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1 Introduction

The city structure, its land uses, and the materials used in buildings or streets can all alter the ambient temperature experienced in urban areas compared to the temperature registered in surrounding rural areas. Heat islands are a well-studied phenomenon by which urban areas can register temperatures that are several degrees hotter than surrounding rural areas. In this chapter, we describe the heat island phenomenon, its causes, its consequences, and proposed solutions to alleviate it. Appropriate urban planning and architecture are two key elements that can contribute to alleviate this phenomenon. The chapter will also devote a section to health, as exposure to extremely hot or cold temperatures is a well-recognized health threat. We will review what is known on the effects of temperatures on health, how the heat island effect can intensify the health effects, and how several studies have shown that measures to reduce the heat island phenomenon can be translated into health benefits.

2 Heat Island Definition

It has been known for some time that large urban areas can register hotter air temperatures than surrounding rural areas, a phenomenon that is known as the urban heat island, which is driven by the replacement of green or open natural areas with buildings, roads, and parking areas built with materials that absorb and store more heat. Such differences in temperature can reach values of more than 10 °C at nighttime, although in most cities, the daytime difference is between 2 and 4 °C (Table 23.1) (CalEPA 2017). The urban heat island represents one of the most significant human-induced changes to Earth’s surface climate (Zhao et al. 2014).

Table 23.1 Magnitude of urban heat island in different cities around the world. Data obtained from Tzavali et al. (2015) and Santamouris (2015)
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There are several reasons that contribute to the increased temperatures in cities, including a large area covered by heat-absorptive surfaces, the lack of vegetation and moisture, the generation of heat by human activities, and the city geometry, all of which can influence the city energy balance. However, there are many other factors that play a role, such as the size of the city, topography, elevation, latitude, location relative to the sea or vicinity to bodies of water, local climate, land use, presence of urban canyons, presence of industry, building density, and urban air pollution, which make the intensity of the heat island effect a very site-specific issue (Burkart et al. 2016; Heaviside et al. 2017; Koppe et al. 2004; Santamouris 2015; Tzavali et al. 2015). Even within a city, there might be variations in the heat island intensity, with the presence of micro-urban heat islands that are usually located in parts of the city covered by buildings and roadways and with a small presence of green space.

The heat island effect can be present throughout the year and at any time of the day, although it is usually more noticeable at night, and in days with clear skies and light winds, which prevent the dispersal of the warm air accumulated in the cities. The nighttime pattern is explained by the fact that several urban materials, such as concrete or pavement, absorb the sun energy they receive during the day and gradually release it during the night. The surrounding rural areas, without those materials with high heat capacity, cool down faster during the night. The heat island effect is particularly evident during heat waves, i.e., during episodes with several consecutive days of extreme heat. Thus, urban areas are usually hit harder by heat waves than rural areas. The heat island effect is also influenced by humidity, with lower intensity of the phenomenon in regions or days with higher relative humidity (Santamouris 2015).

3 Causes of Heat Islands

As mentioned, heat islands are created by a combination of reasons. In this section, we review some of the most important ones. The built environment of cities usually leads to a reduction of evaporative cooling (e.g., by the reduction of vegetation), which is one of the main factors involved in the heat island phenomenon. The artificial materials used (e.g., dark pavement or roofing) can also reduce the albedo (sun reflectance), thus storing more radiation energy during daytime that can then be released during the night. Street canyon geometry also has influences on the radiation balance and on the redistribution of energy through convection with the atmospheric boundary layer. Other human activities that generate heat, such as those involving engines or generators, including emissions from vehicles, can also contribute the warming of cities.

3.1 Urban Area Materials

The materials used to build urban areas absorb and reflect solar radiation, store heat, emit infrared radiation, and release sensible and latent heat through convection and evaporation processes, and they do that at different rates than rural areas. On hot and sunny days, urban surfaces can become up to 50 °C hotter than the air, while shaded or moist surfaces remain close to air temperatures. For example, dark and dry surfaces can reach temperatures of up to 88 °C when exposed to the sun (Tzavali et al. 2015).

3.2 Vegetation

Differences in vegetation can influence the difference in temperature between rural and urban areas (Burkart et al. 2016; Heaviside et al. 2017; Koppe et al. 2004). Vegetation absorbs solar radiation, so reflected radiation is small. Green spaces also provide cooling through evapotranspiration . Instead of increasing air temperatures, solar energy is used to evaporate water from plants, which cools the plant and reduces the amount of energy that is left to warm the air. In addition, vegetation has lower heat capacity and thermal conductivity than the materials used in buildings. Another important way in which vegetation can contribute to cooling cities is through shading, which reduces the incident shortwave radiation, leads to lower ground and wall surface temperatures, and reduces radiant temperature. Different trees have different shading capacity, which is one of the criteria that urban planners need to take into account when deciding to plant trees in an urban area. Finally, green spaces can also reduce wind speed within a city. This may be undesirable in terms of reducing temperature in the summer, as it leads to reduced cooling through wind. For example, in hot and humid areas, it is recommended to use trees with high canopies that allow for ventilation at ground level (Koppe et al. 2004).

3.3 City Form and Topography

City morphology influences the air temperature of cities in complex ways, by modifying, for example, the area of exposed external surfaces and surface reflectance (Koppe et al. 2004; Zhao et al. 2014). As an example, a cubical form can collect three times more radiation than unbuilt ground. Thus, things like the orientation, layout and width of streets, the height, shape, and location of buildings, and the resulting shading patterns can all influence the heat transfer of the urban area by modifying the magnitude of shortwave radiation reaching the street level and the longwave radiation escaping from the canyon (Santamouris 2015). Although it is complex to capture all these aspects in a single index, the Sky View Factor (SVF) , defined as the fraction of sky visible from the ground up, is a good marker of the heat island effect, as it is related to the long-wave heat radiation losses. Figure 23.1 shows two examples of SVF. Indeed, several studies have documented a negative correlation between SVF and the increase in temperature comparing built-up and open areas (Svensson 2004). It is important to note, though, that wind patterns can modify those associations.

Fig. 23.1
figure 1

Examples of Sky View Factors (SVF) obtained from fisheye photographs in two different locations. Figure adapted from Fig. 1 in: Landscape and Urban Planning; Vol 125; Sookuk Park, Stanton E. Tuller, Myunghee Jo; Application of Universal Thermal Climate Index (UTCI) for microclimatic analysis in urban thermal environments; pages 146–155; Copyright 2014; with permission from Elsevier

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Topography is also important in modulating the heat island effect. Having mountains around can prevent air circulation and heat dispersion and can prevent the circulation of the sea-land breeze. The San Francisco Bay Area is an example of ocean and mountains modulating the heat island effect (CalEPA 2017). The breeze from the ocean cools coastal cities, but the inland mountains trap warm air and displace it to other areas, thus warming areas that are upwind of the heat island. Dark-colored mountains can also absorb shortwave radiation and contribute to the warming of cities, as shown for Muscat (Oman) (Santamouris 2015). It is well-known that areas that are close to large bodies of water show less extreme temperatures, in part because of the land-sea breeze generated by temperature and pressure differences between the land and the sea. The heat island can prevent the inland movement of the sea breeze. Then, the cooling impact of the breeze can only be noticeable at the coastal front. For example, the waterside districts in Lisbon (Portugal) register temperatures that are up to 3–4 °C lower than in the city center (Burkart et al. 2016).

3.4 Size of the Urban Area

Several studies have reported that the larger and the more populated an urban area is, the more pronounced is the urban heat island intensity. In 1973, Oke reported that the maximum urban-rural difference of 2.5 °C for towns of 1000 inhabitants increases to 12 °C for cities of one million inhabitants (Oke 1973).

3.5 Wind

As mentioned, wind can influence heat islands, as in the case of sea breezes or wind systems in mountain valleys (Koppe et al. 2004; Santamouris 2015). Usually, faster winds imply a higher rate of heat dissipation by convective cooling . When the rural areas surrounding the cities are open country, wind speed is lower in the city than in the rural areas and so is the heat dissipation. However, street canyons can create complex airflow patterns that may alter this pattern, increasing wind speeds in the city. Winds can also contribute to warming areas downwind the city. A study in Birmingham (UK) found a warming of 1.2 °C up to 12 km downwind of the city, and this phenomenon can reach up to 40 km for a large city such as London (UK) (Bassett et al. 2016).

3.6 Heat-Generating Activities

Several human activities that generate heat can contribute to the heat island phenomenon, although the anthropogenic heat is much smaller than that received from the solar radiation in summer (Santamouris 2015; Tzavali et al. 2015). Those activities include transportation, industry, or energy consumption at homes. The average anthropogenic heat flux in urban areas has been suggested to be 100 W/m2, although they can reach 250 W/m2 in European and North American cities. In Tokyo (Japan), the anthropogenic heat flux exceeds 400 W/m2, with a maximum reaching 1500 W/m2 in winter (Santamouris 2015). The translation of those fluxes to air temperatures depends on many features of the city, including prevailing climatic conditions and geographical location. In Taipei, the anthropogenic sources were estimated to contribute 0.2 °C to air temperature, while in an industrial zone in Malaysia, they contributed 1.1 °C (Santamouris 2015).

3.7 Building Types

Different housing types have been found to have different risks of overheating, also in part due to their location within a city. For example, in a study in the West Midlands (UK), Macintyre et al. found that the type of housing that is more likely to overheat (flats in buildings) tends to be located in the warmest parts of the city (Macintyre et al. 2018). They registered ambient temperatures around buildings and terraced houses were 0.1 °C higher than the average for all housing types, while detached houses, which tended to be in the suburbs, had 0.2 °C lower temperatures. Interestingly, care homes and hospitals tended to be exposed to higher ambient temperatures than average.

3.8 Daytime Cool Islands

Apart from heat islands , cool islands, i.e., urban areas with cooler temperatures than surrounding rural areas, have also been documented (Santamouris 2015). This is mostly a daytime phenomenon that may occur in deep and dense urban canyons due to the extensive shading provided by the buildings, along with their high heat storage capacity. Thus, deep canyons can present a heat island effect during nighttime and a cool island effect during daytime. In Beijing, urban cool islands were observed during the morning period and were attributed to reduced solar radiation because of high concentrations of aerosols and to the possible advection of warmer air to rural areas.

4 Consequences of Heat Islands

Heat islands have several adverse effects (CalEPA 2017; Heaviside et al. 2017; Koppe et al. 2004; Kyriakodis and Santamouris 2017; Santamouris 2015; Tzavali et al. 2015). The increase in temperature can exacerbate the health consequences of heat waves, as detailed below, and reduce human comfort. In addition, the warmer temperatures favor the creation of ground-level ozone, which is associated with the exacerbation of several health conditions such as respiratory and cardiovascular problems. Air pollution and high temperatures may act synergistically during heat waves to produce stronger health effects. The warmth of urban sites can also favor the spread of vector-borne diseases. Apart from the health-related problems, high temperatures also accelerate the degradation of materials, e.g., road surfaces. In addition, one of the most studied consequences of heat islands is the increase in electricity consumption because of the increasing use of air conditioning.

The widespread use of air conditioning contributes to peak electricity loads. The increase in energy consumption leads to increases in greenhouse gas emissions. It has been calculated that the peak electricity demand increases from 0.45% to 4.6% for each 1 °C increase in maximum temperature during the summer (Kyriakodis and Santamouris 2017; Tzavali et al. 2015). A study based mainly on cities in the USA, Europe, and Australia estimated that the increase of the demand for cooling in buildings has increased by 23% in the last four decades (Tzavali et al. 2015). A study in Athens estimated that the urban heat island may double the cooling load and triple the peak electricity load of buildings designed for having low cooling and heating needs (Koppe et al. 2004). Projections estimate that future energy consumption for cooling in residences will increase up to 750% (Kyriakodis and Santamouris 2017). This is due not only to global warming but also to the increase of population in cities and the increased penetration of air conditioning systems in many parts of the world. An economic study in Los Angeles, USA, estimated that the costs of the heat island were about $150,000 per hour, resulting in $100 million for cooling (Koppe et al. 2004). The use of air conditioning also contributes to the increase of anthropogenic heat in cities. Although the use of air conditioning may be beneficial for health, relying on air conditioning to combat heat can widen inequalities, and it makes the population vulnerable in the event of power outages, which occur in some cities during hot summer days with large energy demands.

On the contrary, the heat island effect can have some benefits in winter. In particular, the study in Athens estimated that the heat island reduced the heating load of the buildings by 30% (Koppe et al. 2004). The beneficial effects of heat islands have received less attention in the literature.

5 Proposed Mitigation Actions

The heat island phenomenon can be moderated by appropriate urban and transport planning actions or by using certain technologies. The most popular measures include replacing urban materials by others that reflect more solar radiation (“cool” materials) and increasing the vegetation in the city. However, there are more potential actions, such as reducing anthropogenic heat. This section reviews some of the suggested actions. More detailed information can be found, for example, in the US Environmental Protection Agency report Reducing Urban Heat Islands: Compendium of Strategies (2008).

5.1 Increasing the Albedo of Cities

Increasing the albedo or reflectance of the city can be achieved in several ways. Simple solutions such as painting roofs or building walls or pavement with white colors can already achieve increases in reflectance that can be translated into lower air temperatures. This is an old solution applied for many years in many South European and North African towns characterized by white houses. The effectiveness of this measure is due to the fact that white surfaces directly reflect much of the sun radiation back to space, unlike dark materials, which absorb the sunlight and then reradiate it at much longer wavelenghts, which is then trapped by the greenhouse gases in the atmosphere.

There are several materials available for “cool roofs,” the term used for the use of highly reflective materials in roofs of buildings (see US Environmental Protection Agency (2008)). While traditional roofing materials in the USA have a reflectance of 5–15% (i.e., they absorb 85–95% of solar radiation), some cool roof materials can reach more than 65% reflectance. In the summer sun, black asphalt roofs can reach temperatures of 85 °C, metallic roofs can reach 165 °C, and cool roofs can stay below 46 °C. This can be translated into reductions in air temperature. A simulation conducted for New York City found that replacing 50% of roofs with cool roofs would achieve reductions of 0.2 °C in the average temperature of the city, with reductions of up to 0.8 °C in some parts of the city (US Environmental Protection Agency 2008). Increasing the albedo of a city would not directly affect the nighttime urban heat island, but it can have indirect effects by reducing the amount of heat that is absorbed during the day and, therefore, the amount of heat that is released at night (Zhao et al. 2014).

Cool roofs are not necessarily more expensive than traditional ones. The degree of success of implementing cool roofs will depend on several conditions, including local climate and the building design, but several studies have reported annual cooling savings ranging from 10% to 69% (US Environmental Protection Agency 2008). A detailed cost-benefit analysis is needed before implementing cool roofs, as, for example, having a cool roof may require consuming more energy for heating in winter. One study found that, in California, cool roofs had a net benefit of up to $0.66 per square foot. Implementing cool roofs may have other unintended consequences for a city. For example, the increased reflectance may favor ozone formation (Heaviside et al. 2017). Thus, appropriate planning of such policies is recommended.

Acting on pavements is another way to improve the reflectance of a city and reduce the heat island effect. For example, a study in Athens evaluated a large-scale intervention to introduce cool asphalt in a major traffic street (Kyriakodis and Santamouris 2017). They concluded that the intervention could reduce the surface temperature by 11.5 °C and the ambient temperature by up to 1.5 °C, although the efficacy of the asphalt will be reduced by aging. Many other studies have evaluated similar interventions and found reductions in temperatures of around 0.6 °C.

5.2 Increasing Vegetation

As explained above, vegetation can reduce the heat island effect through multiple pathways, especially through evapotranspiration, increased shading, and decreasing radiant temperature , which is a key parameter for thermal stress in humans (US Environmental Protection Agency 2008; Koppe et al. 2004). Increasing vegetation in an urban area needs to be carefully planned taking into account issues such as water demand and availability, maintenance costs, the different shading provided by different species, or their allergenic potential. Vegetation can have other benefits apart from reducing the heat island effect, such as reducing air pollution levels through dry deposition; reducing noise levels; carbon sequestration; reducing exposure to UV radiation; increasing physical activity levels of residents around green areas; and reducing the volume of storm water received and thus preventing sewer overflow.

Several cost-benefit analyses have shown a net benefit of planting trees. For example, studies in several cities in the USA have calculated benefits of $1.50–$3.00 per dollar invested, or of around between $15.50 and $85 per tree (US Environmental Protection Agency 2008; Koppe et al. 2004). Other studies in the USA have also documented that increasing the urban tree coverage by 25% can lead to savings of up to 40% in the cooling load of a city, although results depend, among other factors, on the city current spending on cooling (Koppe et al. 2004). Another study estimated that the peak temperature in summer in New York City could be reduced by 0.2 °C by adding trees to increase shade in 6.7% of the city area and estimated a reduction of 0.5 °C if 31% of the city was changed to have trees and green roofs (Mills and Kalkstein 2012).

Green roofs , i.e., having a layer of vegetation on top of roofs, and green walls, the same concept applied to vertical surfaces, are other proposed solutions to counteract the heat island phenomenon. The cooling potential of green roofs depends on several factors, including the local climate, the type of vegetation and soil, irrigation, and maintenance. Green roofs also provide insulating effects in winter (Heaviside et al. 2017). A study in Toronto, Canada, estimated that adding green roofs to 50% of the downtown surfaces would reduce ambient temperatures by between 0.1 and 0.8 °C, and those reductions could be increased to 2 °C by irrigating those green roofs (US Environmental Protection Agency 2008).

Installation of green roofs may have large initial costs, but full life-cycle analyses have shown that green roofs can result in net benefits in densely populated areas, especially through reducing the energy needs of the building . When incorporating the public benefits , those investments are even more favorable.

5.3 Other Strategies

Another strategy that could be considered is trying to reduce the anthropogenic heat of a city. However, a study estimated that such a strategy would result in almost no changes in the heat island effect (Zhao et al. 2014). Other strategies that focus on managing the convection efficiency of the city by changing its morphology could bring important benefits, but they are unfeasible as they would require, for example, citywide changes in building height.

6 Effects of Heat on Health

It is well-known that temperatures influence human health . In particular, it is well documented that episodes of extreme heat and cold result in increases in mortality and hospital admissions (Basu 2009; Basu and Samet 2002; Gasparrini et al. 2015). To cite a few, heat waves produced 5000–10,000 extra deaths in the USA in 1988, 70,000 additional deaths in Western Europe in 2003, 11,000 extra deaths in Russia in 2010, and 3500 additional deaths in India and Pakistan in 2015. However, even non-extreme episodes of heat and cold produce such increases in mortality. Actually, results from an international study using mortality data from 384 locations in 13 countries estimated that 0.42% of all deaths can be attributed to heat every year, even if no heat waves occur (Gasparrini et al. 2015). The relationship between mortality and temperature is U-shaped as displayed in Fig. 23.2 for the city of Barcelona (Spain). The same relationship, with different slopes for the effects of heat and cold, has been documented in many other parts of the world. The temperature of minimum mortality also changes by location, showing the adaptation of the population to its local climate (e.g., the effect of heat on mortality starts to be detected at hotter temperatures in hot cities in comparison to colder cities).

Fig. 23.2
figure 2

Relationship between temperature (°C) and number of daily deaths in Barcelona, Spain. The shaded region indicates 95% confidence intervals

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Humans need to maintain a constant body temperature of around 37 °C regardless of the outdoor temperature. To do so under hot ambient temperatures, the body activates several thermoregulatory mechanisms, such as increased sweating leading to loss of water and minerals, diverting blood from internal organs toward the skin to dissipate body heat, changing blood viscosity, or increasing the heart and respiratory rate (Koppe et al. 2004). These changes are well tolerated in healthy people, but they may put an additional stress to the body of vulnerable populations such as the elderly or those suffering from chronic diseases. Ultimately, a heat episode may trigger acute events such as heart attacks or renal failure in a percentage of that frail population, leading to the observed excess deaths during hot episodes. Other vulnerable populations include children, pregnant women, the obese, those with limited mobility and little social contact, those with psychiatric conditions, those taking medications that alter thermoregulation, and the socially disadvantaged (Basu and Samet 2002; Klein Rosenthal et al. 2014).

Apart from the increases in mortality and hospital admissions, other studies have shown that heat can trigger delivery and thus increase the number of preterm births (Zhang et al. 2017) and that heat increases the likelihood of accidents (including traffic crashes) and injuries to occur (Otte im Kampe et al. 2016).

7 Influence of the Heat Island on Health Effects

Heat islands can aggravate the health consequences of hot temperatures and heat waves. In situations of extreme heat, the increased temperatures of urban areas put an extra burden to the human body. In particular, it has been reported that high nighttime temperatures are especially sensitive for health, as they limit the ability of individuals to cool down. As explained above, the heat island effect is especially noticeable at night. Some studies also have shown that those living on the top floors of buildings are at increased risk of health events during heat waves (Klein Rosenthal et al. 2014; Laaidi et al. 2011). A number of papers have also documented that the hotter parts of the city show increased mortality rates during heat waves, indicating an extra effect of heat islands (Goggins et al. 2012; Heaviside et al. 2016, 2017; Smargiassi et al. 2009; Vandentorren et al. 2006).

A study in Montreal, Canada, found that heat increased mortality by 28% in the areas with highest surface temperatures, while this increase was only 13% in the areas with lowest surface temperatures (Smargiassi et al. 2009). Similarly, a study in Hong Kong found that the increase in mortality for every 1 °C increase in temperature over 29 °C in the areas with high urban heat island index was 4.1%, while it was only 0.7% in areas with low urban heat island index (Goggins et al. 2012). A study in the West Midlands (UK) estimated that 50% of the total heat-related mortality during the 2003 heat wave could be attributed to the heat island effect (Heaviside et al. 2016). In London (UK), however, a study suggested almost complete acclimatization of the heat island effect (Milojevic et al. 2016). A study in the USA estimated that the heat island phenomenon was associated with an increase of 1.1 deaths per million population (Lowe 2016). In terms of projections and economic costs, a study in Melbourne, Australia, estimated that the heat island is expected to add 2.2 days with temperatures over 35 °C per year and an additional heat wave every 10 years compared to the areas that are outside of the heat island. This was associated with an extra cost of $300 million (AECOM Australia Pty Ltd 2012).

As mentioned above, the heat island effect can bring some benefits in winter, although these have been less studied. However, a study in the USA estimated that the urban heat island could reduce cold-related mortality by four deaths per million, in comparison with an increase of 1.1 deaths per million due to heat-related mortality (Lowe 2016).

8 Influence of Green and Blue Spaces on Health Effects

Given the potential of green and blue spaces to reduce ambient temperatures, several studies have investigated the variation of the health effects of heat within a city by these characteristics. A study in Lisbon, Portugal, found that a 1 °C increase in temperature over the 99th percentile was associated with a 14.7% increase in mortality in the area with lowest greenness, while it was associated with a 3% increase in the greener areas. Likewise, in areas that were more than 4 km away from a water body, the increase in mortality was 7.1%, while it was only 2.1% in the areas close to blue spaces (Burkart et al. 2016). Thus, the study supported that both green and blue spaces are able to mitigate the effects of extreme heat on mortality. A study in Seoul, Korea, also found decreases in heat-related mortality from 4.1% to 2.2% as greenness of the area increased (Son et al. 2016). In Michigan (USA), the increase in cardiovascular mortality during extreme heat was 17% in the areas with low green space, while there was no increase in greener areas (Gronlund et al. 2015). A study that included the Medicare beneficiaries in the USA found that the effect of extreme temperature on mortality was 3% higher in areas with less green space (Zanobetti et al. 2013). On the contrary, a study in Australia, one in Philadelphia (USA), and one in Massachusetts (USA) did not find differences on heat-related mortality by vegetation (Madrigano et al. 2013; Uejio et al. 2011; Vaneckova et al. 2010).

Other studies have estimated what the health benefits of different vegetation interventions could be. Although these studies are extremely useful, it is often difficult to validate their estimated impacts. A study in Melbourne, Australia, estimated that reductions in heat-related mortality could go from 5% to 28% if the vegetation cover of the business district increased from 15% to 33% (Chen et al. 2014). Transforming the entire business district into a forested park could reduce heat-related mortality between 37% and 99%. A study in Arizona, USA, estimated a 48% reduction on heat-related emergency service calls associated with simultaneous improvement in emissivity, vegetation, thermal conductivity, and albedo (Silva et al. 2010). Finally, a study based on metropolitan areas in the USA estimated that changes in vegetation cover and surface albedo could reduce the projected increases in heat-related deaths due to global warming by 40–99% (Stone et al. 2014).

9 Conclusions

Heat islands have been documented in many large urban areas around the world. They can contribute 2–4 °C to the ambient temperature of cities during daytime and to over 10 °C in certain places and times, especially at night. Certain actions related to urban planning, such as using cool materials or increasing the vegetation of cities, can have an important influence in ameliorating the intensity of heat islands. These actions have been shown to produce benefits, especially in terms of energy savings and also in the reduction of the health effects of heat. Given the complexities associated with the multiple benefits and costs of such interventions, implementations to correct the effects of urban heat islands should be based on full life-cycle studies involving public planners, climatologists, experts in air pollution, botanists, and public health experts.