India, Nov. 10 -- On Tuesday morning, around 9.30 am, at least 18 stations in Delhi reported an air quality index (AQI) of 500, the highest value on the pollution measurement metric. A few private air quality monitoring platforms even reported an AQI of 999, indicating that the pollution measuring values had maxed out.

What these numbers don't indicate is that concentrations of the ultra-fine and hazardous particulate matter (PM 2.5 or PM 10) in Delhi could be much higher than indicated by the maximum index value of 500.

However, experts say the index is not underestimating the crisis by capping the value at 500.

The PM concentrations may be far higher than the criteria for meeting an AQI of 500. But '500' indicates worst air quality w...