In the 1960s, many people moved to Phoenix, Arizona for its clean, dry, desert air. This was the place to go if you had lung problems, such as severe allergies or asthma. By the 1990s, however, this quality had changed. The city's fast growth brought a lot of construction and many more cars. Dust particles in the air and pollution from the cars created what is called the “brown cloud” that sits over the city. Part of the problem is the location of the city, which is in a desert valley. At night, the cool air traps the pollutants, so they can’t escape into the atmosphere. In 2009, Phoenix was listed as one of the worst cities in the U.S. for air pollution. Asthma rates in Phoenix have increased significantly. Residents of the area all agree that the air quality in Phoenix must be improved, and they have started to work on the problem.