The major environmental problems facing Burkina Faso are recurrent drought and the advance of the northern desert into the savanna. This trend toward desertification has been increased by overgrazing of pasture, slash-and-burn agriculture, and overcutting of wood for fuel. Almost all the trees within 40 km (25 mi) of the capital have been felled. The frequency of droughts in Burkina Faso and its location in the Sahara desert contribute to the nation's water supply problems. The country has 17.5 cu km of renewable water resources, but only 66% percent of the city population and 37% of rural dwellers have access to safe water. According to the World Health Organization, about 80% of all disease in Burkina Faso is caused by unsafe water. Pollution problems result from uncontrolled disposal of sewage and industrial wastes. The Ministry of Environment and Tourism is the principal government agency concerned with the environment. Burkina Faso has 12 national parks and wildlife reserves totaling 2,855,000 hectares. Altogether, 10.4% of its total land area is protected. The country has three Wetlands of International Importance. Of 147 species of mammals, 6 are considered endangered, including the African hunting dog, the chimpanzee, and the African elephant. The Sahara oryx, or white oryx, has become extinct in the wild. One bird species in a total of 335 and one reptile are also threatened.