India, Nov. 6 -- Every winter, as toxic smog cloaks Delhi-NCR, a set of emergency measures comes into force. The scale of the restrictions tightens at each stage under the Graded Response Action Plan (Grap), from one to four, as air quality worsens.

These measures - which include a construction ban and curbs on entries of polluting vehicles into the Capital - along with policies meant to mitigate the region's air pollution crisis rely on air quality data. So they can only be as effective as the data is accurate. It is why the Central Pollution Control Board (CPCB) has a set of rules to guide how many monitoring stations a city must have and where they can be located.

On November 2, Delhi's air quality index was 324, this winter season's...