An increase in air pollution in Indian cities with clean air can increase death rates more than in cities with higher levels of pollution. For example, the same increase in air pollution can increase death rates more in Bengaluru than in Delhi, where air pollution is much higher.
Overall, however, high-pollution cities had a larger share of annual deaths due to air pollution: In Delhi, 11.5% of annual deaths were due to air pollution, compared to 4.8% in Bengaluru. Bengaluru’s population was exposed to 30% more air pollution on a daily basis than the average Delhi resident, according to the first analysis of several cities in India to examine the short-term health effects of air pollution, published in the peer-reviewed journal The Peer Review. Lancet Planet Health on Thursday.
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Nearly 30,000 deaths, or 7.2% of annual deaths in the 10 cities, were due to short-term PM2.5 pollution. The total daily deaths in the 10 cities increased by 1.42% for every 10 μg/m3 increase in average PM2.5 pollution over a two-day period, the study found. The scientists analyzed pollution and death registration data from Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Kolkata, Mumbai, Pune, Shimla and Varanasi.
Between 2008 and 2019, about 12,000 people died annually in Delhi due to air pollution, while Shimla had 59 deaths – the lowest among the ten cities. Bengaluru had 2,102 deaths during the same period.
“Our results also confirmed that the risk of death increased faster at lower PM2.5 levels but leveled off as levels increased. Significantly, we found that the risk of death was very high (2.65%) even when we analyzed days with PM2.5 levels below the current Indian air quality standard of 60 μg/m3,” reported the authors, who include several institutes in Europe and India.
In Frames | Gasping for air
Withering smoke: Stubble being burned behind mustard fields near Ambala
Angry: Charred remains in a field after stubble burning near Ambala.
Ground Zero: A thick layer of smog covers Central Vista in the capital.
Clear signs: The Signature Bridge in New Delhi is shrouded in thick smog.
Risky business: A farm worker pulls a bicycle tube out of burning stubble in a field near the Ambala bypass in Chandigarh
Far from the sun: The setting sun almost disappears in the smoke rising from the fields as a farmer burns stubble in Ambala, Haryana.
Fumigated: An autorickshaw driver takes cover during a fumigation drive in Jangpura, New Delhi
Easy prey: A traffic policeman wears a mask on duty during the smog in New Delhi
Hardly visible: On a winter morning in Gurugram, vehicles are stuck in a traffic jam on the Delhi-Gurugram Expressway with limited visibility due to smog.
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The differences in mortality rates across cities in India mirror the results of similar studies in other countries. A study of 272 cities in China reported an increase of 0.22% for every 10 μg/m³ increase in PM2.5. However, death rates were higher in Greece (2.54%), Japan (1.42%) and Spain (1.96%), where background pollution levels were lower.
“As efforts continue to develop and strengthen air pollution action plans at the state, regional and city levels, the results of this study demonstrate the growing need to address dispersed local sources of air pollution in addition to traditional fixed and linear sources. This work reinforces the message that there is no safe limit for air pollution, even in highly polluted regions,” they added.
The authors obtained annual city-level death counts from city authorities and used a novel approach called the “instrumental variable approach” to isolate the impact of local air pollution on mortality. They did so by identifying three weather-related parameters – planetary boundary layer height or mixing height, wind speed and barometric pressure – that are directly related to variations in daily air pollution but have no relationship to daily deaths except through changes in air pollution. “Using this approach, we isolate the impact of locally generated air pollution because these three instruments are linked to the dispersion and transport of air pollution,” said Bhargav Krishna of the Sustainable Futures Collaborative and one of the study’s authors. The Hindu.
This analysis actually found a “much stronger association” between air pollution and mortality than the traditional approach of correlating annual air pollution levels with mortality, Dr. Krishna said.
He explained the larger increase in mortality in cities with lower pollution levels than in cities with higher pollution levels as an example of the “harvest effect” in epidemiology. “This does not mean that the risk is low at higher levels, it is just that the increase in risk is slowing down. If the increase in risk at 20 to 40 micrograms (μg) is 5%, the increase in risk at 120 to 140 μg is 0.5%. The reason we see this or the harvest effect is because most people who are vulnerable to air pollution are likely to die at lower levels of pollution,” he added.
