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 Throughout the world, the prevalence of particular diseases and other threats to human health depend largely on local climate. Extreme temperatures can directly cause the loss of life. Moreover, several serious diseases only appear in warm areas. Finally, warm temperatures can increase air and water pollution, which in turn harm human health.
The most direct effect of climate change would be the impacts of hotter temperatures themselves. Extremely hot temperatures increase the number of people who die on a given day for many reasons: People with heart problems are vulnerable because one’s cardiovascular system must work harder to keep the body cool during hot weather. Heat exhaustion and some respiratory problems increase.
Higher air temperatures also increase the concentration of ozone at ground level. The natural layer of ozone in the upper atmosphere blocks harmful ultraviolet radiation from reaching the earth’s surface; but in the lower atmosphere, ozone is a harmful pollutant. Ozone damages lung tissue, and causes particular problems for people with asthma and other lung diseases. Even modest exposure to ozone can cause healthy individuals to experience chest pains, nausea, and pulmonary congestion. In much of the nation, a warming of four degrees (F) could increase ozone concentrations by about 5 percent.
Statistics on mortality and hospital admissions show that death rates increase during extremely hot days, particularly among very old and very young people living in cities. In July 1995, a heat wave killed more than 700 people in the Chicago area alone. Studies based on these types of statistics estimate that in Atlanta, for example, even a warming of about two degrees (F) would increase heat-related deaths from 78 today to anywhere from 96 to 247 people per year. If people are able to install air conditioning and otherwise acclimatize themselves to the hotter temperatures, the lower estimate is more likely.
Warmer temperatures may decrease the number of people who die each year from cold weather. However, in the United States, only 1000 people die from the cold each year, while twice that many die from the heat. Moreover, of the ten states with the greatest number of cold-related deaths, Alaska and Illinois are the only northern states. For the most part, cold-related deaths occur during occasional cold spells in areas with mild winters where people prepare less for the cold, or during extreme events like the severe snow storm that struck Colorado in November of 1997. Global warming is unlikely to reduce either of these situations. Finally, deaths due to the heat are more sensitive to temperature changes than deaths due to the cold; the difference between -20°F and -15°F, for example, has a much smaller impact than an increase from 95°F to 100°F.
 Global warming may also increase the risk of some infectious diseases, particularly those diseases that only appear in warm areas. Diseases that are spread by mosquitoes and other insects could become more prevalent if warmer temperatures enabled those insects to become established farther north; such "vector-borne" diseases include malaria, dengue fever, yellow fever, and encephalitis. Some scientists believe that algal blooms could occur more frequently as temperatures warm — particularly in areas with polluted waters — in which case diseases such a cholera that tend to accompany algal blooms could become more frequent.
In spite of these risks, increased mortality is not an inevitable consequence of global warming. Malaria, for example, is rare in the United States even in warmer regions where the mosquito that transmits the disease is found, because this nation has the ability to rapidly identify and contain outbreaks when they appear. Heat-related deaths can be prevented by emergency measures to move vulnerable people to air-conditioned buildings, and by reducing the emissions of photochemical oxidants which cause ground-level ozone. Many of the impacts of climate change on health could be avoided through the maintenance of strong public health programs to monitor, quarantine, and treat the spread of infectious diseases and respond to other health emergencies as they occur. Although air-conditioning and public health programs may impose additional costs on the public and private sectors, they would often be preferable to the impacts on human health that would otherwise occur.
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