NEW DELHI – On 2 November 2023, extreme air pollution caught Delhi off-guard. It shut down schools and led to flight cancellations, curbs on construction and other emergency responses. 

That every little increase in air pollution increases the risk of death has been well-established for some years. But till now the effect of short-term air pollution on mortality has remained largely undocumented in detail in India, where most of the world’s most polluted cities are located. 

In a new, peer-reviewed study published in the Lancet on Thursday (4 July), researchers found that 7.2% (or approximately 33,000) of all deaths each year across 10 cities could be linked to short-term PM 2.5 exposure higher than the World Health Organization (WHO) guideline of 15 micrograms/cubic metre. PM 2.5 is a highly toxic ultrafine particulate matter with a diameter of 2.5 microns, which is far thinner than a strand of human hair. 

This is the first multi-city study in India to assess the link between short-term pollution exposure and death with cities spanning different agro-climatological zones.

The bottom line, the authors say, is that the current Indian government standard for PM 2.5 of 60 µg/m³ is “substantially higher” than it should be as a measure of ‘safe’ air quality. In comparison, WHO’s 24-hour standard is 15 µg/m³.  

The researchers are from 14 organisations including Sustainable Futures Collaborative in Delhi, Institute of Environmental Medicine in Stockholm, Indian Institute of Management Ahmedabad, Public Health Foundation of India, Ashoka University in Sonipat, Mount Sinai in New York, and Boston University. 

To come to that conclusion, the researchers analysed primary data on mortality between 2008 and 2019 in 10 cities of India. They used an advanced system of machine learning-based analysis which, simply put, used factors that would have a bearing on air pollution-related deaths, namely wind speed, atmospheric pressure and mixing height. 

This causal modelling approach enabled the researchers to isolate the effect of locally generated pollution as these factors are linked to the dispersal and transport of air pollution. 

Combined with other data from monitors, satellites and other sources, the study looked at over 3.6 million deaths in the 11-year study period. What the more complex causal modelling shows is that the risk of death is greater than what the earlier basic approach showed. 

No safe level of air pollution

The causal modelling estimated an increase in the risk of death by 3.57% for every rise of just 10 micrograms/cubic metre over two days, compared to 1.42% by a more basic study. 

Apart from the usual high-pollution suspects like Delhi and Varanasi, the study looked at places commonly perceived as having clean air, like the Himalayan city of Shimla and the coastal metropolises of Chennai and Mumbai. The authors say this is the first such multi-city study on the links between short-term air pollution exposure and deaths in India. 

Overall, 7.2% of all daily deaths were attributed to PM 2.5 concentrations higher than the WHO guidelines. Annually there are estimated to be approximately 33,000 deaths in these 10 cities. Over a third are in Delhi, where the average annual PM 2.5 level was 113 micrograms which is more than 22 times the WHO’s safe standard. The share of PM 2.5 linked deaths was also the highest in the Capital, at 11.5% compared to the lowest 3.5% in Shimla. 

The surprises are Mumbai and Kolkata, with deaths numbering around 5,000 each. 

“We are seeing a high level of risk and a high number of deaths even in cities that have moderate levels of air pollution exposure. Mumbai, for instance, despite being a coastal city with around a third of the annual PM 2.5 levels of Delhi is still seeing over 5,000 deaths yearly from air pollution,” a lead author of the report, Dr Bhargav Krishna, Fellow at the Sustainable Futures Collaborative, explained to Health Policy Watch.

“Similarly Hyderabad, Kolkata, Bengaluru, Chennai, Ahmedabad and Pune, all of which have annual PM2.5 levels below the current Indian standard, are still seeing a high number of deaths each year. This should drive us to focus not just on those cities that have high seasonal exposures, but even those that may be considered relatively ‘clean’ but are in reality polluted at a level that causes significant health impacts.”

Mumbai, where remarkably the air pollution has been higher than Delhi on some days, was shown to have almost 5,100 deaths, the second highest after the national capital. That is 5.9% of all deaths in what is India’s financial capital. Here, every increase of 10 micrograms of PM 2.5 was associated with a 2.41% increase in daily deaths. 

Source: Lancet

Shimla proves that there is no safe level of air pollution, according to the report. It has the lowest air pollution level of  the 10 cities, yet that was still a risk as 3.7% of all deaths were attributable to short-term PM 2.5 exposure. 

Impact on policy action 

The findings are crucial for immediate policy action and further studies to cover hundreds more towns and rural areas. Immediate action would cover sources of pollution like diesel generators, waste burning, and transport, sources that are commonly seen in many urban neighbourhoods. 

While much of the headline-grabbing focus is on extreme pollution, these findings show the need to act against all sources of air pollution all year round. Acting only on extremely high days of pollution will only  yield marginal benefits regarding daily mortality.

 In particular, the paper calls out the Graded Response Action Plans, largely used in Delhi and its neighbouring cities. GRAPs are usually cited by local governments as their action to cut pollution but are implemented only in peak pollution season which is from October right through winter. 

Toxicity of local pollution

There are some interesting findings which aren’t widely known. The study confirms that the risk of mortality rose more quickly at lower levels of PM 2.5 but plateaued as levels increased. More studies are needed to understand why exactly this happens. The causal modelling showed that the effects of air pollution on deaths were especially strong in cities with lower levels of pollution such as Bengaluru, Chennai and Shimla. 

The Lancet paper doesn’t look at the cause of death as that level of primary data isn’t always available. However, many studies including recent ones in India have shown that air pollution triggers or aggravates non-communicable diseases – specifically NCDs like heart attacks, strokes, lung cancer, and chronic lung disease – which can turn fatal. Of the total air pollution deaths, NCDs account for nearly 90% of the disease burden. 

The study fills a data and communication gap in addressing the health crisis caused by toxic air. On one hand, it establishes a link between rising air pollution and deaths, on the other, it makes the risks a lot more relatable because it shows the impact over just two days of exposure than over many months or years. 

So what can residents in these cities do to protect themselves? Bhargav says individual responses could include wearing a mask, choosing when to step outdoors (pollution tends to be highest during the morning and evening when temperatures dip), and reducing pollution sources inside their home. 

However, the onus lies on governments and policymakers. 

“The levels of air pollution we see in India are extremely high and this study clearly shows how day-to-day variations in these levels lead to considerable mortality,” said Dr Petter Ljungman of the Karolinska Institute, and one of the researchers involved in the study.

“Interestingly we saw that local pollution sources are likely to be more toxic than more distant sources which has implications for policymakers addressing this highly relevant threat to human health.”

Image Credits: Chetan Bhattacharji.