India's growing population has been accompanied by massive urbanisation. Census of India estimates show that between 1991 and 2011 the urban population in the country increased from 26% to 31%. However, this rapid urbanisation has been accompanied by an alarming rate of environmental degradation and a growing scarcity of natural resources. Estimates from the WHO suggest that India has now replaced China as the country with the largest number of cities (13!) in the world's top 20 most polluted cities in terms of air quality.