India, Oct. 22 -- A review meeting to invoke the next category of measures under the Graded Response Action Plan (Grap) will only take place after the air quality index (AQI) crosses 400, officials aware of the matter have said, even Delhi's air quality deteriorated to the higher end of the 'very poor' category on Tuesday - crossing 350.

Grap stage 1 was implemented on October 14, followed by stage 2 on October 19. Stage-1 of GRAP is generally invoked when the AQI crosses 200; stage-2 (very poor) when the AQI crosses 300; stage-3 (severe) when AQI is 400; and stage 4 (severe plus) AQI above 450.

Last year, the Supreme Court had revised the thresholds for the stages 3 and 4, directing that they be imposed at AQI thresholds of 350 and 400...