Health impacts from climate change can arise via complex processes. The scale of these effects would depend primarily on the size, density, and wealth of human populations or communities (WHO, 1998). Extreme weather variability associated with climate change may add new stress to developing nations that already are vulnerable as a result of environmental degradation, resource depletion, overpopulation, or location (McMichael et al., 1996).

Persistent poverty and population pressure accompanied by inadequate sanitation and inadequate public health infrastructure will limit many populations' capacity to adapt (Kovats et al., 1998; Patz, 1998). Interaction between local environmental degradation and changes on a larger scale—climate change, population growth, and loss of biodiversity—may significantly influence effects on health (Haines and McMichael, 1997; WHO, 1998). The direct impacts of climate change depend mainly on exposure to heat or cold waves or extreme weather events, such as floods and droughts.

14.2.5.1. Effects of Changes in Climate Variables on Health

Kattenberg et al. (1996) made generalized tentative assessments concerning extreme weather and climate events. Studies in temperate and subtropical countries have shown increases in daily death rates associated with extreme outdoor temperatures (see Section 14.1.5; McMichael et al., 1996). Climate change scenarios constructed from three models have been used to estimate human mortality with changes in baseline climate conditions for Buenos Aires, Caracas, San José, and Santiago. In Caracas and San José, where present temperatures are close to the comfort temperature for all months, mortality rates (total, cardiovascular, and respiratory) increase for most of the climate change scenarios employed. However, decreases in winter mortality may offset excess summer mortality in cities with relatively colder climates, such as Santiago (Martens, 1998). People who are more than 65 years old are more temperature sensitive than younger people (Martens, 1998).

There is evidence that people living in poor housing conditions (crowded and poorly ventilated) and urban populations in developing countries are particularly vulnerable to thermal stress enhanced by rapid urbanization (urban heat island) because of few social resources and low preexisting health status (Kilbourne, 1989; Martens, 1998). Furthermore, in rural areas, the relative importance of temperature on mortality may be different from its effect on urban populations (Martens, 1998).

Prolonged heat can enhance production of smog and dispersal of allergens. Both effects have been linked to respiratory symptoms (Epstein, 2000). High temperatures and air pollutants, especially particulates, act synergistically to influence human health. This effect is occurring in large cities, such as Mexico City, Santiago, and more recently Buenos Aires, where such conditions enhance the formation of secondary pollutants (e.g., ozone—Escudero, 1990; Katsouyanni et al., 1993; Canziani, 1994). Saldivia (1994, 1995) has observed an increasing trend in the mortality of elderly people following peaks of air pollution in São Paulo. Daily mortality has been correlated mainly with temperature and ozone concentration, both measured the day before (Sartor et al., 1995). Hyperthermic syndrome (heat stroke) affected children under 2 years old and people over 80 in Peru's coastal regions during the high temperatures of the El Niño phenomenon (Instituto Nacional de Salud, 1998a, 1999).

During droughts, the risk of wildfires increases, causing loss of green areas, property, livestock, and human life, as a result of increasing air pollution from suspended particles (OPS, 1998). The direct effects of wildfires on human health occur from burns and smoke inhalation (Kovats et al., 1999). In Alta Floresta, Brazil, there was a 20-fold increase in outpatient visits for respiratory disease in 1997 during a biomass smoke episode (Brauer, 1998).

Increased ambient temperature may have significant effects on the distribution and overgrowth of allergenic plants. Higher temperatures and lower rainfall at the time of pollen dispersal are likely to result in higher concentrations of airborne pollen during the peak season (Emberlin, 1994; Rosas et al., 1989). A relationship between concentrations of algae and weather parameters (temperature and vapor pressure) in Mexico has been correlated. Dispersion of algae has received much attention during recent years as a result of associated inhalant allergies and other respiratory disorders (Rosas et al., 1989). The high degree of seasonality (dry-rainy season) in air-borne enteric bacterial, basidiomycete spore, particle, and protein concentrations also could be associated with temperature and vapor pressure in Mexico City (Rosas et al., 1994, 1995; Calderón et al., 1995). Thus, future changes in climatic variables might have an important effect in the distribution of air-borne bacteria, fungus, pollen, particles, and proteins whose allergenic properties have to be considered.
In 1998, there was an increase in cholera cases in Peru and Ecuador and other countries affected by weather disasters. Hurricane Mitch affected Guatemala (the number of cholera cases increased four-fold), Belize, and Nicaragua (the number of cholera cases was six times the usual number of reported cases) (OPS, 1999).

Global warming could increase the number and severity of extreme weather events, such as storms, floods, droughts, and hurricanes, along with related landslides and wildfires (IPCC, 1996). Such events tend to increase death and disease rates—directly through injuries or indirectly through infectious diseases brought about by damage to agriculture and sanitary infrastructure and potable water supplies (PAHO, 1998a). Floods and droughts could permanently or semi-permanently displace entire populations in developing countries, leading to overcrowding and diseases connected with it, such as tuberculosis and other air-borne and crowd diseases; adverse psychological effects; and other stresses (IPCC, 1996; McMichael and Kovats, 1998a,b; Epstein, 2000). Slums and shantytowns located on hills (e.g., Rio de Janeiro), as well as human settlements located in flood-prone areas, are particularly subject to periodic natural disasters that adversely affect human health and sanitary infrastructure (IPCC, 1996).

In developing countries, populations are becoming more rather than less vulnerable to disasters (McMichael and Kovats, 1998b). Natural disasters may be responsible for outbreaks of cholera, leptospirosis, malaria, and dengue (Moreira, 1986; PAHO, 1998a,b). Hurricane Mitch stalled over Central America in October 1998 for 3 days, claiming 11,000 lives. It was the most deadly hurricane to strike the western hemisphere in 2 centuries (Hellin et al., 1999). After Mitch, Honduras reported thousands of cases of cholera, malaria, and dengue fever (PAHO, 1998a,b; OPS, 1999; Epstein, 2000). As the risk of flooding increases with climate change, so does the importance of the major drainage system, which will determine whether floodwaters drain in minutes, hours, or days (McMichael and Kovats, 1998b). Floodwaters can cause the release of dangerous chemicals from storage and waste disposal sites and precipitate outbreaks of vector- and water-borne diseases (Patz, 1998).

Indirect effects of disasters can damage the health care sector. After Hurricane Mitch, some countries in Central America regressed decades in health services and transport infrastructure, thereby making it more difficult to assist the affected population (OPS, 1999). Environmental refugees could present the most serious health consequences of climate change. Risks that stem from overcrowding include virtually absent sanitation; scarcity of shelter, food, and safe water; and heightened tensions—potentially leading to social conflicts (Patz, 1998). Weather disasters cause many deaths and have long-term impacts on communities, including psychological effects such as post-traumatic stress disorder (Kovats et al., 1998). In 1999, heavy rains on the coast of Venezuela displaced 80,000-100,000 people and caused 20,000-50,000 deaths, as well as enormous damage in infrastructure (PNUD/CAF, 2000).

IPCC Homepage