Delhi, Kolkata, Mumbai among world’s top 20 most polluted cities: Basics Explained

New Delhi, Kolkata, and Mumbai are among the top 20 most polluted cities in the world in terms of PM2.5 levels, according to the latest State of Global Air report on air quality and health in cities. While New Delhi and Kolkata rank first and second, respectively, Mumbai stands 14th on the list.

The study ranked cities on the basis of two major air pollutants found — fine particulate matter (PM2.5) and nitrogen dioxide (NO2). PM2.5 is an atmospheric particulate matter of a diameter of fewer than 2.5 micrometers, which is around 3 percent the diameter of human hair. It causes respiratory problems and reduces visibility.

Meanwhile, road traffic is among the primary outdoor contributors of NO2, long exposure to which is linked to deaths.

Delhi, India (110)

Kolkata, India (84)

Kano, Nigeria (83.6)

Lima, Peru (73.2)

Dhaka, Bangladesh (71.4)

Jakarta, Indonesia (67.3)

Lagos, Nigeria (66.9)

Karachi, Pakistan (63.6)

Beijing, China (55)

Accra, Ghana (51.9)


PM2.5 means particulate matter in the air – caused by motor exhaust or anything combustible – that is less than 2.5 micrometers.

         Airborne particles are sometimes referred to as ‘particulate matter’ or ‘PM’. They include dust, dirt, soot, smoke, and liquid droplets. Some particles are emitted directly into the air from a variety of sources that are either natural or related to human activity. Natural sources include bushfires, dust storms, pollens and sea spray.

           Those related to human activity include motor vehicle emissions, industrial processes (eg electricity generation, incinerators and stone crushing), unpaved roads and wood heaters.

Particles can be classified on the basis of their size, referred to as their ‘aerodynamic diameter’. ‘Coarse particles’ are those between 10 and 2.5 micrometres (µm) in diameter; ‘fine particles’ are smaller than 2.5 µm; and ‘ultrafine particles’ are smaller than 0.1 µm. Studies have linked exposure to particle pollution to a number of health problems including respiratory illnesses (such as asthma and bronchitis) and cardiovascular disease.


India has set standards for what it thinks are appropriate warnings for a particular level of pollutant. Air Quality Index(AQI) help in comparing pollution levels at a glance with a color code and a numerical value. In India, AQIs are determined based on the concentrations of pollutants, including PM2.5 (fine, respirable particles), sulphur dioxide (SO2), nitrogen dioxide (NO2) and carbon monoxide (CO).There are six AQI categories, namely: Good, Satisfactory, Moderately polluted, Poor, Very poor and Severe.

                    The Index is centered around five chief pollutants – Particulate Matter with a diameter less than 10 micrometers (PM10), Particulate Matter with a diameter of less than 2.5 micrometers (PM2.5), ozone (O3), Nitrogen Dioxide (NO2), and Carbon Monoxide (CO).


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