Understanding the numbers

Media now routinely provides air quality information along with the weather for many Indian cities.

The main measure of air quality is the concentration of fine particles or Particulate Matter that has a diameter less than 2.5 micrometers — it is called PM2.5. When it rises above 60 micrograms per cubic meter of air, averaged over a day, it is considered unhealthy. In many  Indian cities this number is routinely  150- 300. The media also uses something called an air quality index or AQI, a number in a scale of 0-500, that takes into account the concentration of particles as well as other harmful gases. An AQI between 150 and  200 is unhealthy; between 350 and 500 is classified as very unhealthy to hazardous and can trigger respiratory illness on prolonged exposure.