Non-metallic and metallic mineral mining together probably disturb some 500,000 hectares of land per year world-wide. Land impact includes excavation, waste dumping and water table interference or pollution, with consequent damage to plants, and the ecological niches for animals. Air pollution is caused by dust, sulphur oxides, nitrogen oxides and hydrogen sulphide. Sulphur oxides cause acid rain. The human environment may be contaminated by proximity to some mines, mills and smelters; settlements may be moved as mines expand; and the natural landscape may be destroyed.