Winter mortality and its causes.

In the 1970s scientific research focussed for the first time on dramatic rises in mortality every winter, and on smaller rises in unusually hot weather. Following the recent decline in influenza epidemics, approximately half of excess winter deaths are due to coronary thrombosis. These peak about two days after the peak of a cold spell. Approximately half the remaining winter deaths are caused by respiratory disease, and these peak about 12 days after peak cold. The rapid coronary deaths are due mainly to haemoconcentration resulting from fluid shifts during cold exposure; some later coronary deaths are secondary to respiratory disease. Heat related deaths often result from haemoconcentration resulting from loss of salt and water in sweat. With the possible exception of some tropical countries, global warming can be expected to reduce cold related deaths more than it increases the rarer heat related deaths, but statistics on populations in different climates suggest that, given time, people will adjust to global warming with little change in either mortality. Some measures may be needed to control insect borne diseases during global warming, but current indications are that cold will remain the main environmental cause of illness and death. Air pollution in cities may also still be causing some deaths, but these are hard to differentiate from the more numerous deaths due to associated cold weather, and clear identification of pollution deaths may need more extensive data than is currently available.