India now grades air quality along a colour-coded chart based on pollutant levels.

Delhi’s severely high pollution levels during the winter months drives it to India’s worst annual air quality levels, but on individual days, several Indian cities often experience worse air quality. For the first week of April, for example, the urban area with the worst air quality of 17 locations analysed by The Hindu was the BTM Layout region of south Bangalore. The new National Air Quality Index gives current as well as 24-hour average data on particulate matter – PM2.5 or very fine particles smaller than 2.5 micrometres in diameter, and PM 10 which are less than 10 micrometers in diameter – as well as other pollutants including nitrogen dioxide, ozone and carbon monoxide. PM 2.5 levels are commonly used as the best indicator of severe air pollution, while PM 10 particles are also a cause of public health concern, but less lethal. India now grades air quality along a colour-coded chart based on pollutant levels.

Using data from the new Index, as well as data from the Central Pollution Control Board (CPCB) and state pollution control boards, The Hindu found that Bangalore’s BTM Layout area has had only one “moderate” air quality day in the last ten days, while all the rest were “poor” or “very poor”. Other parts of the city did far better.Delhi, the focus of global attention for its consistently poor air quality, on the other hand, had “moderate” air quality over the first week of April for the three monitoring stations for which PM 2.5 was available - Dilshad Gardens, Dwarka and Shadipur. Dwarka was the only location with more “poor” or “very poor” days than “good” or “satisfactory” ones.

CPCB officials warned that the quality of monitoring stations was mixed across the country, and Delhi was likely to have the most rigorous data. “There is still a lot of standardisation work we need to do to get all cities and all monitoring stations comparable. The new index should be seen as indicative,” the official said, asking not to be named. Moreover, while comparable data for these 17 locations is available only for the last few weeks, particulate matter is heavily influenced by weather patterns. Anumita Roychowdhury, head of the Centre for Science and Environment’s air pollution and clean transportation programmes, compiled data for Delhi from October 2014 onwards. The data shows that while there were “moderate” days in October, February and March, the second halves of November and December, and the first half of January were consistently “very poor”.

In 2014, the World Health Organisation compiled average annual PM 2.5 numbers for over 1600 cities across the world, including 124 from India. Delhi had the worst air quality in the world by that estimate, but 12 other Indian cities were among the world’s worst 20 – Patna, Gwalior, Raipur, Ahmedabad, Lucknow, Firozabad, Kanpur, Amritsar, Ludhiana, Allahabad, Agra and Khanna.