24-hour exposure
4. See a run-down of city-specific findings below  

Overall, the study holds key insights for policy including:

1. Air pollution policy currently focuses on ‘non-attainment cities’ which fail to meet the current Indian standard. Since these standards are 4X the current WHO guideline value of 15 μg/m³, people in many more regions beyond these cities are also vulnerable. This study shows the need to expand remedial action beyond current ‘non-attainment cities’
2. Increase focus on action year-round and not just on seasonal extremes. A large proportion of mortality risk is concentrated at low to moderate PM_2.5 levels with risk tapering off at higher levels. Our current policy instruments such as Graded Response Action Plans largely focus on pollution extremes and must be recalibrated to focus on year-round action
3. Develop policy instruments to better address dispersed local sources of air pollution. These are the sources most people are exposed to on a regular basis, but are also the most complex to address. Doing so, however, would bring with it substantial health gains. This would require focusing not just on ambient concentrations, but those sources people are most exposed to

Dr. Bhargav Krishna, Fellow at the Sustainable Futures Collaborative and a lead author on the study said “The results of this first-of-its-kind multi-city study show us that reducing air pollution is a nationwide challenge. Our analyses highlight air pollution’s substantial effects on death even in cities previously considered less polluted such as Mumbai, Bengaluru, Kolkata and Chennai. The significant effects observed below the current Indian standards have implications for what we consider acceptable air quality, and emphasise the need for stringent year-round action across the whole country”.

Dr. Jeroen de Bont, Postdoctoral researcher at Karolinska Institutet and a lead author, emphasised the significance of our first multi-city study. “Spanning data from 2008 to 2019 across India’s largest cities, our findings support the evidence that approximately 7.2% of all deaths in India are attributable to daily PM_2.5 exposure. As efforts to combat air pollution intensify, addressing dispersed local sources becomes crucial alongside existing strategies”, he said.

“This unique study accounted for a diverse air quality profile across 10 Indian cities and demonstrates for the first time that the risk of mortality is significant even at lower levels of air pollution. The insights signal an urgent need to revisit our air quality management strategies that currently focus only on ‘non-attainment cities’, rethink current air quality standards accounting for the lower risk thresholds and shift from addressing regional to local sources to effectively protect human health”, said Dr. Poornima Prabhakaran, Director of the Centre for Health Analytics Research and Trends (CHART) at the Trivedi School of Biosciences, Ashoka University and India lead of the CHAIR-India consortium. 

“The levels of air pollution we see in India are really extremely high and this study clearly shows how day-to-day variations in these levels leads to considerable mortality. Interestingly we saw that local pollution sources are likely to be more toxic than more distant sources which has implications for policy makers addressing this highly relevant threat to human health.” said Dr. Petter Ljungman, Associate Professor at the Karolinska Institutet, and one of the researchers involved in the study. 

Professor Joel Schwartz of Harvard University, a co-author of the study said “Lowering and enforcing Indian air quality standards will save tens of thousands of lives per year. Methods for controlling pollution exist and are used elsewhere. They urgently need to be applied in India.”

Link to the study

For more information on this study, please reach out to Dr. Bhargav Krishna (bhargav@sustainablefutures.org; 99585 82769) or Dr. Poornima Prabhakaran (poornima.prabhakaran@ashoka.edu.in; 98106 11752). 

City-specific data points:

1. Ahmedabad – During the study period, 5.6% of all deaths (~2,500 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 1.33% increase in daily deaths. This estimate increased to 6.06% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
2. Bengaluru – During the study period, 4.8% of all deaths (~2,100 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 3.06% increase in daily deaths. This estimate nearly tripled to 9.12% when we used a causal modelling approach, indicating the need for a greater focus on local air pollution sources in Bengaluru.
3. Chennai – During the study period, 4.9% of all deaths (~2,900 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 0.97% increase in daily deaths. This estimate increased five-fold to 5.31% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
4. Delhi – During the study period, 11.5% of all deaths (~12,000 each year; the highest across all cities studied) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 0.31% increase in daily deaths. This estimate increased to 0.78% when we used a causal modelling approach, indicating the need for greater attention on local air pollution sources.
5. Hyderabad – During the study period, 5.6% of all deaths (~1,600 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 1.22% increase in daily deaths. This estimate increased to 4.55% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
6. Kolkata – During the study period, 7.3% of all deaths (~4,700 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 1.77% increase in daily deaths. This estimate nearly doubled to 3.22% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
7. Mumbai – During the study period, 5.6% of all deaths (~5,100 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. This was the second-highest after Delhi. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 2.41% increase in daily deaths. 
8. Pune – During the study period, 5.9% of all deaths (~1,400 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 0.8% increase in daily deaths. This estimate more than doubled to 1.78% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
9. Varanasi – During the study period, 10.2% of all deaths (~830 each year) were attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. Each 10 μg/m³ increase in short-term PM_2.5 exposure was associated with a 1.17% increase in daily deaths. This estimate increased to 3.80% when we used a causal modelling approach, indicating the strong effect of local air pollution in the city.
10. Shimla – Shimla had the lowest air pollution levels recorded across all the cities in the study. However, air pollution was still a risk here with 3.7% of all deaths (59 per year) attributable to short-term PM_2.5 exposure higher than the WHO guideline value of 15 μg/m³. The results from Shimla add credence to global evidence that there is no safe level of air pollution exposure.