There’s a pretty fundamental question that those of us who live in metro Phoenix have to ask ourselves in order to live here: How much heat can the body handle?
We’ve all got our own answers to that question — but scientists are working hard to come up with a definitive answer. Rob Meade, a research fellow at the Harvard T.H. Chan School of Public Health, is one of those scientists.
Recently, Meade conducted what he calls a “heat chamber” experiment. The goal was to test the human body’s ability to cope with extreme heat by exposing test subjects to temperatures at which they could no longer cool themselves — or “thermoregulate.”
Full conversation
ROB MEADE: Yes, the thermoregulation just simply is how the body regulates it's temperature. So you can think of it basically at any given time, you know, the amount of heat being produced in the body needs to be balanced by how much heat the body is losing.
So, when we're exposed to hot conditions and we start to gain heat or we exercise and we start to generate heat within the body, the body essentially needs to use strategies — sweating being one of them to — dissipate heat so that we can stay, you know, within the range of temperatures that's conducive to normal bodily function.
SAM DINGMAN: Got it. And so the idea behind the heat chamber study was to see at what point the body's natural mitigating abilities were no longer sufficient.
MEADE: Yeah, exactly. The point at which we couldn't actually generate enough heat loss via sweating to counteract the amount of heat we're gaining from the environment.
DINGMAN: And what did this look like exactly? Did you — was there a literal heat chamber that you put people in?
MEADE: Yeah, so, quite a large heat chamber at the University of Ottawa. Essentially what we did is we brought a group of young adults in, you can think of it like we stuck them in the chamber. And then we slowly increased the humidity, to see which humidity at which, you know, core temperature would start to rise very rapidly.
But we made our project different was then we brought them back in for prolonged visits — so about nine hours where we exposed people to conditions just above or just below that threshold, we identified in the first part of the trial. So we could see, you know, above the estimated limit. Did their core temperature stream upwards? And then below the limit, was it able to stabilize?
DINGMAN: So, were you like visually observing the participants while this was happening? Can you tell us what you saw?
MEADE: Yes. So, what we saw on the participants is, you know, the cardinal signs that someone is heat stressed ... flushed red faces. Obviously, they were sweating quite profusely.
And then in that condition that I mentioned where conditions were hotter than each person's estimated threshold, what we saw was there'd be kind of a delay. You get in the heat and it takes a little bit of time for the heat to start entering your body. But once it started, core temperature would just kind of stream upwards. And then most — right, of the 12 participants — we actually had to pull out of the heat. At a core temperature of 39.2 [degrees C], which is about 103 degrees F. This was our kind of safety threshold for when to stop the trial.
DINGMAN: Gotcha. Can I ask, I mean, did you have the opportunity to talk to any of the participants about what this was like, you know, just anecdotally and individually for them? Because it sounds, I have to say, really intense. [LAUGHS]
What kinds of things did they say when they came out of the chamber?
MEADE: So, I should add that I was also participated in the trial myself. ... I have a kind of rule that if I'm gonna ask someone to sit in these kind of conditions, I should probably also know what they feel like.
DINGMAN: Oh, well then you can tell us from your experience then.
MEADE: Yeah, so everyone responds to heat a little bit differently. I would say, agitation is a pretty common across the board. ... Time starts to move very slow. But the best way to describe it is you just feel your brain is just screaming, "We need to get to a cool environment. We shouldn't be in this, in this heat."
DINGMAN: So, Rob, what were your findings based on the heat chamber study in terms of where that threshold in the human body is, after which thermoregulation is no longer effective?
MEADE: Yeah, so, you know, the actual limits for thermoregulation for prolonged heat tolerance in humans are probably quite a bit lower than the theoretical 95 °F — sorry, upper limit.
The important thing to consider though, when we're thinking about these limits is most of the research we've done on defining these new upper limits for thermal regulation has been conducted in relatively cool areas. So, for example, our study was in Ottawa, Ontario, Canada. Most of the research has actually come out of Penn State University. So quite a temperate environment and all of these studies use unacclimated participants.
So I think where the real open question is, is how well do these new limits actually translate to areas that are more regularly heat exposed? Places like Phoenix. But how much flexibility there is for acclimatization and regular heat exposure, I think is something we need to figure out pretty soon.
DINGMAN: Yeah, and just to be clear, the way that you were able to theorize that the limits were lower than previously thought is because the subjects of your study were starting to lose the ability to thermoregulate at these lower limits.
MEADE: Yeah, exactly. So, our limit in the particular combination of conditions we used, it was around 32 degrees C, about 89 degrees F. And exposing people just above this threshold, so not 35 [degrees], it was around, you know, maybe a degree higher. The core temperatures just streamed up and up and up, until we had to pull eight of 12 of them before they reached dangerous levels.
DINGMAN: Yeah. So, now that you have done this test, what do you think the implications are?
MEADE: For one, it does actually seem like ... the temperatures at which humans can be exposed to for prolonged periods are much lower than previously thought. Phoenix is actually a really important case because dry environments it seems like could be potentially even more dangerous. And Phoenix obviously is quite dry.
DINGMAN: And what is the reason for the higher level of danger in a drier environment?
MEADE: So, basically the two things that are going to affect how much sweat you can dissipate is, again, the humidity in the air, as well as, you know, how much sweat you can actually produce. ... So you can kind of think of it simply as what this assumes is that you're completely coating the skin with sweat. And in dry environments, what we're limited by is our ability to sweat. And it turns out that humans actually can't, I guess, for all intents and purposes, coat the entire skin with sweat. Some of it will drip off, and any drip sweat doesn't actually contribute to heat loss.
So, it's because the basically we're limited by how much sweat we can produce in dry environments, whereas in kind of warm but very humid environments, it's more of how much that sweat is evaporating.
DINGMAN: Got it. Did you look at all in this study at, if somebody is in a very hot environment, did shade play any role in the study or do you have any thoughts on it's ability to mitigate some of these effects?
MEADE: So in our specific study, we didn't consider shade or, you know, radiative heating type of heat gain we get from the sun.
So our limits — and most of the limits actually you see in this kind of studies like ours — are usually done not considering shade. Though shade is obviously very important. Because in most scenarios, individuals are exposed to sunlight, they are exposed to radiant heat sources, whether you know, you're outdoors or you're getting sunlight through a window. And, actually, sun can heat the body very quickly.
Basically, the way you can think about it is these are limits in the shade, and what we evaluated. But exposed to sunlight, these limits probably dropped considerably.
DINGMAN: When you look at the results of this study, what does it make you think about?
MEADE: You know, we're already experiencing very extreme impacts of heat. The planet, by all accounts, is still warming. And if there's a chance that we can, you know, be exposed in the summer for prolonged periods to these high temperatures, you know, that we know, we probably can't be exposed to for prolonged periods without adverse impacts. You know, this is something we really need to consider because if these temperatures are coming, you know, this is potentially a very large public health problem.
DINGMAN: Well, terrifying but important research, from our guest Rob Meade, who is a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health. Rob, thank you so much.
MEADE: Yeah, thanks for having me.
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