Death rates from air pollution spikes vary across cities: Study

Delhi had about 12,000 deaths per year, while the lowest rate among the 10 cities surveyed is Shimla, with 59 deaths per year between 2008-19

Updated - July 04, 2024 02:47 pm IST

Published - July 03, 2024 08:25 pm IST - NEW DELHI

From air pollution, Delhi had about 12,000 deaths per year. File.

From air pollution, Delhi had about 12,000 deaths per year. File. | Photo Credit: Sushil Kumar Verma

A spike in air pollution in Indian cities that have clean air may raise death rates higher than in cities that have higher pollution loads. Thus, the same increase in air pollution in, say, Bengaluru, can raise death rates more than in Delhi, which has much higher background levels of air pollution.

Overall, however, cities that had high pollution loads saw a greater fraction of annual deaths attributable to air pollution, with 11.5% of Delhi’s annual deaths attributable to air pollution, and 4.8% in Bengaluru. The latter’s population had 30% the exposure to daily air pollution than the average Delhi resident, says a first-of-its kind multi-city analysis in India that studied the health effects of short-term exposure to air pollution published in the peer-reviewed Lancet Planet Health on Thursday.

Explained | How can India monitor air pollution-related illnesses effectively?

Nearly 30,000 deaths or 7.2% of the 10 cities’ annual deaths were due to short-term PM 2.5. Daily deaths totalled across the 10 cities rose by 1.42% for every 10 μg/m3 increase in the average PM2.5 exposure over a two-day period, the study found. The scientists analysed pollution and death registry data from Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Kolkata, Mumbai, Pune, Shimla, and Varanasi.

From air pollution, Delhi had about 12,000 deaths per year while Shimla had 59 deaths per year between 2008-19 — the lowest among the 10 cities. In the same period, Bengaluru had 2,102 deaths.

“Our findings also confirmed that the risk of mortality rose more quickly at lower PM2.5 levels but plateaued as levels increased. Significantly, we found mortality risk to be very high (2.65%) even when analysing days with PM2.5 levels below the current Indian national air quality standard of 60 μg/m3,” the authors, who span multiple institutes in Europe and India, reported.

The variation in mortality in different cities in India mirrors findings from similar studies in other countries. A 272-city study in China reporting a 0·22% increase, per 10 μg/m³ increase in PM2·5. However, death rates were higher in Greece (2·54%), Japan (1·42%), and Spain (1·96%), which had lower base pollution levels.

“As efforts to develop and strengthen air pollution action plans at state, district, and city levels continue, the results of this study show the growing need to address dispersed local sources of air pollution in addition to traditional fixed and line sources. This work reinforces the message that there is no safe level of exposure to air pollution, even in highly polluted regions,” they added.

The authors sourced city-level annual death numbers from municipal authorities, and to specifically isolate the impact of local air pollution on mortality, employed a novel approach called an ‘instrumental variable approach’. For this, they identified three weather-related parameters — planetary boundary layer height or mixing height; wind speed; and atmospheric pressure — that are directly related to variations in daily air pollution, but are unrelated to daily deaths except through air pollution changes. “Through this approach, we isolate the effect of locally generated air pollution since these three instruments are linked to dispersal and transport of air pollution,” Bhargav Krishna, of the Sustainable Futures Collaborative and one of the authors of the study, told The Hindu.

This analysis actually found a “much stronger link” between air pollution and mortality than the traditional approach of correlating annual air pollution readings with mortality, Dr. Krishna said.

He explained the sharper mortality spike in cities with lower pollution loads than more polluted cities as an example of the ‘harvesting effect’, in epidemiology. “This does not mean that risk is low at higher levels, it is just that the increase in risk slows down. If the increase in risk from 20 to 40 micrograms (μg) is 5%, the increase in risk from 120-140 μg is 0.5%. Why we see this, or the harvesting effect, is that most people who are vulnerable to air pollution are probably dying at lower levels of exposure,” he added.

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