Chennai had the highest proportion of ‘severe’ days based on AQI

Official air quality data for the first half of this year shows that while Delhi does indeed face high levels of air pollution, Kanpur, Varanasi and Chennai are worse off.

Averaging across the ten pollution monitoring stations in the city, a little over a quarter of the days from January to June this year in Delhi had an Air Quality Index (AQI) value of severe (3.2 per cent), very poor (8.7 per cent) or poor (14.4 per cent). In comparison, nearly half of all days in Kanpur were of severe, very poor or poor air quality. Varanasi had no ‘severe’ days, but nearly half of all days were either of very poor or poor air quality. Averaging across its three monitoring stations, Chennai had the highest proportion of ‘severe’ days (17.7 per cent), and a third of all days were either of severe, very poor or poor air quality.

The average AQI value for the last six months for these three cities as well as Lucknow was also higher than Delhi’s.

SO2, CO prominent pollutants in Chennai

In the first week of April, the union government unveiled a colour-coded index of air quality based on recommendations from the Indian Institute of Technology Kanpur. The government’s tracking website currently has live data for 11 cities across the country. Central Pollution Control Board (CPCB) monitoring stations are able to read concentration levels for one or more pollutants of the following - Particulate Matter with a diameter less than 10 micrometres (PM10), Particulate Matter with a diameter of less than 2.5 micrometers (PM2.5), ozone (O3), Nitrogen Dioxide (NO2), Carbon Monoxide (CO), and Sulphur Dioxide (SO2). The values for any of these pollutants the station is able to pick up are converted into an AQI value. The pollutant with the highest AQI value at the time for that city is determined to be the ‘prominent pollutant’ and its AQI value becomes the station’s overall AQI value.

According to the index, days when the AQI value is between 401 and 500 are ‘severe’ and “may cause respiratory impact even on healthy people, and serious health impacts on people with lung/heart disease. The health impacts may be experienced even during light physical activity.” Among all 26 monitoring stations for which data is available, the IIT Chennai monitoring station has by far the most ‘severe’ air quality days – over 47 per cent of all days in the last six months. Two monitoring stations in Lucknow – Central School and Lalbagh West Lucknow – have the second and fourth highest proportion of severe days, while Anand Vihar in Delhi is third, and Nehru Nagar in Kanpur is fifth.

In Delhi, high AQI values are driven primarily by PM 2.5, while in Chennai, they are driven by sulphur dioxide and carbon monoxide. In Bengaluru, ozone is additionally often the prominent pollutant. The AQI is normalised for the concentration levels of different pollutants, with a common scale, and so comparing AQI values, even if the prominent pollutant is different, is valid, a senior official connected with the AQI told The Hindu. Similar AQI values, even if for different pollutants, indicate that the health impact on humans will be similar, and also suggest which pollutant that city needs to curb, the official added.

“Northern India tends to have higher PM levels than southern India, and the impression in the south sometimes is that there is less pollution there. What you’ve found shows that there is high concentration of gaseous pollutants in other cities, and all pollutants have health risks,” Anumita Roychowdhury, head of the Centre for Science and Environment’s air pollution and clean transportation programmes, told The Hindu. This indicates high levels of combustion and vehicular pollutants, she added.