8.3.9. Human Health

Climate change is likely to have wide-ranging and mostly adverse impacts on human health. These impacts would arise by direct pathways (e.g., exposure to thermal stress and extreme weather events) and indirect pathways (increases in some air pollutants, pollens, and mold spores; malnutrition; increases in the potential transmission of vector-borne and waterborne diseases; and general public health infrastructural damage) (IPCC 1996, WG II Sections 18.2 and 18.3, and Figure 18-1). Climate change also could jeopardize access to traditional foods garnered from land and water (such as game, wild birds, fish, and berries), leading to diet-related problems such as obesity, cardiovascular disorders, and diabetes among northern populations of indigenous peoples as they make new food choices (Government of Canada, 1996).

8.3.9.1. Thermal Extremes

Temperate regions such as North America are expected to warm disproportionately more than tropical and subtropical zones (IPCC 1996, WG I). The frequency of very hot days in temperate climates is expected to approximately double for an increase of 2-3°C in the average summer temperature (CDC, 1989; Climate Change Impacts Review Group, 1991). Heat waves cause excess deaths (Kilbourne, 1992), many of which are caused by increased demand on the cardiovascular system required for physiological cooling. Heat also aggravates existing medical problems in vulnerable populations-particularly the elderly, the young, and the chronically ill (CDC, 1995; Canadian Global Change Program, 1995). For example, mortality during oppressively hot weather is associated predominately with preexisting cardiovascular, cerebrovascular, and respiratory disorders, as well as accidents (Haines, 1993; IPCC 1996, WG II, Section 18.2.1). In addition to mortality, morbidity such as heat exhaustion, heat cramps, heat syncope or fainting, and heat rash also result from heat waves. People living in hot regions, such as the southern United States, cope with excessive heat through adaptations in lifestyle, physiological acclimatization, and adoption of a particular mental approach (Ellis, 1972; Rotton, 1983). In temperate regions, however, periods of excessive heat occur less frequently, and populations accordingly are less prepared with responsive adaptive options (WHO, 1996).

Data in cities in the United States and Canada show that overall death rates increase during heat waves (Kalkstein and Smoyer, 1993), particularly when the temperature rises above the local population's temperature threshold. In addition to the 1980 heat wave that resulted in 1,700 heat-related deaths, heat waves in 1983 and 1988 in the United States killed 566 and 454 people, respectively (CDC, 1995). More recently, in July 1995, a heat wave caused as many as 765 heat-related deaths in the Chicago area alone (Phelps, 1996). Tavares (1996) examined the relationship between weather and heat-related morbidity for Toronto for the years 1979-89 and found that 14% of the variability for all morbidity in persons 0-65 years of age was related to weather conditions.

Death rates in temperate and subtropical zones appear to be higher in winter than in summer (Kilbourne, 1992). Comparative analyses of the causes of differences between summer versus winter weather-related mortalities are lacking, however. The United States averaged 367 deaths per year due to cold in the period 1979-94 (Parrish, 1997), whereas the annual average number of Canadians dying of excessive cold is 110 (Phillips, 1990). It has been suggested that winter mortality rates, which appear to be more related to infectious diseases than to extremely cold temperatures, will be little impacted by climate change. Any global warming-induced increases in heat-related mortality, therefore, are unlikely to be of similar magnitude to decreases in winter mortality (Kalkstein and Smoyer, 1993).

Mortality from extreme heat is increased by concomitant conditions of low wind, high humidity, and intense solar radiation (Kilbourne, 1992). In Ontario, the number of days annually with temperatures above 30°C could increase fivefold (from 10 to 50 days per year) under doubled CO₂ scenarios (Environment Canada et al., 1995).

Several studies (e.g., WHO, 1996) have found that future heat-related mortality rates would significantly increase under climate change. Table 8-9 shows projected changes in heat-related deaths for selected cities in North America under two climate change scenarios. Acclimatization of populations, however, may reduce the predicted heat-related morbidity and mortality. Kalkstein et al. (1993) found that people in Montreal and Toronto might acclimatize somewhat to global warming conditions. People in Ottawa, on the other hand, showed no signs of potential acclimatization. It is important to note that acclimatization to increasing temperatures occurs gradually, particularly among the elderly, and may be slower than the rate of ambient temperature change.

Air conditioning and adequate warning systems also may reduce heat-related morbidity and mortality in a warmer North America. It has been suggested that air conditioning could reduce heat-related deaths by 25% (Phelps, 1996). A warning system such as the Philadelphia Hot Weather-Health Watch/Warning System (PWWS) that alerts the public when oppressive air masses (e.g., extended periods of extreme high temperatures, high humidity, moderate to strong southWesterly winds, and high pressure) may occur might further reduce heat-related mortality (Kalkstein and Smoyer, 1993). The PWWS is a three-tiered system that produces a health watch, health alert, or health warning and then accordingly initiates a series of interventions, including media announcements, promotion of a "buddy system," home visits, nursing and personal care intervention, increased emergency medical service staffing, and provision of air-conditioned facilities (Kalkstein et al., 1995).

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