How Does the Human Body Cope with Heat? According to the Associated Press, 27 people died from the high temperatures that gripped New York City last week. How does heat affect the human body? And what can scientists discover about extreme heat by studying soldiers?
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How Does the Human Body Cope with Heat?

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How Does the Human Body Cope with Heat?

How Does the Human Body Cope with Heat?

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JOE PALCA, host:

For the rest of this hour, a look at heat. Those of us in the East are glad to finally have escaped the blistering heat wave we've experienced a few weeks back, but folks in the central part of the country continue to suffer. Temperatures are expected to reach 103 in Dallas, and not much cooler in Oklahoma and parts of the central plains.

If past heat waves are any measure, this current one will be deadly. According to the Associated Press, at least 27 people died from the high temperatures that gripped New York last week.

Joining me now to talk more about what happens to our bodies in the heat our my guests, Michael Sawka. He's the chief of the Thermal and Mountain Medicine Division at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts. He joins us by phone. Thanks for being on the program today, Dr. Sawka.

Mr. MICHAEL SAWKA (U.S. Army Research Institute of Environmental Medicine): Good afternoon.

PALCA: And also Larry Sonna is assistant professor in the Division of Pulmonary and Critical Care Medicine at the University of Maryland in Baltimore, and he's sitting right across from me here in Studio 3A. Welcome to Science Friday.

Professor LARRY SONNA (University of Maryland, Baltimore): Good to see you.

PALCA: So let's start with you, Michael Sawka. What happens? What does the body do when it gets hot? How do we respond to that?

Mr. SAWKA: Well, the body has two avenues, two ways to lose body heat to the environment. The first is putting blood to the skin, and if the environmental temperature is low enough, basically you can radiate heat from the skin to the environment to dissipate heat. But as the temperature of the environment gets higher than the body temperature, you can't use that way of losing heat anymore, so you secrete sweat to the skin and the sweat then evaporates and that causes evaporative cooling to the blood in the skin and therefore allows you to lower body temperature.

PALCA: So how hot can people tolerate? How much heat can they tolerate?

Mr. SAWKA: Well, it depends on the person. A highly conditioned athletic who's acclimatized to the heat can get body temperatures well over 40 degrees - over 41 degrees often reported - and they'll have adverse consequences. Whereas perhaps a frail older individual may have body temperatures not much over 40 degrees, and may have problems.

But it's not just a function of high body temperature, it's also the cardiovascular strain placed on the person. As I had said, putting blood to the skin requires additional blood flow and stress on the heart, and also as you secrete sweat and that evaporates, that can also cause dehydration, causing more cardiovascular strain.

PALCA: We're talking about how human bodies respond to the heat, and we're taking your calls at 800-989-8255, that's 800-989-TALK.

And Larry Sonna, maybe I can turn to you, you're interested in not so much these, you know, whole body physiological responses to heat, but more on the molecular level. What happens to cells when we get exposed to the heat?

Prof. SONNA: Well, as it turns out, cells throughout the evolutionary tree have mechanisms to compensate for exposure to high temperatures. A variety of biochemical processes take place, one of which includes the expression of certain types of proteins known as heat shock proteins. These heat shock proteins have many functions, one of the important ones of which is to serve as sort of the 911 response of yourself.

PALCA: Mmm-hmm.

Prof. SONNA: One of the things that they do that seems to be important is they help proteins that have been unfolded by the heat refold back into a state that's normal. That's of course one of the best understood processes, but there are many others as well that are still under active investigation.

PALCA: And - well, first I should say we're talking with Larry Sonna from the University of Maryland, and Michael Sawka from the U.S. Army Research Institute of Environmental Medicine about how the body responds to the heat.

I'm Joe PALCA, and this is TALK OF THE NATION from NPR News.

PALCA: So Larry Sonna, you and Michael Sawka have collaborated on some projects. Why is someone like Michael Sawka who studies the whole body response to heat, what does your research have that would help him?

Prof. SONNA: Well, it turns out that we respond to environmental stress in general, and heat stress in particular at all levels. At the behavioral level people will move to cool themselves down.

Dr. Sawka has just described what happens at the level of systems physiology: you divert blood flow to the skin, you help lose heat. And as you move down to tissues and even cells, these responses at the lower level help protect cells from subsequent stresses, and respond to the stress that they're under.

And so in fact, as we've looked at different levels within the organism, it seems to be part of an integrated whole, starting at the level of molecules and ranging all the way up to the level of behavior.

PALCA: Hmm. And Michael Sawka, I just want to come back to your for a second because I heard a story earlier this week that there was a 17th century physiologist who went into a chamber that was heated over 200 degrees and was able to survive. Is that true, as far as you know?

Mr. SAWKA: Yes, it's possible. Depends on how long you're in there.

PALCA: How long do you think someone could stand that?

Mr. SAWKA: Two hundred degrees? Probably not very long. But

PALCA: Probably a hard experiment to do, I should think today, with human protection rules and such.

Mr. SAWKA: But just to expand a little bit farther from what Larry had said...

PALCA: Mmm-Hmm.

Mr. SAWKA: ...that as we understand how the body responds psychologically, the key is really tying in to the molecular adaptations, in that, as we look at heat injury, we had always looked at it very simplistically just due to an overexposure, but now we're starting to learn that, one, in terms of protection of the mechanism of the body, it's not only your ability to dissipate heat, but the ability for cells to adapt - and we don't really fully understand - so that they can stand higher levels of body temperature and greater reductions in blood flow to them.

On the other hand, we're starting to understand that some people under certain conditions are becoming more susceptible to injury, and that it may not be in heat loss responses, but on particular days or particular events, whether it's due to sickness or in a young and healthy - such as athletes - from events preceding, that maybe, may or may not affect the heat loss responses which we've traditionally looked at in science, but may actually deactivate a lot of those molecule systems, or may do both together.

So on some instances, people may not only store more heat, or may not, but they also, at the same time, may be unlocking these protective mechanisms, making a given person on a given day more susceptible to very serious heat injury.

PALCA: All right. Let's take a call now from Layla(ph) in Iowa City. Layla, welcome to SCIENCE FRIDAY.

LAYLA (Caller): Well, thanks for taking my call.

PALCA: Sure.

LAYLA: My question is, what mechanisms does the brain have that keeps it cooler than the rest of your body? Because your body can tolerate a higher change in temperature than your brain can. What's the deal?

PALCA: Interesting. I guess Michael Sawka?

Mr. SAWKA: Well, I'll answer that. First of all, the brain doesn't have - recently we just found out what brain temperature is relative to body temperature. As physiologists, most mainstream physiologists always thought that brain temperature was approximately equal to arterial blood temperature. And there were just a few minority individuals that actually thought humans could have brain temperature lower.

But as it turns out, brain temperature's actually slightly higher than arterial temperature, which are some very recent experiments, and the brain does have a lot of very good protective mechanisms, because as I said, the temperature's higher.

But the same molecular responses that protect the brain actually can be decoupled and cause injury. So the brain basically is about the same as our ulterior blood temperature, or slightly higher. It's reasonably well protected, but it's not a lower temperature.

PALCA: Hmm. Even when it gets extremely warm?

Mr. SAWKA: Well, the experiments that have actually been able to directly - get a close to direct measurement of brain temperature have been with physical exercise. But it seems that the brain has higher temperatures for two reasons. One is, is that at least when you have the high blood flows associated with exercise, blood flow to the brain may decrease slightly.

PALCA: Michael Sawka, I am sorry, I shouldn't have asked that question when I did, because we're right at a break. But I'll let you finish it when we come back. You're listening to TALK OF THE NATION from NPR News.

(Soundbite of music)

PALCA: Here today we're talking about heat and how our bodies deal with the heat, and we're talking with Larry Sonna. He an assistant professor in the division of pulmonary and critical care medicine at the University of Maryland in Baltimore, and Michael Sawka, chief of the Thermal and Mountain Medicine Division at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts. And just before the break, I asked Michael Sawka about how the brain keeps cool in extremely hot environments. And maybe you can just go back over the two points that you were making.

Mr. SAWKA: Yeah, what I was saying was that the brain is not any cooler than the rest of the body, according to our latest scientific evidence, and that in fact with heat stress associated with physical exercise, the brain is probably slightly warmer. And the reasons for that is a reduction in brain blood flow, which would also probably carry over to arresting heat exposure, because you have to support the high skin blood flow. And second is, there's an increase in metabolism in the brain, so increase heat production in the brain, at least during physical exercise, which contributes perhaps the slightly higher temperatures, and we don't know whether that occurs with arresting heat exposure.

PALCA: Okay. Well, here's an interesting phenomenon that you may not have thought about. But when the temperature climbs into the high 90s in Philadelphia or New York, why are there likely to be more deaths than in, say, Phoenix or Miami, where the temperatures are routinely in the 90s plus range?

Joining me now to talk about that curious question is Larry Kalkstein -sorry, Kalkstein. He's president of Applied Climatologists Incorporated. He joins us by phone from Marco(ph) Island, Florida. Thanks for talking with us today.

Mr. LARRY KALKSTEIN (Applied Climatologists Inc.): Hi, Joe, good to be here.

PALCA: So what's the answer then? Why - why would people in Phoenix be less susceptible than people in Philly?

Mr. KALKSTEIN: Well, first of all, let me tell you that when you have a heat wave in Philadelphia or New York, it is considerably hotter than what you get down here in Marco Island or in Miami. Every day here is basically the same. When you live in South Florida or even New Orleans or even somewhat in Phoenix, although less so, our weather is the same day after day. We get up to 89 or 90, get down into the mid-70s and so on.

That is not going to kill you. What you need to kill you is high variability in the weather, and that is what happens in cities like New York and Philadelphia. For example, you have a large number of days maybe in the low to mid-80s, and then all of a sudden you get one of those weeks where the temperature shoots to near 100 or above, and those are the killing situations.

So rather than the extremity of the heat, it is the high variability that's most important, and that's why more people die in Philadelphia from the heat than they do in Phoenix, Arizona.

PALCA: Hmm. And so you've been trying to work with the National Weather Service on tweaking at least a little bit the nature of heat warnings for different cities. What's that all about?

Mr. KALKSTEIN: Well, the National Weather Service is the agency that's designated the call the excessive heat warnings. Whenever you hear one on the radio or television, it's the National Weather Service that calls this.

For years they've been using a very strict set of criteria, usually being a heat index of over 105 degrees for a couple of days, and sometimes the minimum temperature of 80, and that's when they call the heat index - what they call an excessive heat warning, whether you live in Boston or Atlanta or wherever.

Our research shows that our response to heat is very relative, and you react very differently to heat in Boston than you do, let's say in Atlanta or New Orleans, and so our systems are locally tailored for each individual city, and we look for thresholds beyond which we see increases in human mortality.

So ours is based on a health outcome rather than just some arbitrary number like 105 degree heat index.

PALCA: So you went to hospital records and found out how many deaths were related to the heat and what the temperature and humidity and all that was, and that how you used to come up with these heat indexes for different cities?

Mr. KALKSTEIN: Something like that. We have compiled everyone who has died in the United States for the last 20 to 30 years, and the data are compiled by the National Center for Health Statistics. So we can tell every day's total deaths for any given area. We can't talk about individual people and what Mary Jones died from, because that's private, but we do have total numbers of deaths.

So we know what weather conditions lead to increases in mortality, and basically we don't just look at temperature and humidity, we look at a wide variety of weather parameters.

For example, if it's 95 degrees and clear out, that's more dangerous than it being 100 degrees and cloudy because the heat load on a building when it's 95 and clear would be much more extreme, and you get many more deaths under that circumstance.

PALCA: So these heat warnings are for information purposes, but I mean they don't change the fact that you're still stuck in a hot environment.

Mr. KALKSTEIN: Yeah, but they are much more important than just for information. For example, all the health departments are clued in to this in every major city - emergency managers, utilities, and so on.

And also, there are - along with the warnings are given information to the public as to what they should do to lessen their vulnerability to the heat. One of the bits of advice we tell them not to do is to sit in front of a fan in a hot apartment, because it has a convection effect. But let's say you are a poor person without air conditioning, you live in an upper story of a tenement in Philadelphia. Your best bet is to go into a bathtub and sit there and remain cool during much of the hottest part of the day, or to get out of the house and go to an air-conditioned mall.

That advice is dispensed, and all of these people like the health departments and utilities, they do something whenever excessive a heat warning is called. In Philadelphia they activate an 800 line that people can call and get information if they're starting to feel dizzy or whatever. They actually take the homeless off the streets, open air-conditioned shelters. And in some cities, utilities, when people don't pay their bill, they cut off their service. But if an excessive heat warning is called, they do not - they suspend these disconnects. In other words, those people can maintain their utilities even if they haven't paid during this excessive heat event.

PALCA: All right. Well, Dr. Kalkstein, thanks for describing some of these measures that you've been working on.

Mr. KALKSTEIN: Been my pleasure. Have a good day.

PALCA: Thank you, Dr. - I mean Larry Kalkstein is the president of Applied Climatologists Incorporated, and he spoke with us from San Marco, Florida.

Now let me bring back my other guests, Michael Sawka from the U.S. Army Research Institute of Environmental Medicine in Natick, Mass, and Larry Sonna from the University of Maryland in Baltimore. And let's take a call now from Lenaya(ph). Is that right, Lenaya in San Diego?

LENAYA (Caller): That's right.

PALCA: Okay, good. What's your question?

LENAYA: I live in Southern California. I live in San Diego, and my question is, I'm heat intolerant. I'm 46 years old, and take, for instance, the other day in Escondido, where I work on the Blood Mobile, on the pavement it got 115. And I can't sweat. I'm wondering, why am I heat intolerant, and actually cold intolerant, and (unintelligible) intolerant, by the way, too.

PALCA: Okay, let's ask - I'm sorry. Let's ask Larry Sonna if there's any - at least a molecular basis for this kind of response.

Prof. SONNA: Well, there may be. In fact, I'm looking at an article right now by a group in Israel that has looked at individuals who are highly heat intolerant versus people who are normally tolerant, and there do appear to be biochemical changes that are not proceeding normally in people who are heat intolerant.

I don't think this group has had the final word on it, but the observation that the expression of certain proteins that may be protective against this kind of stress differs from people who are normal from heat intolerant.

It's certainly a very - it's a very interesting observation and I think one that will give us a lot of work in the next five to 10 years to follow up on and follow forward.

PALCA: How would you use this information. I mean, if you found some molecular marker that said you were heat intolerant, what could you do with that?

Prof. SONNA: Well, there is sort of several levels at which you can look at it, beyond just simply the interest in science itself. One is, if you could in fact find a practical marker - right now to diagnose someone as heat intolerant, one of the things you have to do is either stress them in the heat and observe their core temperature relative to normal, which is sort of what the - one of the research ways of doing about it. Or the other way is, sadly, to be someone who is prone to having incidents of heat exhaustion or heat stroke, much more so than the people around you.

Wouldn't it be great if we could find a cellular marker or a biochemical marker or a gene expression marker that could tell you before you got to that point that, you know, you're someone who's at risk for this kind of injury, and perhaps you should take extra precautions during the heat.

PALCA: I like it. I can't come to work today. It's too hot and I'm heat intolerant. I think - there's nothing bad about that. I think it's interesting.

Let's take another call now and this time go to Bob in Gloucester, Massachusetts. Bob, welcome to the program.

BOB (Caller): Yes. I was - we're moving from Gloucester, Massachusetts to South Carolina, and I was wondering - it sounds like we can actually cope with the heat on a molecular basis. I'm wondering how long do those kind of changes take, or is there any research - when can I expect to be able to tolerate the heat in South Carolina?

PALCA: Well, Michael Sawka, maybe it's more of a practical question, since the molecular answer to that is probably not going to help Bob very much. But what do you think, Michael Sawka?

Prof. SAWKA: Well, it's nice that you ask that question. We just finished up a study working with a group at the University of Maryland, looking at heat acclimatization and some of the molecular adaptations.

What's kind of interesting is we don't understand the molecular adaptations to heat quite as well, but if you look at the time span of heat acclimatization, that's the physiological adjustments that reduce the strain, minimize the strain of the body for a given exposure, generally, for the average person, most of the - a large majority of your adaptations can occur over five days, or about a week, and most of your - you're almost fully adapted after about two weeks, roughly. Okay?

And if you look at some of the molecular adaptations, most of those adaptations occur over several days and probably follow a very similar time span. So as you look at adaptation, you're looking at it, one, of an adaptation of improving heat loss so the strain on the body, the cardiovascular system, the rise in body temperature is not as high, and at that same time you have these molecular adaptations so that within the cells, as they get hot, it's less likely to produce injury.

PALCA: Did you - I'm just thinking, since you're at the U.S. Army Research Institute of Environmental Medicine, were there any special instructions given to U.S. troops who might have come from Gloucester, Massachusetts and suddenly found themselves in Iraq?

Mr. SAWKA: Well, what we do is we had very extensive instructions and very extensive preventive medicine guides that we've developed over many decades, and these fall around principles that are used for risk management of heat exposure, so that you can maximize adaptation, so that - and also do other risk management procedures so that you can, one, maximize performance, which is particularly important to the U.S. armed forces so that they can have peak operational effectiveness, but also at the same time minimize the effects of injury. So yeah, there's very extensive guidance.

PALCA: Any main thing that you warn them about?

Mr. SAWKA: Well, I think for the general population, as we look at risk reduction, I think that, you know, you basically would fall under a broad category of minimizing exposure, and these are all common sense types of things that I'm going to tell you right now: minimizing exposure, acclimatizing, that, you know, gradually increasing the amount of heat or physical work or exposure that you have over a number of days so that you can allow the adaptive responses, you know, to occur. And also it's important - and you certainly hear a lot about this in the press - is maintaining hydration because the cardiovascular system's important, so you want to have adequate blood volume.

So it's important that people drink, and they also have high sweat rates, so you want to replace that water lost. But you know, as we talk about heat injury, and we've had this outbreak on people's minds in terms of deaths, it's really important to realize that there's really two different types of heat strokes that we see, and two different populations at risk.

We have what's called classical heat stroke, which are the young and the elderly, the frail populations. And we have exertional, which we see in athletes and thought-to-be-healthy populations.

PALCA: I see. Sorry. This is Joe Palca. And - I'm Joe Palca and this is TALK OF THE NATION from NPR News. Let's take another call now from, how about Dave in Kansas City, Missouri. Thanks very - Dave, welcome to the program.

DAVE (Caller): Thanks, Joe, for taking my call. I have, I guess, a question really. I have a friend - a little bit of history of myself. I work in the HVAC industry. We're on the roofs, where it's relatively 123 to 142 degrees, depending on, you know, how the sun's hitting, black roof, white roof, that type of thing.

But we're always told to hydrate, and always drink plenty of fluids, and like you say, replace that. But I actually ran into, last year, a friend of mine that drank too much water, and the older doctor that he went to called it water intoxication. It was to the point where he was drinking so much fluids, it was so hot, and he was so thirsty, that he actually washed out the salt content or some kind of an electrolyte in his body that actually made him pass out.

PALCA: Larry Sonna, I see you nodding. What about this question of too much water?

Prof. SONNA: Well, Mike Sawka is actually the subject-matter expert on that because a similar problem was picked up in the Army some years ago of overdrinking in the setting of heat. And your caller is absolutely right. Overdrinking in this kind of setting can lead to profound disturbances and even death from what we call hyponatremia, which is low concentrations of body sodium. There is such a thing as too much being detrimental. Mike, would you care to comment on that?

Mr. SAWKA: Yeah. Hyponatremia, it's been seen quite a bit in long-distance athletic events, occasionally in marathons, with increasing severity, and in ultra-marathon events. And basically what it is, is it's - as said, it's basically diluting the sodium in your body, and it can result from one or two things. One is deficits of sodium to begin with. And two, that combined with over-drinking of dilute fluids or just massive over-drinking in general.

Usually for a general, a healthy population, eating the U.S. - the normal U.S. diet, it's not a problem. It starts becoming a problem as you're getting into a lot more endurance events, and particularly you see quite a bit of it as made very popular in the recent Boston Marathon (unintelligible) death, particularly cooler marathons, where you don't have that much of a sodium loss, but people are just still consuming very large volumes of hypo-tonic or fluids.

PALCA: You know, Larry Sonna, I wonder if I could ask you this. It's somewhat personal. I guess you're not supposed to ask questions about your family. But my son had a fever this week. How does someone with a fever cope with heat? Is it different?

Prof. SONNA: Do you want the molecular answer?

(Soundbite of laughter)

PALCA: I guess if that's what your expertise is, that's what I'll take.

Prof. SONNA: What we've been finding is that some of the mechanisms that have evolved to cope with heat also come into play when someone has a fever. Now, for many, many years, the tools, the biochemical tools that we use to detect changes in cells in response to thermal stress were fairly insensitive and required very high delivery of temperatures.

But as we have become more sensitive in our tools and our ability to detect these changes and become better, we find that even under febrile conditions, or what we call febrile-range hyperthermia...

PALCA: That's fever, febrile.

Prof. SONNA: Fever, fever - you will see some of the similar molecular changes. How beneficial or how detrimental they are is still an active research question, but they do in fact come into play.

PALCA: Okay, well, I'm afraid that's where we have to leave things. I'd like to thank my guests this hour. Michael Sawka is the chief of the Thermal and Mountain Medicine Division at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts; and Larry Sonna is assistant professor in the Division of Pulmonary and Critical Care Medicine at the University of Maryland in Baltimore.

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