The natural environment—air, water and soil—is not only essential for supporting life, but is also the medium through which the technological waste products of society are consumed and restored. At lower levels of industrialization and lower population densities than are now prevalent, the capacity of the natural environment to absorb and recycle waste appeared limitless. Increasingly, however, nature's capacity to assimilate waste is being threatened because of the rise in industrial production which has resulted in a much larger volume of emissions of organic and inorganic compounds from mining, refining, chemical production and manufacturing activities. In addition, certain industries (such as aluminium smelting plants and electricity generating stations) require large amounts of cooling water and discharge large quantities of heat into streams and rivers. Technology is also responsible for production of discharges of fine particulates, sulphur dioxide and nitrogen oxides, into the atmosphere, and for the great number of undegradable waste products from the plastics, metallurgical, glass and petrochemicals industries. Further impacts are the increased number of motor vehicles and rising volume of traffic, leading to higher level emissions of carbon monoxide, nitrogen oxides and lead; and the increased use of incineration, both by municipalities and industries, as a means of disposal of solid wastes, leading to the emission of a complex mixture of pollutants.