Burning of stubble from the rice harvest to clear land for the next year's cultivation in the Punjab creates 25% of the pollution over New Delhi. Automobile pollution adds more pollutants. This toxic mix of pollutants is trapped by the cooler air during the winter over much of northern India. At one point the pollution index reached 643 in New Delhi, by comparison it was hard to breather with an index of 116 from the wildfires in Canada's northern region that sent smoke all the way into Washington D.C.