India, Nov. 9 -- There is something deeply unsettling about the fact that each year, Delhi enters Diwali, a festival that celebrates the triumph of light over darkness, under a sky through which even sunlight struggles to penetrate. The irony is difficult to ignore. Yet the more troubling question is not what the festival does to Delhi's air, but why the city's atmosphere is already so compromised even before the celebrations begin.

On November 6, two days before Diwali, Delhi's Air Quality Index had reached 382, prompting the imposition of Stage III restrictions under the Graded Response Action Plan. By Diwali morning, several monitoring stations had crossed into the 'severe' category. The deterioration preceded the fireworks. This dist...