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3. Impact of Cold Weather
a. General Relationships
Many studies have provided evidence that mortality rates increase during periods of cold weather. In general, total mortality is about 15% higher on an average winter day than on an average summer day (National Center for Health Statistics, 1978). However , increases in mortality during exceedingly cold periods are less dramatic than their hot weather counterparts (Kalkstein, 1984). The impact of cold on human well-being is highly variable. Not only is cold weather responsible for direct causes of death su ch as hypothermia, influenza, and pneumonia, it is also a factor in a number of indirect ways. Death and injury from falls, accidents, carbon monoxide poisoning, and house fires are all partially attributable to cold (U.S. Department of Commerce, 1984).
Hypothermia occurs when the core body temperature falls below 35deg.C (Centers for Disease Control, 1982). Certain sectors of the population appear more susceptible to hypothermia than others. Most victims fall in one or more of the following categories: the elderly, newborns, the unconscious, alcoholics, and people on medications (Fitzgerald and Jessop, 1982; Lewin et al., 1981; Hudson and Conn, 1974; Bristow et al., 1977; Massachusetts General Hospital, 1982). In addition, malnourishment, inadequate hou sing, and high blood ethanol levels increase the incidence of hypothermia (Centers for Disease Control, 1982).
Sex and race appear to be related to susceptibility to hypothermia. Nonwhite elderly men generally constitute the highest risk group, while white women comprise the lowest risk group (Rango, 1984; Centers for Disease Control, 1982). Women possess a higher skin temperature to core temperature gradient, suggesting that they are better able to maintain a higher body core temperature during periods of cold stress (Cunningham et al., 1978; Hardy and DuBois, 1940; Wyndham et al., 1964; Graham, 1983). Some studi es contend that the difference in the response of men and women to cold is related to the amount of subcutaneous fat within the body (Hardy and DuBois, 1940; Wyndham et al., 1964), but other studies have failed to confirm this hypothesis (Bernstein et al. , 1956; Gallow et al., 1984; Veicsteinas et al., 1982). Although women are less susceptible to hypothermia, they appear to be more susceptible to peripheral cold injuries such as frostbite (Graham and Lougheed, 1985).
Age appears to have an even greater impact upon hypothermia sensitivity than gender, and the elderly display the highest mortality rates of all groups. Vasoconstriction and shivering, two primary cold adaptive measures, appear to be reduced in many elderl y persons (Collins et al. 1977; Collins and Easton et al. 1981; Wagner et al., 1974). In addition, many of the elderly do not discriminate changes in temperature well and are thus less able to adjust to them (Collins and Exton-Smith et al., 1981).
One of the first efforts to predict the impact of a severe cold wave was published by NOAA using algorithms developed by Kalkstein. Seven cities in the eastern and southern United States exhibited significant relationships between winter weather and morta lity, and the following regression equations were developed for each:
Atlanta: MORT = C - .11 MT Chicago: MORT = C - .08 MT Cincinnati: MORT = C - .21 MT - .01 CDH + .13 HRS Dallas: MORT = C - .12 MT - .13 MIN - .02 CDH Detroit: MORT = C - .11 MT Oklahoma City: MORT = C - .16 MT Philadelphia: MORT = C + .09 MD + .01 CDH + .06 WAM - .08 WPM,
where MORT is the daily standard deviation increase in mortality above the mean, C is a constant (different for each city), MT is daily maximum temperature, HRS is the total hours in the day with temperatures below 32deg.F, MIN is daily minimum temperatur e, MD is daily minimum dewpoint, WAM is 3AM windspeed, WPM is 3PM windspeed, and CDH is a measure of the day's coldness and is calculated as follows:
b. Adaptation
It appears that adaptation to cold temperatures can occur through repeated exposures. Radomski and Boutelier (1982) noted that men who had bathed in 15deg.C water for one-half hour over nine consecutive days before a trip to the Arctic showed less signs o f cold-induced stress than non-treated men.
There appears to be a cold-adaptive mechanism influencing mortality as well. In a study comparing winter mortality rates for 13 cities in different climates around the U.S., a large differential response was noted. The southern cities seemed to exhibit th e greatest increases in mortality during cold weather, while little or no response was found in northern cities (Kalkstein, 1984). In a city such as Minneapolis, no increase in mortality was noted at temperatures down to -40deg.C, but in Atlanta, mortalit y increases were evident if the maximum temperature did not exceed 0deg.C (Kalkstein ant Davis, 1985). Of the 13 cities studied, 7 demonstrated a statistically significant relationship between winter cold and mortality. The six non-significant cities incl uded cold weather locations (Minneapolis) and mild West Coast locations where very cold weather is virtually unknown (Los Angeles and San Francisco). "Threshold temperatures," which represent temperatures below which notable increases in mortality occur, were established for the seven cities (Kalkstein and Davis, 1985). The threshold temperatures were comparatively mild for the more southerly cities (0deg.C for Atlanta; 1deg.C for Dallas) and somewhat colder for the more northerly cities (-5deg.C for Phil adelphia). This differential geographical response seems to add credence to the importance of relative, rather than absolute weather conditions.
There is evidence that a lag time of two to three days exists between the offending cold weather and the ultimate mortality response (Kalkstein, 1984). Deaths did not necessarily rise on the day of the coldest temperatures, but in many cases, the sharpest increases were noted three days after the coldest weather occurred. A similar lag time was not noted after extremely hot summer days; the impact appears more immediate in summer.
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