Delhi, the capital of India (in the Indo-Gangetic Plains), is one of the most polluted cities in the world. 1 The air quality of Delhi and Beijing is often compared. The cause of poor air quality in the two mega cities is similar, and is mainly attributed to the growing population, economic growth, coal-based power plants, vehicular emissions, growing construction activities, and crop residue burning. The westerly winds bring polluted air mass from western parts of India (including Pakistan) to the Indo-Gangetic Plains and the pollution is carried over the Indo-Gangetic Plains depending on the meteorological conditions. The extent of the pollution in the Indo-Gangetic Plains depends on the wind speed. Delhi and Beijing have different geographical settings. Delhi is surrounded by the towering Himalayas in the north, whereas Beijing is located in the plains. The poor air quality in Delhi and surroundings have a serious impact on people living in the northern parts of India compared with those in central and southern India, although no official health data are available. Rice crop residue burning2–6 during mid-October to November every year is becoming a serious health threat because of increased burning by farmers in the states of Punjab, Haryana, and western Uttar Pradesh in northern India. Crop residue burning2–6 started in the late 1980s with the start of mechanised harvesting in Punjab. Farmers found burning to be an economical way of cleaning crop stalk residues that are left over by mechanised harvesters. In doing so, farmers ignore the impact of this practice on health and air quality3–6 in the cities located in the Indo-Gangetic Plains. In the winter season, the severity of this problem increases as dispersion of smoke plumes is slowed down because of cold temperatures, whereas during the summer season the problem does not exist because of fast dispersion of plumes from burning. The practice of crop residue burning that started in Punjab has slowly spread to other adjoining states in northern India. We have found that the impact of crop residue burning on air quality is not restricted to cities in the Indo-Gangetic Plains alone, but is spreading to the far eastern parts of the Indo-Gangetic Plains and over central and southern parts of India, including parts of Bihar, Jharkhand,