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In Hot Water: Health Implications of Extreme Heat

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According to Spotify Wrapped, I listened to Kelly Clarkson’s “Heat” a grand total of 297 times in 2021. For well over 15 total hours (not including time spent watching YouTube covers and live versions), I listened to the “best voice in the history of pop music” plead insistently to her lover that she needed “more heat” from them. It’s a catchy song, I swear, but you can be the judge of that (and me).

I proffer this somewhat embarrassing confession with good reason — to explain why I gravitated naturally to the subject of this article, extreme heat and climate change. After listening to that song so many times, seeing the word “heat” pop up in the New England Journal of Medicine (NEJM) automatically caught my attention, and here we are.

Climate change analysis has often focused on its manifold downstream effects – extreme weather events, biodiversity loss, declines in agricultural yields and even a rise in waterborne disease. I’d argue that this kind of analysis isn’t just valid, but necessary, because perhaps the most alarming truth about climate change is how wide-ranging its impacts are and will be.

But sometimes, we don’t need to look downstream. Climate change, fundamentally, is characterized by an increase in average global temperatures. Simply put, the world is getting hotter. So, take away all the fancy analysis, big words and Ph.D. dissertations, and just ask anyone how they feel walking outside during a typically sweltering August afternoon in Baltimore. They’ll tell you — heat is a problem in and of itself.

Bell et al.’s new review article in the NEJM explores this very topic, exploring the drivers of extreme heat, along with health risks and mitigation options. Perhaps the most important takeaway is the simple fact that heat is dangerous. This may seem like basic, self-evident truth, but it’s an absolutely critical one. Unlike a hurricane or forest fire, heat waves do not generate the same fear or shock value as other manifestations of climate change. But in reality, heat waves kill more Americans than any other type of extreme weather. In a recent horrific example, during the June 2021 heat wave in the Pacific Northwest, 486 people in British Columbia, 116 people in Oregon and 78 people in Washington died.

This lethality is tied to a simple principle — humans must thermoregulate, or maintain their body temperature within a narrow range around 37 degrees Celsius (98.6 degrees Fahrenheit). The stable temperature is essential, all the way down to the cellular level. Enzymes and immune cells, located in every part of the body, depend on normal human body temperature to function, and the human body has evolved many compensatory mechanisms to maintain this temperature. As the body is subjected to heat, blood vessels dilate and move more blood toward the skin in an attempt to dissipate heat from the body into the air. You might remember from physics that pressure is equal to force divided by area. This dilation of blood vessels increases the denominator of that equation by expanding the cross-sectional area that blood travels through, thereby diminishing blood pressure.

Sweating is another thermoregulatory tool in the body’s toolbox. The evaporation of sweat on the skin surface cools the body, as heat is dissipated to vaporize liquid sweat. Naturally though, sweating dehydrates the body, causing a drop in the total volume of blood in the body, which drops blood pressure further. This traps the body between a rock and a hard place. Low blood pressure is dangerous, and can decrease perfusion of the kidneys and other organs, causing damage to the kidneys and other organs. To avoid this, the body can compensate by forcing the heart to pump faster and harder to maintain cardiac output, which can exacerbate cardiovascular disease. Heat also worsens breathing difficulties, and can worsen cerebrovascular disease and diabetes-related conditions.

Many of these consequences revolve around the costs of maintaining body temperature despite external heat. But what happens when this thermoregulation fails altogether? That is the basic pathophysiology of heat stroke, an extremely dangerous, acute medical condition where core body temperature exceeds 40 degrees Celsius (104 F). At this temperature, the aforementioned enzymes and immune cells that function at human body temperature become dysregulated and precipitate a systemic (body-wide) inflammatory response. Dysfunction of platelets and electrolytes (minerals and cells in body fluid) further disrupts basic physiological processes. The effect is systemic and often deadly. Patients with heat stroke experience central nervous system dysfunction, and can incur serious damage to essentially every organ system in the body. This explains why the mortality rate for untreated classic heat stroke approaches 80%.

These physiological ramifications help explain the epidemiological evidence covered in Bell et al.'s review article, namely the documented increased risk of death from cardiovascular disease, respiratory conditions and kidney disease during periods of elevated temperature. The article cites a study that found a 7.8% increase in the relative risk of having to visit the emergency room for any cause during days with temperatures that met or exceeded the 95th percentile of local summer temperatures. This epidemiological evidence makes it clear that people don’t need to have a heat stroke to be seriously impacted by heat. The strain placed on the body and the need to regulate body temperatures is undeniable, and can be the domino that tips the body into cardiovascular, renal or respiratory disease states. Additionally, heat exposure has been linked to anxiety, depression, aggressive behavior and violence, adding another compounding risk that further increases population-level morbidity and mortality during periods of extreme heat.

So it’s clear that extreme heat is dangerous, and unfortunately, it’s only projected to become more common. Compared to the 1980s, Americans now experience twice the number of heat waves annually, and the Environmental Protection Agency expects that heat waves will continue to become more frequent and intense. The cause is simple — man-made climate change. Fossil fuel emissions trap heat in our atmosphere, which elevates average temperatures. Given that, you might check indicators like this one from NASA that tells us temperatures have risen on average by 2.11 degrees Fahrenheit globally, and figure that I’m just being a worrywart. You might tell me, Ethan, come on, 2 degrees is nothing. If my weather app told me it was 87 outside versus 85, it wouldn’t matter, I’m still wearing shorts and going out. But the operative word in all of this is average. An average rise of 2 degrees does not necessarily mean that every day is 2 degrees warmer, rather, it only means it averages out to 2 degrees. The Earth’s climate is extremely complex, and doesn’t function like your thermostat. Rises in global temperature are not distributed evenly, and will likely continue to precipitate extreme heat events of greater intensity, duration and frequency. In short — climate change elevates both average and extreme temperatures, which means that the future health risks of extreme heat will only continue to grow.

Given these health risks, mitigation is crucial. A key player in this area is an obvious one — air conditioning. Providing access to air-conditioned cooling centers during times of extreme heat directly helps shield individuals from exposure to heat and its consequences. Efforts like this 24/7 cooling center in Phoenix will become increasingly important as nighttime temperatures increase, too. However, as temperatures rise, the energy demand and general need for air conditioning also rises, meaning that power infrastructure becomes even more important. Unfortunately, our current electric grid doesn't appear to be up to the task. A Reuters investigation characterized the U.S. grid as “decrepit,” with aging components that contributed to double the number of power outages from 2015-2020 compared to the six years prior. In a heat wave, a power outage causing loss of air conditioning could be deadly. Investments in the grid will be necessary to respond to extreme heat.

Urban areas will also require additional infrastructure-level mitigations. Cities are vulnerable to the urban heat island effect — a number of physical factors including limited green space, heat absorption by building materials, heat released by human/economic activity in densely populated areas — which cause increased air temperatures in cities compared to rural areas. Efforts like installing reflective roofs on buildings, increasing vegetation and tree cover and installing mist sprayers throughout cities can all make a difference in mitigating heat in urban areas.

State and local governments also should collaborate with public health authorities to ensure that plans are in place to respond to heat waves. Currently, only 12 states have existing mitigation plans for extreme heat. Governments can monitor conditions, alert citizens about impending heat waves, mobilize public health resources during heat waves and conduct health checks of vulnerable populations. These actions can all be part of mitigation plans, and these plans should be prepared and ready nationwide. Further, public health authorities can educate citizens about heat — namely the risks of heat exposure and individual mitigations such as staying hydrated, wearing loose, light clothes and using cooling devices.

Additionally, any mitigation effort must grapple with the fact that exposure and vulnerability to extreme heat is not uniform. Special care must be placed on medically vulnerable populations who are more susceptible to heat-related illness. Further, in the status quo, access to air- conditioned buildings, green spaces and other cooler environments is not universal, and the divide falls largely along divisions of race and socioeconomic status. Compounding this is the fact that marginalized groups often have more frequent exposure to heat through their jobs (factory or outdoor work), are more likely to have chronic medical conditions that render them more vulnerable to heat, and have comparatively lesser access to medical care. Efforts must be made to target and intensify efforts in under-resourced regions and populations that bear the brunt of heat exposure.

The final and perhaps most important mitigation is to address climate change itself. A rapid transition away from fossil fuels is of utmost importance in confronting the issue of extreme heat, and the reality is that we are nowhere close to target. Air conditioning, public health campaigns and everything I’ve discussed so far only treat symptoms, not the root cause. Further, they are not perfect, and will fail if temperatures become even more extreme as a result of runaway climate change. For example, standard air conditioners are generally only able to cool spaces by 20 degrees Fahrenheit, meaning that heat exposure risk may persist, even in air-conditioned spaces, if outdoor temperatures are extreme enough. Humans and human society possess so much power and technological advancement, but the simple truth is that we are organisms that require a narrow range of habitable conditions to survive. Our cleverness can only take us so far in the face of unforgiving physical realities like heat.

Writing this article inspired me to listen to Kelly Clarkson’s “Heat” again. Like a favorite, well-worn pair of shoes, I comfortably slipped back into the song’s punchy beat, effusive chorus and the unabashed power of Clarkson’s vocals. With three years between now and when I was obsessed with this song, I can admit that it may not be truly great, nor her best work. But as I played the song on loop, one line stuck with me, a line that I also believe applies to the current state of extreme heat mitigation in the U.S. After reminding her lover that she loves them, she closes the song with a simple observation:

“We can do much better than that.”

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