New Delhi, Oct. 23 -- 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. By some measures popular on social media, parts of Delhi crossed the index value of 1,000 at times-even 1,500-at 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 ...