Short-term exposure to air pollution levels that meet India’s prescribed safety standards is causing 33,000 human deaths per year in 10 cities, a new study in the journal Lancet Planetary Health has found.

The cities are Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Kolkata, Mumbai, Pune, Shimla and Varanasi.

A significant number of deaths were observed in cities with relatively low levels of air pollution, such as Mumbai, Bengaluru, Kolkata and Chennai, the study noted.

Conducted between 2008 and 2019, it is the first study in India to assess the link between short-term air pollution exposure and mortality across cities with diverse environments.

The researchers relied on data obtained from death registries in the 10 cities for periods of three to seven years, analysing over 3.6 million deaths between 2008 and 2019. They also used data from air quality monitors, satellites, meteorological instruments and other sources to find out the cities’ exposure to PM2.5 pollutants, in volume and over time.

PM2.5 refers to respirable airborne particulate matter not more than 0.0025 mm in width.

India’s National Ambient Air Quality Standards prescribe a “safe” PM2.5 limit of 60 μg/m3 (micrograms per cubic metre of air) over a 24-hour period. The World Health Organization prescribes 15 μg/m3.

“We found that 7.2% of all deaths across all 10 cities could be linked to PM2.5 exposure surpassing the WHO guideline value of 15 μg/m3,” the study said. “Delhi had the highest proportion at 11.5% [equating to ~12,000 deaths per year], while Shimla had the lowest at 3.7% [59 deaths per year] between 2008-’19.”

For every 10 μg/m3 increase in average PM2.5 exposure over two-days, the daily number of deaths rose 1.42% across the 10 cities.

The researchers also used a modelling technique to isolate a city’s exposure to “local sources” of air pollution – transport, waste burning and diesel generators – and found that when taking only these into account, the daily number of deaths rose 3.45% across the 10 cities.

An increase in short-term exposure to PM2.5 in cities with relatively cleaner air had a more pronounced effect on daily mortality than in more polluted cities.

“This is likely due to the sharp increase in risk at lower levels of exposure which plateaus at higher levels which are unlikely to be experienced in these cities,” the study notes.

“The lowest PM2.5 concentration observed during the entire study period across any of the cities was 17.1 μg/m3,” the study said. This is more than the World Health Organization’s prescribed threshold for human exposure.

Deaths due to short-term PM2.5 exposure by city

City Deaths per year  % of total deaths 
Ahmedabad ~2,500 5.6%
Bengaluru ~2,100 4.8%
Chennai ~2,900 4.9%
Delhi ~12,000 11.5%
Hyderabad ~1,600 5.6%
Kolkata ~4,700  7.3%
Mumbai ~5,100  5.6% 
Pune ~1,400 5.9%
Shimla 59 3.7% 
Varanasi ~830 10.2%
*for the nine-year study period between 2008 and 2019. Source: Ambient air pollution and daily mortality in ten cities of India: a causal modelling study

The study’s authors include researchers from the Sustainable Futures Collaborative, Ashoka University and the Centre for Chronic Disease Control in India, Harvard University and Boston University in the United States, and Karolinska Institutet in Sweden.