Chang et al. (2004) analyzed data from the World Health Organization's study of cardiovascular disease and steroid hormone contraception (WHO, 1995) to determine the effects of temperature on rates of hospitalization for arterial stroke and acute myocardial infarction (AMI) among young women aged 15-49 from seventeen different countries in Africa, Asia, Europe, Latin America and the Caribbean.  Their work revealed that "the rate of hospitalized AMI, and to a lesser extent stroke, was higher with lower mean environmental air temperature."  More specifically, they say that "on average, a 5°C reduction in mean air temperature was associated with a 7 and 12% increase in the expected hospitalization rates of stroke and AMI, respectively."  They also note that "the findings of an inverse association between mean air temperature and hospitalization rate of AMI in this study are in agreement with several other studies," citing those of Douglas et al. (1990), Douglas et al. (1991), Mackenbach et al. (1992), Douglas et al. (1995), Seto et al. (1998), Danet et al. (1999) and Crawford et al. (2003).  In addition, they note that "lagging the effects of temperature suggested that these effects were relatively acute, within a period of a month."

In another multi-continent study that focused on (1) North Carolina, USA, (2) South Finland, comprising all of Finland except the northern provinces of Oulu and Lapland, and (3) Southeast England, comprising Greater London, Essex, Kent, Sussex, Hampshire, Surrey, Berkshire, Oxfordshire, Buckinghamshire and Bedfordshire, Donaldson et al. (2003) determined the mean daily May-August 3°C temperature band in which deaths of people aged 55 and above were at a minimum.  Then, they compared the numbers of heat- and cold-related deaths that occurred at temperatures above and below this optimum temperature interval for each region, after which they determined how heat-related deaths in the three areas changed between 1971 and 1997 in response to (1) the 1.0°C temperature rise experienced in North Carolina over this period, starting from an initial temperature of 23.5°C), (2) the 2.1°C temperature rise experienced in Southeast England, starting from an initial temperature of 14.9°C, and (3) the unchanging 13.5°C temperature of South Finland.

Donaldson et al.'s work revealed that the 3°C temperature band at which mortality was at its local minimum was lowest for the coldest region (South Finland), highest for the warmest region (North Carolina), and in between for the "in between" region (Southeast England), which indicates that the populations of these three regions are somewhat acclimated to their respective thermal climates.  It was also determined, for each region, that cold-related mortality was greater than heat-related mortality.  Last of all, with respect to changes in heat-related mortality from 1971 to 1997, it was determined for the coldest of the three regions (South Finland, where there was no change in temperature over the study period) that heat-related deaths per million inhabitants in the 55-and-above age group declined from 382 to 99.  In somewhat warmer Southeast England, however, where it warmed by a whopping 2.1°C over the study period, heat-related deaths per million of the at-risk age cohort still declined, but this time from only 111 to 108.  Last of all, in the warmest of the three regions (North Carolina, where mean daily May-August temperature rose by 1.0°C over the study period), corresponding heat-related deaths also fell, and this time from 228 to a mere 16 per million.

What do these findings imply?  First of all, they suggest that people can adapt to both warmer and cooler climates to some degree.  Beyond that, local cooling tends to produce many more deaths than does local warming in all three of the areas studied.  As for the dramatic decline in the number of heat-related deaths over a period of warming in the hottest area of the study (North Carolina), Donaldson et al. attribute this phenomenon to "the increase of air conditioning in the South Atlantic region of the U.S.A.," where they note that "the percentage of households with some form of air conditioning in that region rose from 57% in 1978 to 72% in 1997."  With respect to the declining heat-related deaths in the other two regions, they say "the explanation is likely to lie in the fact that both regions shared with North Carolina an increase in prosperity, which could be expected to increase opportunities for avoiding heat stress."

A third study with globe-girdling implications is the literature review of Keatinge and Donaldson (2004), who begin the main body of their text with a clear declaration of the relative dangers of heat and cold when it comes to human mortality: "cold-related deaths are far more numerous than heat-related deaths in the United States, Europe, and almost all countries outside the tropics, and almost all of them are due to common illnesses that are increased by cold."  Expanding on this statement, they report that coronary and cerebral thrombosis account for about half of all cold-related deaths, and that respiratory diseases account for approximately half the rest.  With respect to the first of these sets of problems, they say that cold stress causes an increase in arterial thrombosis "because the blood becomes more concentrated, and so more liable to clot during exposure to cold."  The sequence of events, as they describe it, is that "the body's first adjustment to cold stress is to shut down blood flow to the skin to conserve body heat," which "produces an excess of blood in central parts of the body," and that to correct for this effect, "salt and water are moved out from the blood into tissue spaces," leaving behind "increased levels of red cells, white cells, platelets and fibrinogen" that lead to increased viscosity of the blood and a greater risk of clotting.

With respect to respiratory-related deaths, the British scientists report that the infections that cause them spread more readily in cold weather as people "crowd together in poorly ventilated spaces when it is cold."  In addition, they say that "breathing of cold air stimulates coughing and running of the nose, and this helps to spread respiratory viruses and bacteria."  The "train of events leading to respiratory deaths," as they continue, "often starts with a cold or some other minor infection of the upper airways," which "spreads to the bronchi and to the lungs," whereupon "secondary infection often follows and can lead to pneumonia."  They also note that cold stress "tends to suppress immune responses to infections," and that respiratory infections typically "increase the plasma level of fibrinogen, and this contributes to the rise in arterial thrombosis in winter."

Another interesting thing about cold-related deaths, as Keatinge and Donaldson describe it, is that "cold spells are closely associated with sharp increases in mortality rates," and that "deaths continue for many days after a cold spell ends."  On the other hand, they report that "increased deaths during a few days of hot weather are followed by a lower than normal mortality rate," because "many of those dying in the heat are already seriously ill and even without heat stress would have died within the next 2 or 3 weeks."

So what are the implications of global warming for human mortality? Keatinge and Donaldson state that "since heat-related deaths are generally much fewer than cold-related deaths" - and, we note, are comprised primarily of deaths that typically would have occurred a few weeks later even in the absence of excess heat - "the overall effect of global warming on health can be expected to be a beneficial one."  As an example, and even including the heat-harvesting of naturally-expected deaths, they report that "the rise in temperature of 3.6°F expected over the next 50 years would increase heat-related deaths in Britain by about 2,000 but reduce cold-related deaths by about 20,000."

In concluding their treatise, Keatinge and Donaldson report that "even in climates as warm as southern Europe or North Carolina [USA], cold weather causes more deaths than hot weather."  They say that "global warming will reduce this at first, but the improvement is not likely to continue without action to promote defenses against cold."  For example, they say that "people in regions with mild winters become careless about cold stress, protect themselves less effectively against cold, and generally have more winter deaths than people in colder regions [our italics]."  Continuing, they say that "climatic warming therefore calls for action to control cold stress as well as heat stress," and they say that "if this is taken, rising temperatures could reduce overall mortality rates."

In closing, we consider one other health concern that was raised early in the global warming debate, and that is that diseases that are transmitted by insects might spread to cooler regions of the world in response to rising temperatures and become major problems there.  However, in the words of Keatinge and Donaldson, "closer examination showed that this was unlikely to happen to a serious extent."  In the case of malaria, for example, this scourge of humanity - which they remind us "was once prevalent in most of Europe and even in Russia" - was largely eliminated from these regions in concert with the earth's recovery from the cold temperatures of the Little Ice Age.  Why?  In the words of the two medical sleuths, "the main reason was that modern farming methods and changes in human living conditions had reduced the number of the mosquitoes that spread the disease and had reduced their access to people."

All things considered, therefore, it should be clear to most logically-thinking people, in the words of Keatinge and Donaldson, that "the overall effect of global warming on health can be expected to be a beneficial one."

References
Chang, C.L., Shipley, M., Marmot, M. and Poulter, N.  2004.  Lower ambient temperature was associated with an increased risk of hospitalization for stroke and acute myocardial infarction in young women.  Journal of Clinical Epidemiology 57: 749-757.

Crawford, V.L.S., McCann, M. and Stout, R.W.  2003.  Changes in seasonal deaths from myocardial infarction.  Quarterly Journal of Medicine 96: 45-52.

Danet, S., Richard, F., Montaye, M., Beauchant, S., Lemaire, B., Graux, C., Cottel, D., Marecaux, N. and Amouyel, P.  1999.  Unhealthy effects of atmospheric temperature and pressure on the occurrence of myocardial infarction and coronary deaths - a 10-year survey - the Lille WHO-MONICA Project.  Circulation 100: E1-7.

Donaldson, G.C., Keatinge, W.R. and Nayha, S.  2003.  Changes in summer temperature and heat-related mortality since 1971 in North Carolina, South Finland, and Southeast England.  Environmental Research 91: 1-7.

Douglas, A.S., Allan, T.M. and Rawles, J.M.  1991.  Composition of seasonality of disease.  Scottish Medical Journal 36: 76-82.

Douglas, A.S., Dunnigan, M.G., Allan, T.M. and Rawles, J.M.  1995.  Seasonal variation in coronary heart disease in Scotland.  Journal of Epidemiology and Community Health 49: 575-582.

Douglas, A.S., Russell, D. and Allan, T.M.  1990.  Seasonal, regional and secular variations of cardiovascular and cerebrovascular mortality in New Zealand.  Australia and New Zealand Journal of Medicine 20: 669-676.

Keatinge, W.R. and Donaldson, G.C.  2004.  The impact of global warming on health and mortality.  Southern Medical Journal 97: 1093-1099.

Mackenbach, J.P., Kunst, A.E. and Looman, C.W.N.  1992.  Seasonal variation in mortality in the Netherlands.  Journal of Epidemiology and Community Health 46: 261-265.

Seto, T.B., Mittleman, M.A., Davis, R.B., Taira, D.A. and Kawachi, I.  1998.  Seasonal variation in coronary artery disease mortality in Hawaii: observational study.  British Medical Journal 16: 1946-47.

WHO.  1995.  WHO Collaborative Study of Cardiovascular Disease and Steroid Hormone Contraception.  A multinational case-control study of cardiovascular disease and steroid hormone contraceptives: description and validation of methods.  Journal of Clinical Epidemiology 48: 1513-1547.