The chief environmental problem is soil erosion and degradation, particularly because of overgrazing. Population growth and the increased demand for fuel has threatened the country's forests, and the resulting deforestation has contributed to the loss of valuable soil. Swaziland has 4 protected areas for wildlife—2 wildlife sanctuaries and 2 nature reserves—totaling 40,045 ha (98,953 acres), all in the northern half of the country. As of 1994,2.3% of the nation's total land area was protected. Another significant environmental problem in Swaziland is air pollution from transportation vehicles and emissions from other countries in the area. Water pollution from industrial and agricultural sources is also a problem, as well as contamination by untreated sewage, which contributes to the spread of life-threatening diseases. Swaziland has about 1.7 cu mi of water, of which about 93% is used for farming and 2% for industrial activity. Access to safe drinking water is available to only 41% of urban dwellers and 44% of the population. As of 1994, none of Swaziland's 46 animal species were endangered, but 5 of the nation's bird species and 25 types of plants were threatened with extinction. Burchell's zebra has become extinct. Threatened marine species include the Baltic sturgeon, Danube salmon, and marsh snail.