“Have building staff check on at-risk residents every few hours.” (City of Toronto, Heat Alerts and Extreme Heat Alerts, 2010)
Heat health warnings regularly incorporate the message that specific groups of people are more vulnerable to hot weather mortality and morbidity. Among groups cited are children, older adults, the socially isolated, and people who live in poverty. However, the rationale for targeting these demographic groups is not always clear. As heat-related illness is largely preventable, understanding who is vulnerable, and why, allows public health practitioners to best prioritize actions and effectively create public health prevention measures.
There is an overwhelming base of evidence suggesting that older adults are at greater risk for mortality during hot weather.1 In research studies, greater risk of heat-related mortality has been variably demonstrated in persons of at least 75 years2; 70 years3; and 65 years of age.4-12
What physiological changes occur with age that can place people more at risk?
Aging is associated with physiological changes that may increase susceptibility to the adverse health effects of high ambient temperatures. Aging reduces cardiac output and the capacity to redistribute blood to the skin and to the intestinal and renal circulatory beds.13 The decline in cardiac reserve and age-related reduction in vascularity decreases the ability to move blood to the peripheral circulation, thereby reducing the efficiency with which heat can be dissipated.14 Aging also reduces the number of sweat glands and sweat gland response.15 Lifetime ultraviolet exposure and other environmental factors contribute to reduction of sweat gland responsiveness.16
Is there any clear demarcation of a specific age or physical status that confers a status of at risk for heat illness?
There is a lack of information pertaining to a discrete age at which older adults experience physiological changes.17 In research on sweat gland function, a progressive age-related decline has been suggested.16,18 However, other research suggests that sweat gland function declines among persons in their 70s and 80s.19 Additionally, research relating to age-temperature vulnerability does not differentiate the effects of age from other confounding factors (such as, changes in body composition, chronic disease, obesity, inactivity among older adults, or a combination of factors).17
Is risk inherent or a combination of concurrent factors associated with aging?
In epidemiologic studies, age may appear as either a confounder or an effect modifier in the relationship between temperature and mortality. A longitudinal cohort study of elderly residents of Rome, Italy during the summers of 2005-2007 suggests that underlying mechanisms may influence age-related susceptibility.20 Specifically, excess mortality among the 65-74 year age group was associated with previous hospitalization for chronic pulmonary disease, while excess mortality among the older age group (75 years plus) was significantly higher among females and unmarried subjects.20 Overall, the older age group experienced higher mortality impacts; researchers suggest concurrent factors such as pre-existing chronic illness, low physical fitness level, and physiological response changes may be associated with this finding.20 Besides the physical factors reviewed by Schifano et al.,20 aging is also associated with a number of behavioural and social changes. Other important confounding variables may include social isolation, being unable to care for oneself independently, being housebound, being less mobile and less active, and living in poverty. Further analysis is required to better understand the basis for susceptibility among older adults.
Rare cases of child death associated with hot weather have been reported in France (during the 2003 and 2006 heat waves) and observed in Portugal (during the 1981 and 1991 heat waves).21 Collected research identifies children under the age of 15 years,11,22 five years and younger,5 and infants one year and under5,23 as being at greater risk of mortality during hot weather. Children are often considered more vulnerable to environmental hazards as they are dependent on others; however, the opposite may also occur, children who are well cared for may be protected from hazards.
In comparison with adults, children have traditionally been considered more physiologically vulnerable to heat-related illnesses associated with physical activity.24 Proposed reasons include greater surface area to body mass ratio than adults, blunted thirst response, production of more metabolic heat per kilogram of body weight, and lower cardiac output.24 However, Rowland25 indicates that when relative exercise intensity is taken into account, research does not indicate thermoregulatory differences in heat dissipation between children and adults. Rowland25 suggests that greater risk of heat-related illnesses among infants and young children (< 4 years of age) is likely to be associated with dependency factors and pre-existing illness.
Anecdotal reports and media outlets describe fatalities associated with children left in parked cars during hot weather.26,27 Temperatures in motor vehicles can increase rapidly and can become very hot, even at moderate outdoor temperatures (especially when parked in direct sunlight).28-30 McLaren et al.28 report that between 1998 and 2002, in the United States, an average of 29 children died per year under such conditions. No such data is available in Canada. Krous et al.31 investigated cases of heat-related deaths among infants and children found in motor vehicles and beds, in the United States and Australia. The researchers describe rare events in which young children are left in vehicles because they were sleeping, forgotten, or because it was mistakenly assumed they could get out of the vehicle without assistance.31 As well, a number of fatal cases were associated with children entering a vehicle to play without the parent’s knowledge, becoming trapped, disoriented, or falling asleep.31 Researchers also described rare cases of infant heat-related deaths. Generally, infants are more susceptible to heat-related deaths in bedroom environments, due to their high metabolic rates and inability to remove excess clothing and blankets.31
Adults Under Age 65
Research demonstrates that adults under 65 years of age can also be at risk for temperature-related mortality. Gouveia et al.11 found that in Sao Paulo, Brazil, while the greatest increase in temperature mortality risk was observed among persons under the age of 15 and over the age of 65, relative increases in risk were also observed among persons between 15 and 64 years. In a preliminary analysis of heat event-related mortality in British Columbia, the greatest proportional increase in mortality was observed among the 45-64 year age group.32
An investigation of heat-related deaths in the United States from 1999-2003, including all deaths where hyperthermia was listed as a contributing factor or an underlying cause, found that 7% of deaths were among children less than 15 years, 53% among persons between the ages of 15 and 64, and 40% in those over the age of 65.33 Heat-related deaths represented a considerable burden of Potential Years of Life Lost (PYLL) as 60% of deaths occurred among persons under age 65. The majority of cases among those in the 15 to 64 age category occurred among men.33 Possible explanations, including exposure to heat during outdoor work or high levels of physical activity, require further research.
Investigation in the United States found that cardiovascular disease was recorded as the underlying cause of death in the majority (57%) of cases in which hyperthermia was a contributing factor.33 Many, approximately 70%, of these heat-related cardiovascular deaths occurred among persons with chronic ischemic heart disease.33 Risk of cardiovascular disease increases with age and particularly affects men over the age of 45 and women over the age of 55.34 Risk of heat-related mortality among persons with cardiovascular disease in various age groups requires further exploration.
People Who Live in Poverty
Evidence of the association between low socioeconomic status and heat-related mortality has been mixed. Research in the United States has demonstrated greater mortality among people of low socioeconomic status compared with those of higher socioeconomic levels.35-37 However, research in Barcelona,38 Italy39 and Latin America6,11 found little or no association between socioeconomic status and heat-related mortality.
Mechanisms for observed increased risk of mortality among those of low socioeconomic status are not well explored in hot weather literature. People who live in poverty may lack the awareness, means, and opportunity to take protective health measures. For example, use of air conditioning has been illustrated as being protective during heat waves. However, lack of air conditioning has been strongly associated with poverty in the United States.9,35,40 Persons of low socioeconomic status are also more likely to have a chronic disease or other medical risk factor that increases their risk during hot weather events.21 As well, persons of low socioeconomic status may also be more likely to live in poor quality housing (without windows or insulation) or in high-rise apartment buildings and their exposure to heat may increase.40 It has also been hypothesized that persons of low socioeconomic status may avoid opening windows or utilizing screen doors because of personal safety concerns in their residential area.41
Rey et al.42 sought to clarify the relationship between socioeconomic status, spatial heterogeneity of heat exposure, and heat-related mortality. Researchers found that more densely populated regions of Paris experienced more extreme heat exposures. In areas more exposed to heat, excess mortality was twice as high in the most deprived regions than in the least deprived regions. No relationship between heat exposure and deprivation was observed in regions less exposed to heat or in regions where the level of deprivation was less heterogeneous. This research illustrates that, in situations of extreme heat exposure, deprivation at a regional level can be associated with increased heat wave mortality.
A social analysis of the 1995 Chicago heat wave described socially-isolated older adult males as the principle victims of heat-related mortality.41 Case control studies, following the 1995 and 1999 Chicago heat wave, found that persons who lived alone and who did not leave their residence daily were at greater risk of heat-related mortality.40,43 However, living alone did not alter the risk of heat-related death in Modena, Italy44 or in England and Wales.9 Further research is required to better understand the context-specific variables, including social networking or behavioural tendencies of social isolation, that may increase vulnerability.
Measuring aspects of social isolation can be difficult. Some researchers have explored marital status as a proxy for social isolation. Stafoggia et al.39 and Fouillet et al.45 found that marriage reduced the risk of heat-related mortality in both Italy and France. Research indicates underlying social components associated with gender; one study in Paris found that risk of heat-related mortality was higher for unmarried men, but not for unmarried women.46
Are There Gender Differences in Vulnerability?
The majority of research investigating heat waves in Europe found that women are more at risk, in both relative and absolute terms, for heat-related mortality.21 A social analysis, following the 2003 heat wave in Paris, described older women as particularly vulnerable due to social isolation and economic poverty (e.g., living alone in small, urban, high-rise dwellings) and thus had increased exposure to heat.47 Other research has concluded that women are more physiologically susceptible to heat-related illness, due to thermoregulatory responses.48,49 However, evidence from the United States illustrates that men more often experience heat-related mortality.33,50 Higher levels of physical activity among working-age American men may increase vulnerability.33 As well, vulnerability may be associated with greater levels of social isolation.41 The inconsistent findings across different populations illustrate the potential for social and contextual factors to influence mortality and morbidity. Social factors of gender (e.g., level of isolation, marital status) and behavioural (e.g., physical activity) are important determinants of heat-related mortality risk.21
Protecting Vulnerable Populations
Heat-related morbidity and mortality can be prevented.51 For example, emergency measures and community outreach can be activated in response to harmful weather conditions. The most effective interventions target and tailor their approaches to reach the most vulnerable populations.52 Identifying and exploring the attributes of demographic population groups most at-risk is a first step in developing appropriate interventions. Further research is required to understand the effectiveness of targeted interventions in reducing morbidity and mortality during extreme heat.
Observational studies, individual case investigations, and some physiological experiments identify certain demographic groups as vulnerable to heat-related illness and mortality. The identification of population attributes is a required first step in tailoring strategies to prevent heat-related mortality and morbidity. In summary:
- Aging is associated with multiple factors that increase vulnerability to heat-related illness and death, including: physiological changes, presence of chronic illnesses, use of certain medications, low levels of physical fitness, lower mobility, socioeconomic vulnerability, social isolation, and being unable to care for oneself independently.
- Heat-related child deaths are rare events, often associated with factors of dependency.
- Observational studies suggest that persons under the age of 65 may be at risk of heat-related illness, often associated with chronic illnesses and greater exposure to heat, due to physical activity or outdoor work. Further research and exploration is required.
- Evidence regarding the association between heat-related illness and socioeconomic status is mixed. Recent investigations illustrate an association between heat-related mortality and deprivation in regions of extreme heat exposure.
- Assessing the association between heat-related illness and social isolation is difficult. Aspects of social isolation are context-specific and difficult to measure.
- Vulnerabilities associated with gender are contextual and associated with behavioural factors (e.g., physical activity) as well as social factors (e.g., level of isolation, marital status, socioeconomic status).
Questions & Gaps
- Are healthy older adults at risk during hot weather events?
- What mechanisms increase vulnerability among persons of low socioeconomic status? among persons who are socially isolated?
- What explains the high proportional mortality among adult men in the United States?
- What are the gender and hormonal differences in thermoregulatory changes associated with aging?
- What hot weather interventions are most effective in reducing morbidity and mortality among specific vulnerable groups during extreme heat events?
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