Thermal pollution is the unfavourable product of man's actions; the major sources are heated effluents and solar heating. The major cause is the extension of the thermal electrical power industry. Through this disposal of waste heat, the temperature of surface waters throughout the world is being changed. Uncontrolled heat releases may destroy, dislodge, or debilitate positions of aquatic biota. Oxygen requirements of most aquatic life increase with a temperature rise; metabolic activity rises, and a point is reached where survival rapidly drops. With temperature rise, toxicity of pollutants increases, chemical reactions speed up, flocculation of finely suspended particles is hastened, and salinity increases. The alteration in behaviour, distribution, and migration of anadromous or schooling fishes can occur, and organisms can be killed from shock, or their life cycles can be affected.
Over cities, thermal inversions occur when heat radiates upward on clear nights and the ground layer cools. Over valley cities, or when high-pressure air masses stagnate over the city, the cool air layer is trapped by the warm air above. Gaseous pollutants (nitrogen oxides, sulphur dioxide, smog) collect in the cool air. Thus, thermal air pollution can augment the adverse effects of other air contaminants. The most serious thermal pollution stems from the use of water to cool industrial installations, especially fossil fuel and nuclear generating plants. Water taken from lakes, rivers, or estuaries to cool reactors is sprayed into the air in cooling towers or is returned to the water body. If this water is too hot, it becomes a pollutant.
Electric power, iron and steel, oil refining, paper and wood pulp, and synthetic rubber industries account for 89% of industrial water use in the USA. A thermal electric plant converts heat energy from fuel into electricity, with only 32% average efficiency – the rest is waste. Vast quantities of water are now used to cool power plants, and it has been estimated that at least 25% of all fresh water flow in the USA will be used for this purpose by the year 2000.