According to a recent report titled Air Quality and Health in Cities, published by the United States-based Health Effects Institute, Delhi and Kolkata are the top two most polluted cities in terms of exposure to harmful fine particulate matter (PM2.5).


About the report

The report examines pollution and global health implications in over 7,000 cities worldwide, focussing on two of the most dangerous pollutants – fine particulate matter (PM2.5) and nitrogen dioxide (NO2).


Findings of the report

  • The report, using data from 2010 to 2019, found that global patterns for exposures to the two key air pollutants (PM 2.5 and NO2) were strikingly different.
      • Particulate Matter (PM) 2.5 refers to a category of particulate pollutant that is 2.5 microns or smaller in size.
      • PM 2.5 is considered especially dangerous to human health because they bypass many of our body’s defences (nose hair, mucus) and can get into our lungs, from where they can eventually enter the bloodstream.
      • Nitrogen dioxide (NO2) is a gaseous air pollutant, which forms when fossil fuels such as coal, oil, gas or diesel are burned at high temperatures.
      • As city residents tend to live closer to busy roads with dense traffic, they are often exposed to higher NO2 pollution than residents of rural areas.
  • According to the report, while exposures to PM2.5 pollution tend to be higher in cities located in low and middle-income countries, exposure to NO2 is high across cities in high-income as well as low- and middle-income countries.
  • The report found most global cities far exceed World Health Organisation’s (WHO) air pollution guidelines, posing serious health risk.
      • In 2019, 86% of the cities analysed exceeded the WHO’s 10 µg/m3 (microgram per cubic metre) guideline for NO2, impacting about 2.6 billion people.
  • 41 of the 50 cities with the highest increase in PM2.5 are in India, with 9 in Indonesia.
  • On the other hand, all 20 cities with the highest reduction in PM2.5 pollution from 2010 to 2019 are in China.
  • India specific findings –
    • Delhi and Kolkata were ranked first and second in the list of top 10 most polluted cities when PM2.5 levels were compared.
      • In terms of impact, Delhi and Kolkata ranked sixth and eighth for PM2.5 related disease burden, reporting 106 deaths and 99 deaths per lakh of population, respectively due to exposure to PM2.5 in 2019.
    • However, no Indian city appeared in the list of top 20 polluted cities when N02 levels were compared (Shanghai at the top with an average annual exposure of 41 µg/m3).


Why is there such a contradiction in India?

  • According to the report, ground monitoring of air quality remains insufficient in many parts of the world, particularly in low and middle-income nations, concealing the true extent of NO2 pollution in places like India.
      • For example, based on the 2022 WHO Air Quality Database, of the 20 cities with the steepest increases in PM2.5 exposures in the report, only two (Satna and Varanasi in India) have an official monitoring station at ground level
  • According to the experts, this paradoxical situation (difference in PM2.5 and NO2) in India was likely due to the relatively lower adoption of high-efficiency engine vehicles.
      • Complete combustion of fuel results in higher NOx (nitrogen oxides) where incomplete combustion sees other kinds of emissions.
      • Due to their highly reactive nature, nitrogen oxides also contributed to the formation of other pollutants, including ozone and particulate matter.
  • NO2 also has a shorter lifetime compared with PM2.5 and other air pollutants.
      • As a result, NO2 levels show very high variability in space and time and levels can vary significantly even across a few kilometres within the city.
      • In comparison, PM2.5 levels tend to show less spatial variation at the fine scale.


Way ahead for Indian cities

  • More developed cities who have controlled PM2.5 exposures are now in the grip of NO2 problem.
  • Therefore, the Centre for Science and Environment (CSE) suggests that Delhi and Kolkata need the most stringent time bound multi-sector action to meet the clean air benchmark.
  • The Indian cities must also learn from Beijing, which still has the largest PM2.5-related disease burden (despite significantly lowering its PM2.5 exposures) due to expanding and ageing populations.