A city with miles of dark hot pavement absorbs and radiates heat into the surrounding atmosphere at a greater rate than a natural landscapes do. This is urban heat island effect causing an increase in air temperatures. The EPA estimates that the average air temperature of a city with one million people or more can be  warmer than surrounding areas. Higher temperatures reduce air quality by increasing smog. In Los Angeles, a 1 degree temperature increase makes the air roughly 3 percent more smog. Green roofs and other forms of green infrastructure help improve air quality and reduce smog through their use of vegetation. Plants not only provide shade for cooling, but also absorb pollutants like carbon dioxide and help reduce air temperatures through evaporation and evapotranspiration.