Although urbanizing the Mexico City region was intended to improve the economy and establish a political centre, it has come at an ecological cost that threatens the region's future. The government's decision to build new roads and houses has required the clear-cutting of trees for road construction and housing development. This practice has had severe environmental consequences. Mexico City has one of the most polluted air environments in the world. Outdoor pollution comes from vehicles burning gasoline and diesel fuel, which emit carbon monoxide, nitrogen oxides, benzene, toluene, xylene, and other chemicals. Indoor pollution comes from cooking with solid fuels such as coal or wood, which produces carbon monoxide, nitrogen dioxide, and smoke. The heavy traffic flow in Mexico City has led to high levels of air pollution. In fact, according to research published by the Mexican Social Science Council (CSO), Mexico City has the highest rate of mortality due to air pollution among large cities around the world. The clearing of forests for farmland has also caused climate change. Deforestation leads to more heat-absorbing surfaces like concrete and asphalt that can increase global temperatures even more than water vapor or carbon dioxide emissions, which can lead to forest fires and floods.