Though almost the entire debate on air pollution has been centered around Delhi, there are several cities and towns in India which have much worse air quality than the national capital. Just outside Delhi, Ghaziabad is one of the most polluted cities in the country.

The Central Pollution Control Board has been running a National Air Quality Monitoring Programme since 1984 under which three pollutants — sulphur dioxide (SO2), nitrogen dioxide (NO2) and particulate matter of less than 10 micron (one micron is a millionth of a metre) (PM10) — are monitored. This programme now covers 254 towns and cities in 29 states and five union territories. There are 612 operating monitoring stations in these places which continuously collect data on these three pollutants.

There are 46 cities in the country with a population of one million or more. These are also amongst the most polluted. We have featured four of these cities here. Based on the average concentration of PM10 in 2013, the last year for which full data is available, Raipur in Chhattisgarh was the worst polluted million-plus city in the country, followed by Ghaziabad, Allahabad and Delhi. The acceptable limit of average annual PM10 concentration is only 60 micrograms per cubic metre (mg/cu m) but Raipur had a concentration of 305 mg per cubic metre in 2013.

It was the third consecutive year that Raipur had recorded a 300-plus reading of PM10. Chhattisgarh has large deposits of iron ore and there are several sponge iron factories in and around Raipur, spewing particulate matter into the atmosphere.

Data at these 600-plus monitoring stations is collected manually, on a piece of paper, and sent to the respective state pollution control board which are mandated to send it to the CPCB every month. But many SPCBs don’t send this data on time, sometimes delaying it by even six months. As such, even the 2014 data is not entirely verified and released as of now.

In April 2015, the CPCB developed an index of air quality (AQI) based on concentrations of eight pollutants. Data of at least three pollutants, one of which was particulate matter concentration, was required to calculate the index. Real-time monitoring of these pollutants started at at least one monitoring station in 12 cities and an AQI was calculated. The programme has now expanded to cover 21 cities. Ultimately, the idea is to have a real-time AQI for every monitoring station in the country.

The AQI of several monitoring stations outside of Delhi are seen to be routinely worse than many of those in Delhi. Here is how the AQI at some of these stations looked at 7 pm on the first day of the new year.