Lomborg considers pollution from different angles. He notes that air pollution in wealthy nations has steadily decreased in recent decades. He finds that air pollution levels are highly linked to economic development, with moderately developed countries polluting most. Again, Lomborg argues that faster growth in emerging countries would help them reduce their air pollution levels. Lomborg suggests that devoting resources to reduce the levels of specific air pollutants would provide the greatest health benefits and save the largest number of lives (per amount of money spent), continuing an already decades-long improvement in air quality in most developed countries. Concerning water pollution, Lomborg notes again that it is connected with economic progress. He also notes that water pollution in major Western rivers decreased rapidly after the use of sewage systems became widespread. Concerning waste, Lomborg notes once again that fears are overblown, as the entire waste produced by the United States in the 21st century could fit into a square 100 feet thick and 28 km along each side, or 0.009% of the total surface of the United States.