Like other environmental stressors, climate change can be expected to have differential effects on different subpopulations, depending on a variety of susceptibility factors. Biological sensitivity, socioeconomic factors, and geography may each contribute to heightened risk for climate-sensitive health outcomes, which include heat stress, air pollution health effects, extreme weather event health effects, water-, food-, and vector-borne illnesses. The term “vulnerability” connotes the balance between susceptibility factors and factors that increase the resilience of populations to environmental stressors. Particularly vulnerable subpopulations include children, pregnant women, older adults, impoverished populations, people with chronic conditions and mobility and cognitive constraints, outdoor workers, and those in coastal and low-lying riverine zones. For public health planning, it is critical to recognize those populations that may experience synergistic effects of multiple risk factors for health problems, both related to climate change and to other temporal trends.
Identifying Vulnerable Subpopulations
In adapting the IPCC's definitions1 to considerations of public health, “vulnerability” can be defined as the summation of all risk and protective factors that ultimately determine whether an individual or subpopulation experiences adverse health outcomes, and “sensitivity” can be defined as an individual's or subpopulation's increased responsiveness, primarily for biological reasons, to a given exposure. Thus, specific subpopulations may experience heightened vulnerability for climate-related health effects for a wide variety of reasons. Biological sensitivity may be related to the developmental stage; pre-existing chronic medical conditions (such as the sensitivity of people with chronic heart conditions to heat-related illness); acquired factors (such as immunity); and genetic factors (such as metabolic enzyme subtypes that play a role in sensitivity to air pollution effects). Socioeconomic factors also play a critical role in altering vulnerability and sensitivity to environmentally-mediated factors. They may increase likelihood of exposure to harmful agents, interact with biological factors that mediate risk (such as nutritional status), and/or lead to differences in the ability to adapt or respond to exposures or early phases of illness and injury.
Populations living in certain regions of the United States may also experience increased risks for specific climate-sensitive health outcomes due to their regions' baseline climate, abundance of natural resources such as fertile soil and fresh water supplies, elevation, dependence on private wells for drinking water, and/or vulnerability to coastal surges or riverine flooding. Some regions' populations may in fact experience multiple climate-sensitive health problems simultaneously. One approach to identifying such areas is to map regions currently experiencing increased rates of climate-sensitive health outcomes or other indicators of increased climate risk, as illustrated in Fig. 1.
Specific Subpopulations at Risk
Vulnerable subpopulations may be categorized according to specific health endpoints (Table 1). Although this is typically the way scientific literature reports risk factors for adverse health affects, this section discusses vulnerability for a variety of climate-sensitive health endpoints one subpopulation at a time.
Children's small body mass to surface area ratio and greater susceptibility to dehydration make them more vulnerable to heat-related morbidity and mortality (Kovats and Hajat, 2008).2 In an analysis of warm season mortality in California, children under 5 and infants under 1 year of age suffered higher mortality with increasing daily temperatures (Basu and Ostro, 2008).3 Infant mortality increased 4.9% for a 10 degree increase in mean daily apparent temperature and children under five experienced a 4.2% increase, although the latter finding failed to achieve statistical significance. This study did not analyze temperature associations with specific causes of death in children. Epidemiologic analyses of heatwave events, however, have generally failed to find significantly increased mortality among children and infants (Kovats and Hajat, 2008).4
Children's increased breathing rates relative to body size, greater amount of time spent outdoors, and developing respiratory tracts heighten their sensitivity to harm from ozone air pollution.5 They are also susceptible to developing atopy and allergies related to aeroallergen exposures, which are anticipated to increase with increasing CO2 concentrations and longer pollen seasons (Bunyanavich et al, 2003).6–8
Children are especially vulnerable to a variety of infectious diseases. Their relatively naïve immune systems increase the risk of serious consequences from water- and food-borne diseases.9 Specific developmental factors make them more vulnerable to complications from severe infections like E. Coli O157:H7.10 Children's lack of immunity also plays a role in higher risk of mortality from malaria11; conversely, maternal antibodies to dengue in infants convey increased risk of developing dengue hemorrhagic syndromes. A second peak of greater risk of complications from dengue appears in children between the ages of 3 and 5 (Guzman and Khouri, 2002).12
Children may also be more vulnerable to psychological complications of extreme weather events related to climate change. Following two floods in Europe in the 1990 seconds, children demonstrated moderate to severe stress symptoms and long-term post-traumatic stress disorder (PTSD), depression, and dissatisfaction with ongoing life.13
Pregnant women are likely to be vulnerable to adverse health effects in the aftermath of extreme weather events, as they may have exposure to environmental toxins, limited access to safe food and water, psychological stress, and disrupted health care access. One review suggested an increased incidence of adverse reproductive outcomes after Hurricane Katrina.14
Studies of increased risk of adverse reproductive outcomes associated with air pollutants such as particulate matter and associated polycyclic aromatic hydrocarbons suggest that pregnant women constitute a vulnerable subpopulation for climate or climate change policy-associated changes in these types of air pollutants (Perera, 2008; Makri and Stilianakis, 2007).15,16
Pregnancy also confers increased susceptibility to a variety of climate-sensitive infectious diseases, including malaria and food-borne infections.9,17
Health effects associated with climate change pose significant risks for the elderly. Older adults are more sensitive to temperature extremes, particularly heat18,19; individuals 65 years of age and older comprised 72% of the heat-related deaths in the 1995 Chicago heatwave (Whitman et al, 1997).20 The elderly are also more likely to have preexisting medical conditions, including cardiovascular and respiratory illnesses as well as limited mobility, which may put them at greater risk of severe morbidity or mortality from climate-related events or conditions. For example, a 2004 rapid needs assessment of older adults in Florida found that Hurricane Charley exacerbated preexisting, physician-diagnosed medical conditions in 24% to 32% of elderly households.21
The elderly have been demonstrated to be more susceptible to adverse effects of air pollution. A recent study found people over 65 experienced a 1.1% additional increase in mortality for each 10 ppb increase in 8 hour daily mean ozone concentration.22 Effects of ambient particulate matter on daily mortality also tend to be greatest in older age groups.23
In the US, as around the globe, the greatest health burdens related to climate change are likely to fall on those with the lowest socioeconomic status.24 Most affected are individuals with inadequate shelter or resources to find alternative shelter in the event their community is disrupted. Although quantitative methods to assess the increase in risk related to these social and economic factors are not well-developed, qualitative insights can be gained by examining risk factors for mortality and morbidity from recent weather-related extreme events such as the 1995 heatwave in Chicago and Hurricane Katrina in 2005 (Box 1).
Box 1: Hurricane Katrina
In 2005, Hurricane Katrina caused more than 1500 deaths along the Gulf Coast, and many of these victims were members of vulnerable subpopulations, such as hospital and nursing-home patients, older adults who required care within their homes, and individuals with disabilities.31 According to the Louisiana Department of Health and Hospitals, more than 45% of the state's identified victims were 75 years of age or older; 69% were above age 60.32 In Mississippi, 67% of the victims whose deaths were directly, indirectly, or possibly related to Katrina were 55 years of age or older.33
At hurricane evacuation centers in Louisiana, Mississippi, Arkansas, and Texas, chronic illness was the most commonly reported health problem, accounting for 33% or 4786 of 14,531 visits.34 Six of the fifteen deaths indirectly related to the hurricane and its immediate aftermath in Alabama were associated with preexisting cardiovascular disease (CDC, 2006 days),35 and the storm disrupted an estimated 100,000 diabetic evacuees across the region from obtaining appropriate care and medication.36 One study suggested that the hurricane had a negative effect on reproductive outcomes among pregnant women and infants, who experienced exposure to environmental toxins, limited access to safe food and water, psychological stress, and disrupted health care.14 Other vulnerable individuals included those without personal means of transportation and poor residents in Louisiana and Mississippi who were unable to evacuate in time.31
The tragic loss of life that occurred after Hurricane Katrina underscores the increased vulnerability of special populations and demonstrates that, in the wake of extreme weather events, particularly those that disrupt medical infrastructure and require large-scale evacuation, treating individuals with chronic diseases is of critical concern.37
Studies of heatwaves identify poor housing conditions, including lack of access to air conditioning and living spaces with fewer rooms, as significant risk factors for heat-related mortality.18,25 Higher heat-related mortality has been associated with socioeconomic indicators, such as lacking a high school education and living in poverty.26 Financial stress plays a role, as one study of the 1995 Chicago heatwave found that concern about the affordability of utility bills influenced individuals to limit air conditioning use.27 The risk for exposure and sensitivity to air pollution is also elevated among groups in a lower socioeconomic position.28
Another area of concern for impoverished populations is the impact that climate change may have on food systems and food supply. In the US, food insecurity is a prevalent health risk among the poor and poor children in particular (Cook et al 2007).29 On a global scale, studies suggest that climate change is likely to contribute to food insecurity by reducing crop yield, most significantly at lower latitudes, due to shortened growing periods and decreases in water availability (Parry et al, 2005).30 In the US, changes in the price of food would be likely to contribute to food insecurity to a greater degree than frank scarcity.
People with Chronic Conditions and Mobility and Cognitive Constraints
People with chronic medical conditions have an especially heightened vulnerability for the health impacts of climate change. Extreme heat poses a great risk for individuals with diabetes,38 and extreme cold has an increased effect on individuals with chronic obstructive pulmonary disease.38 People with mobility and cognitive constraints may be at particular risk during heat waves and other extreme weather events (EPA, 2006).39 As noted above, those with chronic medical conditions are also at risk of worsened status as the result of stressors and limited access to medical care during extreme events.
Certain occupational groups, primarily by virtue of spending their working hours outdoors, are at greater risk of climate-related health outcomes. Outdoor workers in rural or suburban areas, such as electricity and pipeline utility workers, are at increased risk of infection with Lyme Disease, although evidence is lacking for greater risk of clinical illness (Piacentino and Schwartz, 2002).40,41 They and other outdoor workers have increased exposures to ozone air pollution and heat stress, especially if work tasks involve heavy exertion.42
Table 2 summarizes the climate-related vulnerability of specific US subpopulations, based on age, underlying medical conditions, and socioeconomic status. Recognition of combined effects will aid efforts at public health intervention and disease prevention.
Assessing vulnerability for climate change related health effects entails combining epidemiologic information on the specific health effects anticipated in a given region, consideration of the unique geography and ecosystems of a region, which will mediate climate-related health effects, and an assessment of the resilience or adaptive capacity of specific subpopulations in a given region. Although there are studies that provide assessments of population and individual risk factors for specific health outcomes, very few associate these health risk factors with geographic risk factors. To date, these assessments have been conducted in a fairly qualitative manner, relying on analogy and fragmented data sources. For the future, improvements in integrated modeling and ongoing research on climate-related health effects will facilitate the development of more quantitative indices for vulnerability that will assist public health planning. In the interim, increased identification of synergies between geography, socioeconomic status, and underlying medical conditions in specific US subpopulations is essential to protecting public health in the face of accelerating climate change.
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