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.