A good place to begin a review of temperature-related mortality is a cold location like Siberia.  Hence, we start with the study of Feigin et al. (2000), who examined the relationship between stroke occurrence and weather parameters in the Russian city of Novosibirsk, which has one of the highest incidence rates of stroke in the entire world.  Analyzing the health records of 2208 patients with a sex and age distribution similar to that of the whole of Russia over the period 1982-93, Feigin et al. found a statistically significant association between stroke occurrence and low ambient temperature.  For ischemic stroke (IS), which accounted for 87% of all strokes recorded, they report that the risk of IS occurrence on days with low ambient temperature is 32% higher than that on days with high ambient temperature.  Hence, they suggested the implementation of "preventive measures ... such as avoiding low temperature."

Hong et al. (2003) observed much the same thing in Incheon, Korea, over the period January 1998 to December 2000, reporting that "decreased ambient temperature was associated with risk of acute ischemic stroke," with the strongest effect being seen on the day after exposure to cold weather, further noting that "even a moderate decrease in temperature can increase the risk of ischemic stroke."  In addition, they note that "risk estimates associated with decreased temperature were greater in winter than in the summer," which suggests, in their words, that "low temperatures as well as temperature changes are associated with the onset of ischemic stroke."

Nafstad et al. (2001) studied another cold place: Oslo, Norway.  Thanks to Norwegian law, which requires that all deaths be examined by a physician who diagnoses cause and reports it on the death certificate, they were able to examine the effects of temperature on mortality due to all forms of cardiovascular disease for citizens of the country's capital over the period 1990 to 1995.  Their analysis showed that the average daily number of cardiovascular-related deaths was 15% higher in the winter months (October-March) than in the summer months (April-September), leading them to conclude that "a milder climate would lead to a substantial reduction in average daily number of deaths."

To see if relationships between cold temperatures and cardiovascular mortality are preceded by an even more general health-temperature relationship, Hajat and Haines (2002) set out to determine if mere cardiovascular-related doctor visits by the elderly bore a similar relationship to cold temperatures.  Based on data obtained between January 1992 and September 1995 for registered patients aged 65 and older from several practices in London, England, they did indeed find that the number of general practitioner consultations was higher in the cool-season months (October-March) than in the warm-season months (April-September) for all cardiovascular diseases.

Of course, one might say, such findings are only to be expected in cold climates.  What about warm climates, where summer maximum temperatures are often extreme, but summer minimum temperatures are typically mild?

In Israel, research conducted by Green et al. (1994) revealed that between 1976 and 1985, mortality from cardiovascular disease was higher by 50% in mid-winter than in mid-summer, both in men and women and in different age groups, in spite of the fact that summer temperatures in the Negev, where much of the work was conducted, often exceed 30°C, while winter temperatures typically do not drop below 10°C.  These findings are also substantiated by other Israeli studies that have been reviewed by Behar (2000), who states that "most of the recent papers on this topic have concluded that a peak of sudden cardiac death, acute myocardial infarction and other cardiovascular conditions is usually observed in low temperature weather during winter."

Evidence of a seasonal variation in cardiac-related mortality has additionally been noted in the relatively mild climate of southern California in the United States.  In a study of all 222,265 death certificates issued by Los Angeles County for deaths caused by coronary artery disease from 1985 through 1996, Kloner et al. (1999) found that death rates in December and January were 33% higher than those observed in the period June through September.  Likewise, based on a study of the Hunter region of New South Wales, Australia, that covered the period 1 July 1985 to 30 June 1990, Enquselassie et al. (1993) determined that "fatal coronary events and non-fatal definite myocardial infarction were 20-40% more common in winter and spring than at other times of year," while with respect to daily temperature effects, they found that "rate ratios for deaths were significantly higher for low temperatures," noting that "on cold days coronary deaths were up to 40% more likely to occur than at moderate temperatures."

In a study of both "hot" and "cold" cities in the United States -- where Atlanta, Georgia; Birmingham, Alabama; and Houston, Texas comprised the "hot" group, and where Canton, Ohio; Chicago, Illinois; Colorado Springs, Colorado; Detroit, Michigan; Minneapolis-St. Paul, Minnesota; New Haven, Connecticut; Pittsburgh, Pennsylvania; and Seattle and Spokane, Washington comprised the "cold" group - Braga et al. (2002) determined both the acute effects and lagged influence of temperature on cardiovascular-related deaths.  Their research revealed that in the hot cities, neither hot nor cold temperatures had much impact on mortality related to cardiovascular disease (CVD).  In the cold cities, on the other hand, they report that both high and low temperatures were associated with increased CVD deaths, with the effect of cold temperatures persisting for days but the effect of high temperatures restricted to the day of the death or the day before.  Of particular interest was the finding that for all CVD deaths the hot-day effect was five times smaller than the cold-day effect.  In addition, the hot-day effect included some "harvesting," where Braga et al. observed a deficit of deaths a few days later, which they did not observe for the cold-day effect.

Working in Sao Paulo, Brazil, with data collected over the period 1991-1994, Gouveia et al. (2003) determined that the number of cardiovascular-related deaths in adults (15-64 years of age) increased by 2.6% for each 1°C decrease in temperature below 20°C, while there was no evidence for any heat-induced deaths due to temperatures rising above 20°C.  In the elderly (65 years of age and above), however, a 1°C warming above 20°C led to a 2% increase in deaths; but a 1°C cooling below 20°C led to a 6.3% increase in deaths, or more than three times as many cardiovascular-related deaths due to cooling than to warming in the elderly.

For the period 1974-1999, McGregor et al. (2004) obtained data on ischaemic heart disease (IHD) and temperature for five English counties aligned on a north-south transect (Tyne and Wear, West Yorkshire, Greater Manchester, West Midlands, and Hampshire) and analyzed them in such a way as to reveal any relationships that might exist between the two parameters.  They report that "the seasonal cycles of temperature and mortality are inversely related," and that "the first harmonic accounts for at least 85% (significant at the 0.01 level) of the variance of temperature and mortality at both the climatological and yearly time scales."  They also note that "years with an exaggerated mortality peak are associated with years characterized by strong temperature seasonality," and that "the timing of the annual mortality peak is positively associated with the timing of the lowest temperatures."  Why?  Because, in the words of McGregor et al., "frequent exposure to cold causes a rise in IHD risk factors (Lloyd, 1991) through increasing blood pressure and viscosity, vasoconstriction, heart rate and angina (Morgan and Moran, 1997)."

Chang et al. (2004) analyzed data from the World Health Organization (WHO) Collaborative Study of CVD and Steroid Hormone Contraception (WHO, 1995) to determine the effects of monthly mean 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.  They report finding that "among young women from 17 countries, 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).  Last of all, they note that "lagging the effects of temperature suggested that these effects were relatively acute, within a period of a month."

Last of all and most recently, Bartzokas et al. (2004) "examined the relationship between hospital admissions for cardiovascular (cardiac in general including heart attacks) and/or respiratory diseases (asthma etc.) in a major hospital in Athens [Greece] and meteorological parameters for an 8-year period."  They report that, over the whole year, "there was a dependence of admissions on temperature," and that low temperatures were "responsible for a higher number of admissions."  Specifically, they say "there was a decrease of cardiovascular or/and respiratory events from low to high values [of temperature], except for the highest temperature class in which a slight increase was recorded."

The results of these several studies clearly demonstrate that global warming is actually beneficial to humanity, in that it reduces the incidence of cardiovascular diseases related to low temperatures and wintry weather by a much greater degree than it increases the incidence of cardiovascular diseases associated with high temperatures and summer heat waves.

References
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