Relation between Elevated Ambient Temperature and Mortality: A Review of the Epidemiologic Evidence

Received for publication July 9, 2002; accepted for publication November 21, 2002. The effect of elevated temperature on mortality is a public health threat of considerable magnitude. Every year, a large number of hospitalizations and deaths occur in association with exposure to elevated ambient temperatures (1, 2). An average of 400 deaths annually are counted as directly related to heat in the United States, with the highest death rates occurring in persons aged 65 years or more (3). The actual magnitude of heat-related mortality may be notably greater than what has been reported, since we do not have widely accepted criteria for determining heat-related death (4, 5–7), and heat may not be listed on the death certificate as causing or contributing to death. Persons living in urban environments may be at particularly increased risk for mortality from ambient heat exposure, since urban areas typically have higher heat indexes (combinations of temperature and humidity (8)) than surrounding suburban or rural areas, a phenomenon known as the “urban heat island effect” (9). Moreover, urban areas retain heat during the night more efficiently (10). Thus, as the US population becomes more urbanized and the number of elderly people continues to increase (11), the threat of heat-related mortality will probably become more severe. Many of these deaths may be preventable with adequate warning and an appropriate response to heat emergencies, but preventive efforts are complicated by the short time interval that may elapse between high temperature exposure and death. Thus, prevention programs must be based around prospective and rapid identification of high-risk conditions and persons. We carried out this review to assess the current epidemiologic evidence available for this purpose. Observations on heat and mortality have been reported since the early decades of the 20th century. Among the classic studies, Gover (12) examined excess deaths associated with elevated ambient temperature exposure in 86 US cities from 1925 to 1937 and summarized findings from other early heat-wave studies. The northern central states (Ohio, Indiana, Illinois, Missouri, Iowa, and Nebraska) were most often affected by mortality subsequent to heat waves, followed by the states in the Northeast; the South and the Pacific Coast were least affected. Further early evidence for an association between ambient heat exposure and mortality came from studies of Army recruits conducted in the 1940s and 1950s (13, 14), and several studies published in the 1970s described possible physiologic mechanisms for heat stroke following exercise during periods of high ambient temperature (15, 16). The courses of several heat waves occurring prior to 1970 have been explored to identify risk factors for heat-related mortality. For example, Bridger et al. (17) examined mortality following heat waves that occurred in St. Louis, Missouri, in 1936, 1953, 1954, 1955, and 1966. For 1936, they found that persons aged 40–80 years had the greatest risk of mortality, whereas in 1966, the majority of deaths occurred among males, Whites, and persons aged 60–90 years, most likely reflecting the increase in life expectancy over the decades. More recent studies have added an emphasis on longer-term relations between weather and mortality to the more focused studies of heat waves. Because of the projected consequences of global warming (18, 19) and the increased frequency and intensity of heat waves (20), heat-related mortality may achieve greater public health significance during the coming decades (21, 22). Over the past century, the overall global surface temperature has increased by 0.7–1.4°F, and the global sea level has risen 4–10 inches (10–25 cm) from the melting of the polar ice caps (18). Major predicted health effects of long-term climatic change include skin and eye damage from increased exposure to ultraviolet radiation, increased incidence of respiratory and cardiovascular diseases, increased incidence of vector-borne and water-borne diseases, and heat-related morbidity and mortality (18). Models of the relation between temperature and mortality are needed to predict the consequences of global warming, particularly for those most vulnerable and least able to adapt. Observational data on temperature and mortality are essential for developing these models. A further use of these models is in predicting dangerous heat conditions so that preventive steps can be taken (23). Because the demographic picture in the United States has changed, particularly since the 1970s—including longer life expectancy, increased racial diversity in cities, and increased availability of air conditioning—we chose to limit the focus of our review to studies conducted after 1970, to more appropriately define current risk characteristics. Additionally, these years include the newer studies that have used time-series methods. The published literature was identified using the MEDLINE database (1970 to the present), covering recent US and international studies examining heat waves or other ambient heat-related exposures and subsequent mortality, and the Johns Hopkins University’s Welch Library. The search was limited to English-language studies of humans in biomedical research; because of the interdisciplinary nature of heat-related mortality, it is covered by a variety of disciplines beyond medicine and public health, including meteorology, and our coverage does not extend comprehensively into those areas. Keywords and terms used for the search included “temperature,” “high ambient temperature,” “heat wave,” “cardiovascular and respiratory mortality,” and “mortality.” Studies focusing only on winter or the effects of cold temperatures were excluded. A total of 98 references that addressed the general issues associated with heat-related mortality were included. Forty-nine studies specifically addressing mortality due to heat waves or high ambient temperatures had been published in peer-reviewed and indexed journals since 1970. In evaluating studies of temperature and mortality, the following methodological issues should be considered: 1) assessment of exposure and outcome measures; 2) selection of an appropriate study design; and 3) use of statistical models to describe the temperature-mortality relation. The heat stress experienced across day and night determines risk for heat-related mortality (24). In the epidemiologic studies, ambient temperature and/or dew point temperature is generally used as the exposure measure, serving as a surrogate indicator of heat stress. The microenvironmental model, which has been widely applied in assessing exposure to air pollution (25), considers personal exposure to pollution as the time-weighted sum of pollutant concentrations in the places where time is spent, referred to as microenvironments. Extending this model to heat or temperature, temperatures in the various locations where people spend their time determines their personal exposure to heat (26). A study of heat stress or temperature effects can also be carried out in a place-based investigation (27), which assesses the stress of being in particular places instead of the stress of particular people in them. Since measuring the daily dynamic ambient temperature exposure of all participants in a study or a representative sample of participants is impractical and expensive, epidemiologists have used ambient temperatures measured at weather stations as an exposure index. These indexes generally combine temperature and humidity, using recorded temperature (dry bulb temperature), wet bulb temperature, dew point temperature, a heat index consisting of temperature and humidity, changes in temperature, or air mass approaches. Outdoor temperature has been used as a surrogate for personal exposure to heat, even though people in developed countries generally spend most of their time indoors (25). This ecologic temperature measure assumes that all persons who are in a specified geographic area experience the same exposure. While this approach is made feasible by readily available data, some degree of misclassification of individual personal exposures is inherent. The extent of misclassification resulting from using temperature exposure data provided by weather stations will depend on the extent to which ambient and microenvironmental temperatures are correlated. Adaptive behaviors such as use of air conditioning will affect the association between ambient temperatures and indoor temperatures and increase variation in heat exposure among persons at a particular ambient temperature. The recorded ecologic temperature of a specified geographic area (e.g., a city) may not accurately represent the actual temperature exposure of each individual in that area (28). However, for the purposes of population-based analyses, ambient temperature is the strongest determinant of variation over time in the exposure of populations to heat. In assessing associations of temperature with mortality, investigators must also consider the temperature variables and lag structure. For example, or average temperature be used and the association of mortality with temperature at lag have used a variety of including the average temperature and the temperature. to lag have explored a number of for the relation between exposure time and of exposure lag are covered in the An of literature on the physiologic consequences of heat exposure and and the evidence that exposures are most appropriate (18, In examining the effect of heat waves, the of these across the must be waves occurring early in the or often in more deaths than heat waves occurring in the Thus, the most vulnerable persons may be affected by heat waves occurring early in the persons in the of vulnerable persons who heat waves the may have physiologic or changes following their exposure to high ambient temperatures However, to temperatures are Since heat-related mortality is to misclassification and generally only a deaths are directly as being by heat, have often used total mortality or mortality as the outcome The use of deaths specifically to heat-related is because of heat-related mortality by and widely accepted criteria for (4, The of deaths as to heat stroke or temperature at the time of death was at least and/or the had a of changes in and elevated of and temperature be a heat-related should be listed as the of death or a contributing the ambient temperature was elevated at the time of death. such heat is often not reported as the of death on death and incidence may be and/or are reported as the of because persons with these are more to death during heat waves (e.g., death from and/or et al. a heat-related death as death in which exposure to high ambient temperature the death or to other and criteria an outcome measure by the exposure (e.g., heat-related may to a resulting in of mortality to heat waves, since deaths are following a heat and are more likely to be reported as with studies of air pollution and mortality, the effect of should be in studies of temperature and mortality. to a of death among persons who are or and at high risk of of and long-term than only time are in which the of is models are applied to the temperature-mortality using various for temperature exposure and mortality These models represent the relation between temperature and other weather and risk of heat-related mortality. In studies, a or has been found appropriate to describe the with elevated mortality being at temperature and mortality at temperatures Thus, statistical models that do not are needed to the association A may be used to model mortality across all it may be to the most appropriate model to the models may be to describe the associated with temperatures and to for the generally with temperature may be an appropriate temperature can be to the effects of and cold the by or also that temperatures were to each A variety of study can be used to assess the effects of mortality following heat waves and to the association between elevated temperature and mortality. studies of heat waves and and studies risk factors for heat waves have been more including the time-series and study have been Studies of heat waves and studies known as ecologic may be used for of ambient temperature and mortality. A heat is an that a of temperature with a high risk of health effects and mortality The effect of ambient temperature for a on may be examined to identify vulnerable populations and high-risk or rates during a heat are generally with rates from several prior to the heat or rates during the same time in the to assess the number of excess deaths to the heat studies rates in to search for that an for or mortality. In to studies of heat waves, studies consider longer time periods of temperature exposure and populations in areas to the relation between ambient heat exposure and mortality in However, they do not the association between temperature and mortality the of mortality associated with a increase in The time-series study is an for examining the temperature-mortality relation for populations in or geographic over a time or rates are the outcome temperature at over time (e.g., or are the variables of factors include and the of air or daily is for exposures and than individual this can be from The study described in as an epidemiologic for assessing the relation between a risk (e.g., air pollution or ambient and an (e.g., or For each the or more time periods are 1) a or the exposure and to the and 2) or more periods the exposure experienced and/or after the during an interval with for exposure. Because the each in the study to as or due to measured and between (e.g., health and physiologic is and effect by individual can be Since people are only included in the study they experience the outcome of more to death may be a known as the A of the study is that each of which may not be readily is for Many investigators have addressed the of heat-related mortality in the United States and in other countries using the study The study outcome and from these studies are described in the and a for mortality risk associated with elevated ambient temperature. total mortality resulting from heat waves in on mortality to cardiovascular and respiratory considers studies conducted in and the findings of temperature-mortality studies other than studies of heat-wave exposure that used and time-series Studies conducted following heat waves in the and evidence of increased mortality from heat waves in various risk factors for vulnerable including and high-risk of the most heat waves, the heat in St. and Missouri, provided population-based data on the effects of ambient heat exposure on health and to for heat-related mortality to these may have to excess mortality in Illinois, following the heat the of studies that examined risk factors for in mortality or rates following heat waves. The effects of and were examined in several studies of heat waves. of and the elderly were reported to be at highest risk for mortality following heat waves In some studies, persons over years were found to be at highest risk other investigators reported the to be those over 65 years of years of or years of In US studies, (4, persons living in cities and persons in and persons living in were most these as for or higher temperature exposure. US of persons aged 65 years or from that urban and were of high risk the found in the studies of heat waves. US heat-wave studies that were more often affected than a heat in and in more deaths A for heat stroke was reported for following the heat in However, other heat-wave studies reported between and or between and Thus, heat-related mortality the and persons of The evidence for of and has been studies were conducted following heat waves that occurred in the in and to identify risk factors associated with heat-related mortality. being to using a not daily living on higher of and being were associated with increased risk of heat-related mortality. The strongest was to air conditioning in the and in other places as as to living in a by or being able to for in such during a heat and In these studies, exposures may have been for be likely to their since they were not affected by the heat and surrogate for (e.g., or not accurately the health behaviors of persons who risk factors were identified for vulnerable populations and for preventive efforts in high-risk For example, the of the population during the heat may have been a of in that the elderly following the heat of of heat waves in the in and reported risk factors An of in from found exposure, and to be factors for heat-related mortality A number of studies have explored of association between temperature exposures over and risk on a particular the lag time or lag structure. lag have been reported for the strongest association of heat with mortality, from the same day to following a heat a of at least of elevated temperature and humidity was associated with increased mortality in The most rates of death from and were with longer-term elevated temperatures as with or elevated temperatures In studies of heat waves and elevated temperature, cardiovascular respiratory and were of death. the findings of studies that examined heat-related mortality to these deaths occurred in persons with such as or respiratory study found that with respiratory were also at risk The is that stress on the cardiovascular and respiratory during periods of high ambient temperature, among elderly persons with limited In elderly the to temperatures is and are generally elevated in with persons heat is greater than to a temperature, from the to the skin and heat is more to the a may increase and and respiratory rates increase (24). as the of several studies examining the effects of ambient heat exposure in populations These provided a to study risk factors associated with heat-related since a persons with have reported that the number of deaths in a increased with daily temperature during a heat and that had the risk of during a heat than the general population air conditioning was readily In study carried out in deaths in air conditioning were greater than studies of in to a for of or an were more affected than by temperatures on at least An association was also found between the incidence of and ambient temperature of were in the on the as with in the and with level of of the and of the as with of the In with cardiovascular and respiratory conditions were found to be at greater risk of during heat waves, with a mortality that was higher during the with the rates Thus, high-risk in air conditioning and appropriate to are essential for mortality during heat waves. Further evidence of heat-related mortality from studies examining the relation between ambient temperature, than heat waves and mortality These studies were able to the general risk of elevated temperature in areas over time using ecologic study such as and time-series In temperature, of these statistical to represent the effect of heat stress. studies have the risk of mortality, developing for studies. For example, et al. found that of mortality and respiratory mortality were associated with a increase in temperature in their time-series study conducted in long-term studies effects of mortality. et al. found effects to with time from 1937 to in and the US particularly with adequate and air by 1970, in the where were likely to be to ambient A long-term study in and found change in or mortality among to in the from to to these mortality most likely because of such as increased to air in mortality were in and the elderly but not in or temperature-mortality studies identified a temperature which effects of temperature on mortality were these temperatures by In a study of US cities, mortality from and stroke was generally found to increase at temperatures greater than temperatures in US states in July and were to higher mortality in a time-series with and effects In the greatest risk of death occurred on daily temperatures with an on the number of A in heat-related deaths was at only in with average temperatures the an increase in deaths of for increase in temperature weather and by a effect of temperature on mortality has been found the United mortality temperature the point of the as the temperature of the mortality in a was to by in a recent time-series study conducted by et al. who in cities at higher such as Illinois, and have mortality people who in cities at (e.g., and have higher for ambient temperature. Thus, people who in the northern United States may be at greater risk of from heat waves. and have provided further into In their and the US areas, that higher mortality was related to the in and particularly in areas where weather is air to define temperature and and that each air mass on the of daily air temperature, dew point temperature, total air and found that a and air mass had more on mortality in the United States than in the where it is more and that it may increase daily mortality by as as deaths in large mortality was predicted to increase in the of to the increased In of air pollution are by the and concentrations of may be associated with temperature. The relations among air pollutant temperature, and mortality have been in several studies These studies have and it is air are and/or effect a of the temperature-mortality association or they have effect on mortality with temperature Thus, studies examining the effect of the relation between air and temperature on mortality are a number of studies have that mortality is increased during heat waves and that ambient heat indexed by are associated with mortality. with heat-wave and studies that provided early evidence of heat-related mortality in cities, the of has to heat and mortality in more populations in areas using time-series and The findings that persons with cardiovascular and respiratory have increased risk of death associated with ambient heat exposure and that risk is higher for several population including the and persons of risk factors include of air of to living using a and living on higher of at lag of have been to the effect of mortality following heat waves, that heat-related mortality is an The studies of risk with related that a of individual and places some people at high risk for heat-related mortality. provided a of the heat of an of high temperature in excess mortality for particular people in in The experience a that prevention efforts and The epidemiologic from heat waves is for persons at high as in et study in The available has been to a for and to for studies have focused on evaluating preventive efforts by effects of heat waves in the same In a of the and heat waves in St. Louis, Missouri, the heat was more with higher heat-related mortality persons in St. at risk for heat-related mortality, increased use of air conditioning and preventive In of heat waves in and occurring in heat-related deaths and were reported to be at least in than in The in associated mortality be to in temperature and preventive efforts may have a in these Thus, for heat-related mortality, those in and may be in heat-related mortality. A “heat for high-risk cities can be developed and with the of health and may 1) of a a on each that elderly and persons are during a heat 2) of a number for persons who are from the 3) to more and to in places with heat and to areas such as during a heat (23). on US heat waves, the current a heat a heat index or and a ambient temperature greater than or to for at least However, these may be for heat-related deaths in cities, since the evidence variation by and should be on the of the and people living in urban who are particularly vulnerable to the effects of heat should be the focus of preventive studies have identified the elderly to be at an increased risk of heat-related mortality (4, risk factors have not been specifically for them. to heat exposure may some heat-related though not during periods of heat. Thus, more is needed to the relation between temperature and mortality, focusing on the elderly Further should consider the of factors that risk for heat-related studies at the population level temperature or weather to mortality not or individual and to heat. the data on or be For example, such have been applied to assess of in models developed for studies of air pollution be to the investigation of heat as to to that they can be addressed by as as by The assessing to cold across countries in model to change and health, in the evidence identified in the recent US to be addressed These include the and/or effect of the relation between air and temperature on mortality, examining changes in resulting from weather and heat exposure assessment studies. the health of exposure to and other air for persons with or other the mechanisms for health effects should be of and are likely to increase during periods of higher temperature and changes in weather (e.g., greater in some areas and in Many effects of elevated ambient temperature have been and these effects will probably with global further is in to these issues to public and to The for to of and from studies risk factors for in mortality following heat waves cardiovascular on from studies risk factors for in mortality following heat waves cardiovascular on from studies of heat-related mortality to cardiovascular and respiratory cardiovascular from studies of heat-related mortality to cardiovascular and respiratory cardiovascular from studies of heat-related mortality in from studies of heat-related mortality in from studies of the temperature-mortality relation other than studies of heat-wave exposure that used and time-series from studies of the temperature-mortality relation other than studies of heat-wave exposure that used and time-series from studies of the effect of the air relation on mortality and A than in from studies of the effect of the air relation on mortality and A than in