Health and Heat Waves: Harbingers of What's to Come with Anthropogenic Climate Change Cady Rancourt '24

Health and Heat Waves: Harbingers of What's to Come with Anthropogenic Climate Change

BY CADY RANCOURT '24

Cover Image: A car dash displays an outside temperature of 106°F. Image Source: Flickr; Creator: Geremy F Introduction

The hottest days of summer bring feelings of exhaustion, discomfort, and longing for cooler days ahead. People flock to beaches and airconditioned buildings seeking respite from the oppressive heat, passing unfavorably updated Smokey the Bear fire danger signs. Even worse, though, is when the hottest days of summer occur consecutively and at abnormally high temperatures as a heat wave. Historically, heat waves have proven powerfully damaging, resulting in thousands of deaths, amplifying drought, and catalyzing wildfires (Union of Concerned Scientists, 2018a). The infamous European heat wave of 2003 took the lives of over 70,000 people and served as a “turning point” for how nations in Europe and beyond perceive, prepare for, and respond to heat waves (Di Napoli et al., 2019). While the 2003 heat wave was extraordinary and triggered a muchneeded response, it also garnered attention as a once-in-a-lifetime event. This mindset, however, is dangerous and unsustainable. Heat waves are becoming more frequent, while simultaneously climbing to increasingly higher temperatures and growing in duration (Rastogi et al., 2020). They are anticipated to continue along that trend, which has been attributed to anthropogenic (human-caused) climate change (Rastogi et al., 2020). Because heat waves are only projected to worsen, the seemingly unbearable heat waves we currently endure are a preview of the kind of temperatures that may one day seem less extreme or even preferable. If—among other lasting and harmful impacts—the heat waves of the past and present have already garnered a dreadful death toll, what is to be expected for heat-related human health outcomes in the coming decades?

The Struggle to Define Heat Waves

The general concept of a heat wave is not difficult to understand and—through lived experiences—is already grasped by many. Even so, problems emerge when researchers seek to study them. Varying definitions of heat waves create incongruence and prevent “comparison and synthesis of results” across literature (Anderson & Bell, 2011). Conceptually, a heat wave is a meteorological event lasting a few

consecutive days characterized by exceptionally high temperatures. But “a few consecutive days” and “exceptionally high temperatures” are phrases far too ambiguous for study, leaving much to question: how many days are “a few,” and what temperatures classify as “exceptionally high”? How many standard deviations away from normal does “exceptionally” demand? One could endlessly descend into a rabbit hole of questions and understand just how daunting a task researchers face.

What is clear, though, is just how malleable to locations a heat wave definition must be. Different parts of the world naturally have vastly different 1) temperature and deviation thresholds that define extreme heat and 2) levels of preparedness to withstand extreme heat. For example, a 4-day, 5-degree increase from normal in Juneau, Alaska might be substantially threatening to citizens if natural variability is low due to coastal proximity, which narrows the range of temperatures experienced (Lutgens & Tarbuck, 2016). Conversely, it might be only a slight inconvenience in inland Omaha, Nebraska, where people may be used to a wider range of temperatures and are likely more prepared to handle various weather conditions, whether that preparedness comes in the form of adequate healthcare, access to air-conditioning, knowledge of handling heat, or otherwise (Union of Concerned Scientists, 2018a).

There have been multiple approaches to defining heat waves for study, each with its own drawbacks. Some researchers, including Kent and colleagues (2014), argued a “mean-temperatureonly” definition as the most suitable for Alabama, where a heat wave was defined as 2 or more consecutive days where the mean temperature was at or above the 98th percentile of non-heatwave control days. Several definitions claim that high daily minimum temperature is the primary indicator of a heat wave, while others say daily maximum temperature is what matters. Some deem the 90th percentile as useful, or the 95th, or 85th, and so on and so forth (Kent et al., 2014). component of a heat wave into account: temperature. Of course, high temperatures constitute the essence of a heat wave, but other factors play a role in how people actually experience heat waves. Wind speeds, solar radiation, and humidity can exacerbate the extent of effects on human health (Di Napoli et al., 2019). Some definitions use heat index or apparent temperature (see Figure 1) (Anderson & Bell, 2011), which comes a step closer to fully representing heat waves by incorporating humidity but still falls short. Because researchers studying heat waves focus on human health impacts, there is a dire need to utilize a comprehensive, health-centered definition (Di Napoli et al., 2019) that takes all those nontemperature factors into account, in addition to analyzing how preparedness of a community (e.g., access to healthcare, spaces equipped with A/C, knowledge of and experience with extreme heat) alters heat-related health outcomes.

One of the most promising health-centered approaches to defining heat waves involves the universal thermal climate index (UTCI). The UTCI is a “bioclimate index elaborated via an advanced model of human thermoregulation that estimates the thermal stress induced by air temperature, wind speed, moisture, and radiation on the human physiology” (Di Napoli et al., 2019). Di Napoli and colleagues (2019) examined 30 years of mortality data from five French cities in conjunction with meteorological data to examine the correlation between excess deaths (the number of deaths over what is considered typical) and UTCI values (Di Napoli et al., 2019). The group discovered that stretches of three or more consecutive days at or above the 95th percentile of UTCI values were most strongly associated with increased “all-cause” deaths (Di Napoli et al., 2019), which covers any type of death, including directly heat-related deaths and deaths from pre-existing conditions that heat worsens (McGregor et al. 2015). Essentially, the group was able to calculate daily UTCI values using the cities’ meteorological data, then crossexamine the daily UTCI values with the cities’ daily excess death values. From there, they were able to find that the best definition for a heat wave in those cities was a stretch of three or more days at or above the 95th percentile of UTCI values. This definition is considered “best” because it defines a heat wave such that under those conditions, people have the most cause for health-related concern, since those conditions are associated with the strongest health effects.

Calculating UTCI values from local weather data and comparing with local health data allows

Figure 1: This table displays the National Weather Service’s heat index, or apparent temperature, by combining temperature and relative humidity to determine a “feels like” temperature and corresponding “likelihood of heat disorders with prolonged exposure or strenuous activity.” Image Source: NOAA

"The group discovered that stretches of three or more consecutive days at or above the 95th percentile of UTCI values were most strongly associated with increased 'all-cause' deaths"

Figure 2: Emergency room visits increase with heat waves, with patients presenting a variety of conditions heat-related and otherwise (McElroy et al., 2020). Image Source: Flickr; Creator: Rob Nguyen

"As expected, heatrelated mortality and morbidity, manifesting in ambulance calls, emergency department visits, and hospitalizations, increase significantly during heat waves"

researchers to find a health-centered definition of a heat wave specific to a community. Under this method, heat wave definitions are standardized through the use of a consistent metric (the UTCI) while remaining loyal to a location’s individualities. Additionally, this method indirectly encapsulates the preparedness of a community to handle extreme heat, since the health data being used is a reflection of the community’s preparedness (because preparedness in any form affects health data, e.g., an area with adequate healthcare will likely result in less negative health outcomes).

The one major drawback of the heat wave definition proposed by Di Napoli and colleagues (2019) is that it only considers mortality, while morbidity—the suffering of any medical condition without resulting in death— hospitalizations also increase during heat waves (Hayhoe et al., 2010). Integrating morbidity into heat wave research remains far more complicated than mortality because of morbidity’s fluctuating definitions between nations and complex nature (Di Napoli et al., 2019). Thus, it is hard to find datasets of morbidity that could be used to find correlations between UTCI values and morbidity in the way that was done with mortality in the five French cities. While deaths are understandably of primary concern, the UTCI used by Di Napoli et al. (2019) still underestimates the overall impact of heat waves on human health by neglecting to account for morbidity.

Heat Waves and Health Impacts

Despite the inconsistency among heat wave definitions, many studies have characterized the health impacts (known as morbidity and mortality) of extreme and sustained heat. In the past, heat waves have put healthcare facilities under pressure due to surging emergency department visits and hospitalizations, with patients facing a variety of ailments (McElroy et al., 2020). In analyzing a 2006 heat wave, researchers found “16,166 excess ED visits and 1,182 excess hospitalizations” across California, a 3.3% increase from normal (Knowlton et al., 2009). A 2011 heat wave in Sydney, Australia produced a 2% increase in all-cause emergency department visits and a 14% increase in all-cause ambulance calls (Shaffer et al., 2012). Tong and colleagues (2010) examined emergency hospital admissions across ten different heat wave definitions and found significant increase in each. While the extent of increase varies across regions due to varying community preparedness, many studies have consistently shown an increase in emergency department visits, ambulance calls, and hospitalizations during heat waves across several heat wave definitions (Li et al., 2015).

Under one umbrella of common heat-related medical conditions—called heat stress—are heat cramps, heat exhaustion, and heat stroke (Union of Concerned Scientists, 2018a). In that order, they go from least to most medically concerning. Heat cramps bring on “muscle pains and spasms,” heat exhaustion causes “dizziness, a weak pulse, nausea, and fainting,” and heat stroke—the most fatal—produces similar symptoms to heat exhaustion and cramps as well as a quick heartbeat and fever over 103°F (Union of Concerned Scientists, 2018a). As expected, heatrelated mortality and morbidity, manifesting in ambulance calls, emergency department visits, and hospitalizations, increase significantly during heat waves (Li et al., 2015).

Aside from heat stress, pre-existing conditions can be intensified with heat. Studies have shown that both morbidity and mortality increase in patients with respiratory and cardiovascular conditions (McGregor et al., 2015). Heart attacks also spike during heat waves (Union of Concerned Scientists, 2018a). In an Alabama study that sought to compare health outcomes across differing heat wave definitions, pregnancies resulting in preterm births rose by as much as 35% under one definition and had positive associations with heat in 9 out of 15 definitions. (Kent et al., 2014). Renal disease and mental illness hospitalizations have also shown increases during heat waves in some studies, although more research is needed to establish a strong association (Li et al., 2015).

There is plenty of evidence to suggest that particularly humid heat waves are the most disastrous to human health (Rastogi et al., 2020). High humidity prevents the sweat on the surface of the skin from evaporating, a process required to cool the body (Union of Concerned Scientists, 2018a). This process is also why it’s often easier to sit for longer periods of time in a 185°F dry sauna compared to a 185°F wet sauna, where

steam fills the air and prevents the body from cooling. If the body cannot cool and regulate temperature properly, negative health effects are bound to occur, whether in a sauna or a heatwave-riddled atmosphere. However, humid heat waves have not always been more harmful to humans when compared to dry heat waves (Montero et al., 2012). According to Montero and colleagues (2012), the highest mortalities in Spain are actually correlated with lower humidity, and the correlation between humidity and mortality varies from city to city in the United States.

Health effects vary considerably among different populations. Elderly people, particularly those aged 75 or higher, remain most vulnerable across most heat-related health impacts (Li et al., 2015). However, in some heat waves and some specific illnesses, children and youth have been most affected (Li et al., 2015). Most heat-related health impacts are experienced more frequently by men of all ages than women, whereas women only outrank men in a few, such as renal disease (Li et al., 2015). Those in predominantly outdoor careers are at high-risk, especially Latino men working for smaller companies less equipped to adequately care for employees during heat waves (e.g., sufficient time off, proper protective gear, informing employees of heat-related risks and how to mitigate them, etc.) (Union of Concerned Scientists, 2018a). Urban residents endure more intense heat wave effects compared to rural residents due to the urban heat island effect, which describes the increased and sustained heat observed in cities on account of their heat-trapping materials like asphalt (Union of Concerned Scientists, 2018a). People with preexisting conditions are more susceptible to heat exacerbating their illnesses, and low-income individuals are also disproportionately affected due to lack of access to both healthcare and airconditioning (Union of Concerned Scientists, 2018a). Understanding the various heat-related health impacts—morbidity and mortality—and how different demographics experience them is key to understanding the future of human health during future heat waves.

Forecasting the Future of Heat Waves

Heat waves are already starting to get longer, hotter, and more frequent; they are only expected to continue along that path (Rastogi et al., 2020).

With that in mind, we can reasonably expect that

Figure 3: NOAA graphs display how heat waves have changed in frequency, duration, season length, and intensity since the 1960s.

Image Source: NOAA

"Health effects vary considerably among different populations. Elderly people, particularly those aged 75 or higher, remain most vulnerable across most heat-related health impacts"

Figure 4: Protesters raise signs to bring awareness to and demand action regarding anthropogenic climate change. People in cities—like protesters in London, pictured here—are most affected by high temperatures due to the urban heat island effect (Union of Concerned Scientists, 2018a). Image Source: Flickr; Creator: Valerio Donfrancesco

the health effects of heat waves we observe in the past will persist, but with increasing ferocity as years go on. Likely there will be more deaths, more pre-term births, and more hospitalizations of all kinds. Keeping those most vulnerable to heat wave impacts is also a necessary and interdisciplinary strategy for preventing negative heat-related health outcomes. More people are expected to move from rural areas to cities (Li et al., 2013), which are already more impacted by heat waves due to the urban heat island effect (Union of Concerned Scientists, 2018a) and thus we can anticipate higher rates overall of negative heat-related health outcomes during heat waves. Other hard-hit demographics, like outdoor workers and those in low-income communities will likely continue to be the most affected.

Ultimately, there remains a need for highly localized yet standardized definition methods in research on heat-induced mortality and especially morbidity (Li et al., 2015) to better predict just how severe the coming decades will be on human health. Finding ways to standardize morbidity data would allow for morbidity’s integration into heat wave definitions. Furthermore, heathealth watch warning systems have the potential to significantly reduce heat-related health complications during future heat waves (McElroy et al., 2020). Current systems have proven useful, but much improvement can be made with the use of a better health-oriented heat wave definition (McElroy et al., 2020) like the UTCI. The UTCI “proved successful in detecting hazardous heat stress levels up to 10 days in advance” and, if more widely used in the future, has great potential to help define heat waves and improve warning systems and therefore lessen the worst effects of human-caused climate change. (Di Napoli et al., 2019). Lastly, while definitions and warning systems are necessary tools for understanding and handling heat from anthropogenic climate change, the best tool humans could utilize going forward is slowing the process that started it all and reducing emissions, a strategy that seems— with enough effort and optimism—just as possible as defining heat waves.

References

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