Thermal Stress and Deaths During Heat Waves

During heat waves, death rates, hospital admissions, and other adverse health effects increase. Data indicate that heat waves in Europe during 2003 and Russia during 2010 caused tens of thousands of premature deaths [1, 2]. The elderly and young are more likely to experience adverse health effects because of extreme heat. For example, an analysis of a heat wave in France in 2006 indicated that 80% of excess deaths occurred in the population with an age greater than 55 [3]. Also, people with poor health, the poor who cannot afford air conditioning, and people living in urban centers are more highly susceptible to heat stress [1, 4-6]. 

Climate change is expected to markedly increase the frequency, severity, and duration of heat waves [1, 7-10]. Projections for the U.S. include a five-fold increase in the number and a one day increase in the duration of heat waves in the Eastern U.S. in 2057-2059 compared to the 2003-2004 [8] and more than a two-fold increase in days with extreme heat in most of California [1].

The Intergovernmental Panel on Climate Change (IPCC) [11], Huang, Burnett et al. [12] and Fisk et al. [13] have compiled and reviewed projections of how climate change will modify heat related deaths. Projections vary among locations and depend on the time periods and climate change scenarios. Examples of the projections for the U.S. include the following:

·       up to a ten-fold increase in heat-related deaths in the Eastern U.S. (1400 – 3600 additional deaths) by 2057-2059 [8]

·       for Los Angeles, between 320 and 1200 deaths in 2080 compared to 165 deaths per year in the 1990s [12]

·       for 44 U.S. cities with a population greater than one million, a 70% to more than 100% increase in heat related mortality in 2050 compared to 2020 [12]

·       for the New York city area, an increase in heat-related mortality of 47% to 95% in the 2050s compared to the 1990s [14]

Several analyses indicate that cold-related deaths are expected to decrease with climate change. Two studies in the review by Huang et al. [12], projected that decreases in wintertime mortality for cold weather exceeded increases in heat-related summer mortality, while a third study [15] that performed analyses for 44 large U.S. cities indicated that decreases in winter-mortality would be small related to increases in heat-related mortality. The most recent review of health effects of climate change by the IPCC states that worldwide, the increases in heat stress deaths will greatly outweigh the reductions in cold related deaths [16]. A major U.S. review indicates that declines in cold-related deaths will be smaller than increases in heat-related deaths [9].

Segments of the population most affected by heat waves (young children, elderly, poor health) spend most of their time at home. For example, a survey in the U.S. found that people age 65 or older spend 81% of time indoors at home; however, even for the general U.S. population 69% of time is spent indoors at home [17]. Consequently, much or perhaps most of the adverse health effects of heat waves appear to be a result of heat stress that occurs inside homes during heat waves, as opposed to heat stress outdoors or at work places. The documented increased susceptibility to heat waves of the people living without air conditioning, on the top floor, and in buildings with poor thermal insulation also indicate the importance of heat stress in the home [1, 4-6, 18]. An estimated 54% of deaths during a 2003 heat wave in France occurred in people located at home or in retirement homes [19].

Table 1 below, based in part on Fisk [13], lists a number of adaptation measures that, based on current knowledge, are expected to reduce heat stress in homes; however, associated empirical data are very sparse. Most of these measures should also increase thermal comfort and many will also reduce home energy use in many climates, thus, contributing to climate change mitigation. Missing from the list is wall insulation. Modeling has indicated that adding insulation to the inner surface of solid masonry external walls can sometimes worsen heat stress conditions indoors in homes without air conditioning by reducing the heat exchange that cools the indoor air at night [20-22]. Addition of insulation to the exterior surfaces of walls and insulation of the cavities inside walls may more often reduce indoor heat stress; however, associated data were not identified.