New evidence shows that one in four deaths between 2009 and 2019 is linked to PM 2.5, one of the most dangerous pollutants commonly monitored. 

NEW DELHI – In November Delhi recorded its worst day of air pollution since 2019. As concerned citizens expressed outrage, authorities scrambled for answers – and it seems that the dirty air crisis may be worse than previously reported.

A new study published in The Lancet this week analyses the link between air pollution and deaths in districts (an administrative jurisdiction of a state or province) in India over 11 years from 2009. 

It shows pollution is not just a Delhi problem nor is it a recent problem, estimating that during the time period, 16.6 million deaths are attributable to PM 2.5 pollution. This is the particulate matter pollutant that is much finer than human hair that penetrates deep into the human body. 

The report is timely as the Supreme Court is now expanding the scope of its air pollution hearing from Delhi to cover all of India. 

The study calls for a fundamental rethink of India’s battle against air pollution. Firstly, it shows 24.9% of deaths – almost one in four – are attributable to air pollution, more specifically PM 2.5. 

Secondly, it calls for India’s regulatory standards for air quality to be tightened. The Indian National Ambient Air Quality Standards for annual mean PM2.5 is 40 micrograms per cubic metre (µg/m³) whereas WHO’s guideline is 5 (µg/m³). 

Thirdly, the authors say this report is more relevant to policymakers than previous reports as it is based on data from India. In the past senior Indian government officials have questioned or rejected data linking deaths to air pollution, particularly from global agencies. 

More deaths than previously estimated

The Lancet report found an average of 1.5 million deaths from air pollution between 2009 and 2019, almost a quarter of all deaths.

This is a higher estimate of mortality than earlier studies. For instance, a WHO study reported an average of 830,000 deaths annually in the decade ending 2019, a conservative estimate based on secondary data sources. 

A study for 2019 by the Indian Council of Medical Reseach (ICMA), a government agency, and others estimated 1.7 million deaths. The new study tops that with 1.8 million attributable deaths for that year.

Applying the more relaxed guideline of 40 µg/m³ for PM 2.5 set by the Indian government, the number of deaths is estimated by the Lancet report to be 3.8 million over 11 years, or 300,000 every year. 

The entire population of India breathes air of a quality worse than the WHO’s guideline for an annual average.

New data is ‘more credible’ 

While there have been several large-scale studies globally on the link between PM 2.5 air pollution and deaths, this is the first such one in India which in recent years has the most polluted places. 

One of the authors, Dr Siddhartha Mandal, told Health Policy Watch that studies usually make associations between exposure to PM 2.5 and mortality. However, this report “lends more credibility” to the numbers because it uses a difference-in-difference approach to reach causal estimates. This methodology compares the changes in outcomes over time between a treatment group and a control group.

“We believe that our study provides the most accurate exposure-response function and health impact assessment in India to date based on causal estimations from a state-of-the-art comprehensive exposure assessment and nationwide mortality data collected in India,” according to the authors.

It covers air pollution across 655 districts of India. The authors collected and analysed national counts of annual mortality for the 11 years, and also factored in the population and GDP per capita for each district. 

The authors say they observed stronger associations between annual PM2.5 averages and mortality in poorer districts (in terms of GDP).

Risk of death rises with pollution

In a country racked by an air pollution health crisis, there is one pressing figure for policymakers. Every 10 microgram/cubic meter increase in PM 2.5 leads to an increase in all-cause mortality rates by 8.6%, the study estimated. 

While the AQI at 500 or 1,000 grab headlines, what the research points out in terms of health risks at lower levels is an eye-opener. 

“As the exposure levels increase, a plateauing effect is seen where, if you keep increasing the levels, additional increments in health are likely to be small,” says Mandal, who is affiliated with the Centre for Chronic Disease Control, New Delhi, and the Centre for Health Analytics and Trends, Ashoka University.

Simply put, that means that the risks rise from much lower levels of PM 2.5, when many thought the air quality was fine, but as the levels become extremely high, the risk may plateau or taper. 

How PM 2.5 harms humans

“Due to its size, PM 2.5 can enter the bloodstream and hence gets transported to multiple organs,” Mandal explains.

“Thereafter several common mechanisms such as inflammation and oxidative stress are triggered or exacerbated in the tissues.

“One of the major ways by which PM2.5 affects cardiovascular health is by inducing an imbalance in the autonomic nervous system, which controls several involuntary functions in humans such as cardiac rhythm. So PM2.5 contributes in multiple ways leading to exacerbation or acceleration of these conditions and subsequently death.”

A few days before this report was published, the Indian government reiterated in Parliament that “there are no conclusive data available in the country to establish direct correlation of death/disease exclusively due to air pollution.

“Health effects of air pollution are synergistic manifestation of factors which include food habits, occupational habits, socioeconomic status, medical history, immunity and heredity etc. of the individuals.” 

Union road transport minister #NitinGadkari said he does not feel like visiting #Delhi due to severe air pollution here and often gives it a second thought every time before coming to the capital.

Read here 🔗 https://t.co/7xjzKyiubN pic.twitter.com/0xx3dOONnM

— The Times Of India (@timesofindia) December 4, 2024

However, the authors say that policymakers could first take stock of all the recent work done relating air pollution and health (including mortality) using Indian data.

Experts from multiple domains, including public health, clinicians and engineering disciplines can deliberate on how to incorporate health-related evidence into designing interventions, mitigation strategies as well as revision of air quality standards. 

In parallel, there should be targeted actions backed by scientific evidence in the short and long-term, rather than reactive actions. For example, one could design and test out a public transport based intervention in certain areas within Delhi to assess how it affects pollutant levels across time. 

But they say, “most importantly, we should not wait for a perfect study to emerge and rather utilise available national as well as international evidence to take steps to improve the quality of air in the context of health.”

Image Credits: Chetan Bhattacharji, The Lancet.