The early stages of economic development are generally associated with increases in the level of industrialization. Along with this industrialization comes increases in populations of cities and pollution causing transportation. For all these reasons, the development of a country is usually associated with increases in the amount of pollution. However, after a certain threshold level of income, the quality of the environment becomes important and people are willing to pay for a cleaner environment. After this point the level of pollution is likely to decrease. This hypothesized inverted-U relationship between the amount of pollution and the income of a country is known as the Environmental Kuznets Curve (EKC). It is an important idea because, if true, it would enable people to predict the levels of pollution for the corning years based on a countries expected GDP (Shafik).
Meers '00, Robin, "A Test of the Environmental Kuznets Curve For Local and Global Pollutants" (2000). Honors Projects. Paper 76.