New IMM thesis on how long-term exposure to air pollution and temperature affects mortality in India

This thesis investigated the effects of long-term exposure to air pollution and temperature on annual mortality in India and identified spatiotemporal trends in air pollution levels across the country that can be useful in informing focused interventions. Previously, studies estimating the mortality burden of air pollution in India have been using inappropriate exposure-response functions from less polluted countries. Furthermore, studies on temperature and mortality in India have only investigated short-term exposure to heat during summer months. Hence, the patterns and mortality burden due to these exposures have not previously been comprehensively addressed despite India being the world's largest populated country at the forefront of climate change. With this broader idea, we a) explored air pollution (PM_2.5) hotspots and their temporal trends, b) investigated associations between long term exposure to PM_2.5 and annual all-cause mortality and calculated the attributable burden and exposure-response function for India using our exposure response function from Indian data, and c)  studied the association of long-term (quarterly) exposure to air temperature and annual counts of heat and cold extremes on annual all-cause mortality along with effect modification by age and socio-economic status.

Can you tell us about some interesting results?

The studies of this thesis are based on India's air pollution and temperature across the country for over a decade and district level mortality counts data from the national register of births and deaths. 

In Study I, we examined air pollution (PM_2.5) exposure at a very fine spatial scale (1 km²) across India, in all states, union territories and few selected cities of the country for 12 years (2008-2019). We aimed to identify areas that demonstrate a consistently high, emerging high or declining trend in their air pollution levels in comparison to their neighboring grids. Across the country ~10% of the area with about 250 million inhabitants (~16% of entire population of India) were identified as consistent hotspots. Emerging hotspots were identified across 2.6% of the country’s area including about 75 million inhabitants. Declining hotspots encompassed a similar area of 2.6% as emerging hotspots but with fewer inhabitants (52 million). Overall, from our results we illustrate a significant proportion of India’s population are living in areas with consistently high or increasingly high levels of PM_2.5 exposure and many of these trends were observed outside major cities, which needs monitoring. 

To read more: https://www.sciencedirect.com/science/article/pii/S0013935124021832

In Study II, we investigated annual average air pollution (PM_2.5) exposure across India at the district level from 2009 to 2019 and its association with annual mortality. We examined the effect of long-term exposure to air pollution and mortality using a quasi-experimental study design and estimated the attributable burden and exposure-response function specific to India. The results showed an increased mortality risk by 8.6% for with every additional 10 μg/m³ increment of annual average PM_2.5 corresponding to an attributable burden of 3.8 million deaths (in comparison to Indian National Ambient Air Quality Standards) and 16.6 million deaths (in comparison to revised WHO guidelines) throughout the study period.  We also observed a near linear exposure-response function for PM_2.5 and mortality and leveling off at extremely high exposure levels. 

To read more: https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(24)00248-1/fulltext

In Study III, we examined air temperature exposure across India at the district level from 2009 to 2019, using a 3-month (quarterly) average as well as episodes of extreme temperature and their associations with annual mortality. We used a quasi-experimental study design among six climatic zones of India. The results indicated varying levels of risk for mortality across different climate zones for increasing quarterly temperature and episodes of extremes of heat and cold. In the tropical monsoon climate zone, we observed an 8.6% increased risk of mortality per 1°C increase in quarterly average temperature in quarter 1 (Dec-Feb), 8.3% in quarter 2 (Mar-May), 6.7% in quarter 3 (Jun-Aug), and 17.5% quarter 4 (Sep-Nov). Stronger effects were observed for districts with lower socio-economic status. Among temperate and tropical savannah climate zones, we observed increased risk for mortality with every additional episode of heat extremes.  While in the arid steppe, tropical monsoon and temperate climate zones we observed increased risk for mortality with every additional episode of cold extremes and stronger effects among districts with lower socio-economic status. 

What further research is needed in the area?

Understanding the impact of air pollution and temperature on mortality is fundamental to both protect the most vulnerable groups and reduce the public health burden in India. Through the studies included in this thesis, we elucidated the hotspots of air pollution and their temporal trends across the country at a fine spatial resolution (1km²) within the states and cities. Future research examining the effect of interventions to lower exposure and improve health outcomes in these areas are warranted, not the least in often overlooked rural or semi-urban areas. We also reported increased mortality and the attributable burden of PM_2.5 exposure. However, in real life situation, air pollution is a mixture of components including PM_2.5, hence interactions between other pollutants and temperature need to further be examined. Further, we reported increased risk of mortality with increasing temperatures in specific climatic zones. With increasing climate change-related events, we need to understand the extent of health burden and identify vulnerable groups to target interventions. India is the most populous country in the world with some of the highest air pollution and temperature levels. Therefore, the interplay between these two exposures in this setting needs careful evaluation to mitigate potentially avoidable public health burden. Novel epidemiological designs offer some advantages to making headway when data constraints may otherwise hamper traditional epidemiological methods.

Read the thesis