How to make sense of our hazy air quality data

Air quality indices on your phone's weather app and the one reported by the government do not seem to agree with each other, and those obsessed with the data have too many contradicting numbers to go by.

- Tanay Sukumar

By some measures popular on social media, parts of Delhi crossed the index value of 1,000 at times-even 1,500at their worst.

Look at the official government scale, and it will never cross 500. Why is this so, and what does this mean for us?

First things first-all indices are correct in their own way, except that the devil lies in the details. They are all released by different organizations, which use different scales, methodologies and sources.

The air quality index (AQI) is supposed to measure the safety of the air around you for breathing. All organizations that report the AQI perform the same task: they measure the density of pollutants (fine particles such as PM2.5 and PMIO, and gases such as nitrogen dioxide, ozone and carbon monoxide) in the air at different monitoring stations.

Density (or concentration) is the mass of any material contained in one unit of volume. For pollutants, it is typically measured in micrograms per cubic metre. A higher figure means more of a pollutant in the air.

However, one gram of one pollutant may not be as harmful as one gram of another pollutant in the same amount of air. That would be like saying 10 grams of carbohydrates in a day have the same utility for your body as 10 grams of vitamins!