9.4. Thermal Stress (Heat Waves, Cold Spells)
9.4.1. Heat Waves
Global climate change is likely to be accompanied by an increase in the frequency
and intensity of heat waves, as well as warmer summers and milder winters (see
Table 3-10). The impact of extreme summer heat
on human health may be exacerbated by increases in humidity (Gaffen and Ross,
1998; Gawith et al., 1999).
Daily numbers of deaths increase during very hot weather in temperate regions
(Kunst et al., 1993; Ando, 1998a,b). For example, in 1995, a heat
wave in Chicago caused 514 heat-related deaths (12 per 100,000 population) (Whitman
et al., 1997), and a heat wave in London caused a 15% increase
in all-cause mortality (Rooney et al., 1998). Excess mortality
during heat waves is greatest in the elderly and people with preexisting illness
(Sartor et al., 1995; Semenza et al., 1996; Kilbourne,
1997; Ando et al., 1998a,b). Much of this excess mortality from
heat waves is related to cardiovascular, cerebrovascular, and respiratory disease.
The mortality impact of a heat wave is uncertain in terms of the amount of life
lost; a proportion of deaths occur in susceptible persons who were likely to
have died in the near future. Nevertheless, there is a high level of certainty
that an increase in the frequency and intensity of heat waves would increase
the numbers of additional deaths from hot weather. Heat waves also are associated
with nonfatal impacts such as heat stroke and heat exhaustion (Faunt et
al., 1995; Semenza et al., 1999).
Heat waves have a much bigger health impact in cities than in surrounding suburban
and rural areas (Kilbourne, 1997; Rooney et al., 1998). Urban areas typically
experience higherand nocturnally sustainedtemperatures because of
the "heat island" effect (Oke, 1987; Quattrochi et al., 2000). Air pollution
also is typically higher in urban areas, and elevated pollution levels often
accompany heat waves (Piver et al., 1999) (see also Section
126.96.36.199 and Chapter 8).
The threshold temperature for increases in heat-related mortality depends on
the local climate and is higher in warmer locations. A study based on data from
several European regions suggests that regions with hotter summers do not have
significantly different annual heat-related mortality compared to cold regions
(Keatinge et al., 2000). However, in the United States, cities with colder
climates are more sensitive to hot weather (Chestnut et al., 1998). Populations
will acclimatize to warmer climates via a range of behavioral, physiological,
and technological adaptations. Acclimatization will reduce the impacts of future
increases in heat waves, but it is not known to what extent. Initial physiological
acclimatization to hot environments can occur over a few days, but complete
acclimatization may take several years (Zeisberger et al., 1994).
Weather-health studies have used a variety of derived indicesfor example,
the air mass-based synoptic approach (Kalkstein and Tan, 1995) and perceived
temperature (Jendritzky et al., 2000). Kalkstein and Greene (1997) estimated
future excess mortality under climate change in U.S. cities. Excess summer mortality
attributable to climate change, assuming acclimatization, was estimated to be
500-1,000 for New York and 100-250 for Detroit by 2050, for example.
Because this is an isolated study, based on a particular method of treating
meteorological conditions, the chapter team assigned a medium level of certainty
to this result.
The impact of climate change on mortality from thermal stress in developing
country cities may be significant. Populations in developing countries (e.g.,
in Mexico City, New Delhi, Jakarta) may be especially vulnerable because they
lack the resources to adapt to heat waves. However, most of the published research
refers to urban populations in developed countries; there has been relatively
little research in other populations.
9.4.2. Decreased Mortality Resulting from Milder Winters
In many temperate countries, there is clear seasonal variation in mortality
(Sakamoto-Momiyama, 1977; Khaw, 1995; Laake and Sverre, 1996); death rates during
the winter season are 10-25% higher than those in the summer. Several studies
indicate that decreases in winter mortality may be greater than increases in
summer mortality under climate change (Langford and Bentham, 1995; Martens,
1997; Guest et al., 1999). One study estimates a decrease in annual cold-related
deaths of 20,000 in the UK by the 2050s (a reduction of 25%) (Donaldson et
al., 2001). However, one study estimates that increases in heat-related
deaths will be greater than decreases in cold-related death in the United States
by a factor of three (Kalkstein and Greene, 1997).
Annual outbreaks of winter diseases such as influenza, which have a large effect
on winter mortality rates, are not strongly associated with monthly winter temperatures
(Langford and Bentham, 1995). Social and behavioral adaptations to cold play
an important role in preventing winter deaths in high-latitude countries (Donaldson
et al., 1998). Sensitivity to cold weather (i.e., the percentage increase
in mortality per 1ºC change) is greater in warmer regions (e.g., Athens,
southern United States) than in colder regions (e.g., south Finland, northern
United States) (Eurowinter Group, 1997). One possible reason for this difference
may be failure to wear suitable winter clothing. In North America, an increase
in mortality is associated with snowfall and blizzards (Glass and Zack, 1979;
Spitalnic et al., 1996; Gorjanc et al., 1999) and severe ice storms
(Munich Re, 1999).
The extent of winter-associated mortality that is directly attributable to
stressful weather therefore is difficult to determine and currently is being
debated in the literature. Limited evidence indicates that, in at least some
temperate countries, reduced winter deaths would outnumber increased summer
deaths. The net impact on mortality rates will vary between populations. The
implications of climate change for nonfatal outcomes is not clear because there
is very little literature relating cold weather to health outcomes.