The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change

How Heat Hurts

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I got heat exhaustion today. Unfortunately, this is not an unusual occurrence for me–I seem to be unusually susceptible. I don’t know why. Heat exhaustion is one of several types of heat-related illnesses. It is not, in itself, normally dangerous, but can progress to heat stroke, which can kill you.

Heat is a matter of weather—but it is also a matter of climate. Obviously, global warming means more hot days, but the increase doesn’t work quite the way intuition says it should. Intuitively, an average warming of, say, one degree Fahrenheit, should add one degree on to typical daily temps. So if your normal summer day was 90 degrees, now it’s 91 degrees. Not a big difference. But that’s not how it works.

As I have addressed before, a small increase in average temperature results in a large increase in the frequency of heat waves. This is because there is a well-established link between rarity and severity across many different types of variation, from body height to intelligence to air temperature. A slight increase above average (a few degrees, a few inches….) translates into a dramatic decrease in frequency of occurrence. How often do you see people who are six feet tall? Now often do you see people who are seven feet tall? The difference seems larger than what a mere twelve inches would imply.

A hotter normal means that severe heat waves that used to be very rare become common-place, while the human vulnerability to heat injury remains roughly the same.

My illness today is not particularly a climate change story–it was not one of those events that make it obvious normal has changed. Hot days in mid-June are not new for Maryland. But the experience did inspire me to do some reading about how heat stroke actually works—more people die from heat than from all other natural disasters combined. I decided I wanted to know more about what happens in the body when it gets hot.

Please note that I’m skipping over issues like how to recognize and treat heat-related problems. For that information, look up a public health website maintained by a reputable medical institution (anything else, and you’ll likely be reading content written by free-lance writers who don’t know anything more about medicine than you do).


When I received emergency medical training years ago, I learned that heat exhaustion is essentially a form of aggravated dehydration—the body is not hotter than it should be, but keeping cool is taking too much effort, including loss of so much water through sweat that blood volume drops. The symptoms are mostly the body’s attempt to compensate for lost blood volume in order to keep adequate blood flow to the brain. Heat injury and then heat stroke, in contrast, result when the body’s cooling system fails (sometimes because dehydration has become critical and the sweat response shuts off—when a person who should be sweating isn’t, that’s a very bad sign) and body temperature rises uncontrollably. Doctors then have hours or even minutes to act before the patient literally cooks to death.

Like most simple explanations, this one is not quite right. For example, brain damage in heat stroke is not caused by the brain tissue heating up, as I’d been led to believe–instead, excessive heat causes the blood/brain barrier to become leaky, allowing substances into the brain that should not be there, and that causes damage. Heat stroke, though triggered by heat (either through passive exposure to high temperature or to excessive exercise in hot weather or under too much clothing), actual injury—and often death—is not the direct result of the body cooking. After all, cooking occurs at specific temperatures (that’s why recipes work), but the temperature at which heat injury occurs is variable. There are documented cases of people surviving core temperatures above 107 degrees Fahrenheit, but there are also many cases of people dying at much lower temperatures. The body is a complex system. Heat-related injury and death are the result of complex responses to heat, not the heat itself.

The information in this post, except where noted, is taken from a document produced–or at least presented–by the US Military (service members are at high risk for heat stroke, therefore the military is interested in the issue). The “report date” of the PDF is listed as 2012, although since it is evidently a chapter in a longer book, I don’t know if the report date is earlier or later than the copyright date of the book. I don’t know how old this information is. It’s a dense read, but I’ve attempted to summarize the main points below.

How Heat Stroke Works

Not everyone is equally vulnerable to heat stroke. There are long lists of circumstances that create higher risk, so many that it might seem everybody must belong to at lest one of them—but it’s important to note that some risk factors are a matter of choice (running marathons on hot days) and some are not (being very young, very old, already ill, or poor). There are obvious social justice issues here, as I’ve discussed before.

Interestingly, several risk factors do not involve simple vulnerability to heat (as in our marathon runner, or a home-bound elderly person without an air conditioner) but rather impairments of the body’s ability to respond. A sunburn or a heat rash can impair the body’s ability to cool itself, for example. Illness or inflammation (e.g., pneumonia) makes heat stroke more likely. Heart problems, certain medications, or low potassium or sodium levels also either make heat stroke more likely or more dangerous. These facts alone should suggest the medical complexity of the problem.

Heat stroke is also a much more drawn out process than the idea of cooking would imply. Literal cooking ceases as soon as the object being cooked cools, but heat stroke isn’t over when the victim’s core temperature is brought back to normal. If he or she lives long enough, the bodily changes initiated by the heat will continue to play out. The patient will probably run a fever (which actually helps the body heal), and may also go through periods of abnormally low body temperature.  Kidney failure will probably occur between two and 24 hours after the initial collapse. The liver will likely fail after 24 to 48 hours. Mortality rates often rise about a month after mass heat stroke events (like heat waves), after patients have been discharged. The risk of dying from cardiovascular, kidney, or liver disease can remain elevated for 30 years. There may be long-term cognitive impairment. And and since many illnesses or deaths are either never recognized as related to a patient’s heat-stroke history, or never reported as such, the true prevalence of these problems is likely much higher than the data we have indicate. There has been little research done on how these long-term problems happen, and no one really knows what to do about it yet.

The bottom line is that the number of people who die of a heat wave is much higher than the number of people who die in a heat wave.

Heat stroke is actually several processes, although the whole story is not yet clear even to scientists.

The dominant process may actually be an immune response called Systemic Inflammatory Response Syndrome (SIRS). This is the same–or at least very similar–to what happens when an infection enters the bloodstream, a condition called sepsis or, less technically, “blood poisoning.” Its symptoms include fever and a whole series of both helpful and non-so-helpful biochemical changes.

Heat-induced SIRS is actually not caused directly by heat. Instead, when the body redirects more blood flow to the skin (heat stroke victims are typically bright red), the internal organs necessarily get less. Insufficient blood flow can damage the gut lining, causing it to leak endotoxin into the blood. The endotoxin, in turn, triggers SIRS–if severe enough, the endotoxin or SIRS (I’m actually not clear which–it looks as though scientists might not be sure, either), destroys the major vital organs, causing death.

Injection of endotoxin alone (into animals) triggers the clinical symptoms of heat stroke.

Another important process is DIC, which stands for Disseminated Introvascular Coagulation. Essentially, the blood starts clumping up, leaving the blood remaining in circulation way too thin. DIC can be caused either by tissue damage (sepsis is listed as a common cause, suggesting that DIC can be caused by SIRS–the immune response I just described–although that is not clear to me from the article) or by direct heat injury to the vascular system.  Besides the real risk of bleeding to death, DIC also causes, or helps cause other problems associated with heat stroke.

DIC can cause kidney failure, for example. But kidney failure can the proteins released by muscles damaged by SIRS, or by heat toxicity itself.  It can be difficult to tell which problems are causes and which are results.

Heat stress is one of several possible triggers for the release of cytokines, a class of messenger proteins that in some circumstances are a necessary part of healing–but experimental injection of these proteins triggers heat stroke symptoms including excessive body heat. In other words, the body doesn’t just get sick because it gets too hot—it also gets hot because it’s sick. Exactly what role cytokines play in actual heat stroke isn’t known, yet, but cytokines are involved in many of the processes and subprocesses of heat stroke.

There are several possible treatments for heat stroke being developed based on this more detailed understanding of the malady, but so far, heat stroke is much easier to prevent than to treat. Prevention consists not just of staying cool, but also in becoming adequately acclimatized–general good health and fitness, plus a recent history of being uncomfortably but not dangerously hot fairly often dramatically increase the body’s ability to safely withstand heat. In other words, HAVING a working air conditioner can save your life, but using it often (hiding from summer heat) puts you more at risk for those times when you do have to get by without it–if, for example, there is a power outage during a heat wave.


All of this might sound like unrepentant geeking out on my part. I am, in fact, an unrepentant geek, but my primary motivation for this post is, as I said, to take a close look at a malady likely to become ever more familiar, both to us individually and as a matter of public health policy.

One study that looked at the UK has predicted that, as a result of global warming, the incidence of death from heat stroke in that country will double by 2050. That’s only just over thirty years away.





Author: Caroline Ailanthus

I am a creative science writer. That is, most of my writing is creative rather than technical, but my topic is usually science. I enjoy explaining things and exploring ideas. I have one published novel and another on the way. I have a master's degree in Conservation Biology and I work full-time as a writer.

One thought on “How Heat Hurts

  1. Pingback: Putting Heat in Context | The Climate in Emergency

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