Is The Hindu is Wrong About Chennai’s Air Pollution

Last week a story that originated from the Hindu about Chennai’s air pollution got picked up and cited across the media in India. In The Hindu’s report, it claimed that Chennai had taken the crown from Delhi as the most polluted city in India and therefore the world.

There was one problem with the report. It was dead wrong.

The journalist, was not able to comprehend the data before them and unfortunately took everything at face value. Had they delved a little deeper, asked themselves if the data made sense or cross-checked the data with an expert, they would not have published the story.

What The Hindu Claims About Chennai

In the report, the journalist diligently compiled the data for the all the monitoring stations in India’s cities beginning from December 2014 onwards. This is no easy task and would have taken several days and a lot of patience to complete.

The journalist averaged out the meter readings for each city and discovered that 17.7% of the days in Chennai had a value of Severe – the worst possible pollution level. By contrast, just 3.2% of the days monitored in Delhi were Severe.

By the analysis given above, the journalist concluded that Chennai was more polluted than Delhi.

Where The Hindu Journalist Went Wrong

The first problem comes from the way the National Air Quality Index reports the numbers. The monitoring stations collect data on the levels of Ozone (O3), Sulphur Dioxide (SO2), Carbon Monooxide (CO), Nitrogen Dioxoide (NO2) and PM2.5 particles – the most damaging type that are so small they can even enter the blood stream.

The Air Quality for a monitoring station is reported on whichever pollutant has the highest value over a 24 hour period. One day the O3 levels could have the highest average at 110 and the air quality reading for the day would be 110. The next day PM2.5 particles could have the highest average index of 95 and therefore the reading for the day would be based on the PM2.5 reading of 2.5.

This methodology assumes that all pollutants are equal. They are not. We’d much rather take our chances in a city that has higher (NO2) than very high levels of PM2.5 (actually, we’d rather be in a city with low levels of both).

The IIT-Madras monitoring station reported exceptionally high levels of SO2, while at the same time low levels of PM2.5 particles. This produced a chart which looked like this:


From 22nd January to 2nd May, about three and a half months, the monitoring station reported a Sulphur Dioxide index of between 400 and 500. After the 5th May, the Sulphur Dioxide levels barely exceeded 20.

The largest producer of Sulphur Dioxide are volcanoes followed by factories that specifically manufacture sulphur dioxide and then followed by vehicle emissions.

There is no reasonable explanation why the sulphur dioxide levels are so high at the IIT Madras monitoring station for many months and then droops off to negligible levels – unless of course that active volcano on the IIT campus stopped erupting after 5th May.

India, and in particular, Chennai, has never had a problem with sulphur dioxide compared to other pollutants. A study by NASA in 2013 found that the levels were increasing but not around Chennai.

Unfortunately, it looks as though the IIT-Madras monitoring station was reporting inaccurate data. The Hindu journalist should have investigated the data further rather than taking the numbers at face value.

The media in general needs to take more care with fact checking before reporting on stories. It was only a few months ago that the media reported another inaccurate story about Bangalore having worse air pollution than any other city. That story was quickly retracted after it was discovered the data was wrong.

Chennai is no saint when it comes to air pollution, but it is a long way off from stealing the crown from Delhi as India’s most polluted city.

To get the facts on the air quality in chennai and other Indian cities visit our monthly roundup blog posts here.