Last year, the government of India launched an Air Quality Index (AQI) to better communicate air pollution levels to citizens. An AQI reduces the complex problem of air pollution to a single number that can be understood easily by a layperson. There are eight sub-indices that monitor level of a specific pollutant: PM10, PM2.5, nitrogen dioxide (NO2), sulphur dioxide, ozone, carbon monoxide, ammonia and lead. Based on the concentration of each pollutant, a sub-index is calculated for each. The worst sub-index determines the overall AQI. For instance, if the worst sub-index value for a particular locality is for NO2, then this number will be communicated as the AQI value for that particular day.

Particulate matter (PM) is widely acknowledged as the most serious problem in Indian cities. PM is the most harmful to human health, and real time air quality bulletins from Central Pollution Control Board (CPCB) also show particulate matter as the predominant pollutant in almost all the Indian cities.

Mint looked at city-wise particulate matter data from 1990 onward from CPCB. In the 1990s, particulate matter data was recorded in the form of suspended particulate matter (SPM). By the early 2000s, with the advent of more sophisticated machines that could measure even smaller particles, CPCB started releasing this data in the form of respirable suspended particulate matter (RSPM), a subset of SPM. This gave way to PM10, or particulate matter whose diameter was smaller than 10 microns. From March 2015 onwards, CPCB moved towards actively measuring PM2.5 (particulate matter whose diameter was smaller than 2.5 microns) at most of its monitoring stations. The average diameter of human hair is 75 microns. This comparison gives us an idea about how small PM 2.5 is.

What is the state of pollution in India’s different cities? To compare air pollution across cities, Mint picked up the top 5 cleanest and dirtiest cities for every year between 2007 and 2015. We then narrowed down our selection to the cities that had featured the greatest number of times out of this list, thus ensuring that we picked those cities that had consistently been in the top 5 cleanest or dirtiest cities over this period. Statistics for PM 10 levels from 2007 onwards show that cities in southern part of the country have much cleaner air than those in the north.

Cities like Ghaziabad, Allahabad, and Raipur recorded much worse PM 10 levels than Delhi in the last decade. The north-south divide manifests itself in data going back to the 1990s as well.

What explains this north-south divide in air pollution? Both man-made and natural factors are responsible for this, says AnubhaGoel, an assistant professor at the department of civil engineering at IIT-Kanpur. “With higher population density, and much larger number of vehicles on road, the amount of pollutants emitted in northern India is much more than in the south. The stark temperature difference between winters and summers not only changes wind patterns, it also raises energy consumption, both fossil fuel and biomass, in North India. Biomass burning, a significant contributor of PM2.5 emissions, spikes in North India during winters because of the intense cold. Topographical and meteorological conditions, mainly wind flow characteristics, control both pollutant dispersion and residence time of airborne pollutants. With Himalayas on the north acting as a barrier and limited availability of large water bodies acting as sinks, the dispersion of pollutants is limited in the Northern part of the country, particularly the Indo Gangetic Plain. In contrast, coastal regions in the south have fresh winds coming in and polluted air blown out to the sea which controls pollutant levels significantly. Educational levels being lower in the north probably account for lower environmental awareness which leads to callous attitude towards environmental issues,” she explains.

How do Indian cities compare worldwide? A look at the WHO database of the world’s most polluted cities show that only four Indian cities feature among the top 25 most polluted cities in the world for PM10 pollution. But for PM2.5, twelve Indian cities feature in the top 25 most polluted cities. The country that features the next highest number of cities is China, with 5 cities in the list.

Goel puts down the difference to the differing sources of pollution for both pollutants. “A lot of African and middle-eastern cities show up as the worst in PM10 because PM10 includes larger, coarse particles, which originate from both anthropogenic (human activities) and natural sources. In these locations natural sources, such as dust and desert sand, mostly dominate observed PM10 levels. On the other hand, PM2.5 or finer particles, a subset of PM10, originate primarily from anthropogenic sources. The largest contributors to PM2.5 load in urban regions are vehicles and power plants. It’s thus not surprising that the worst cities in terms of PM2.5 pollution include mostly rapidly urbanising regions in South Asia,” she explains.

Finally, any analysis of city-wide pollution levels based on current and historical data must be taken with a pinch of salt. This is because; quality of air pollution data is dependent on how extensively it is being collected. For example, if a city has just one air quality monitoring station, then its pollution levels can vary drastically depending on whether the station is located in a highly polluted area or not. India lags behind major countries, in terms of number of monitoring stations adjusted for its population. Moreover, only a limited number of Indian cities are monitored by the CPCB, though coverage has steadily increased through the years.