Climate change promises to bring with it longer, hotter summers to many places on the planet. This June turned out to be the fourth-hottest month ever recorded—globally—scientists are reporting. With more heat waves on the horizon, and a big one currently sweeping much of the U.S., the risk of heat-related health problems has also been on the rise.

Heat exhaustion is a relatively common reaction to severe heat and can include symptoms such as dizziness, headache and fainting. It can usually be treated with rest, a cool environment and hydration (including refueling of electrolytes, which are necessary for muscle and other body functions). Heat stroke is more severe and requires medical attention—it is often accompanied by dry skin, a body temperature above 103 degrees Fahrenheit, confusion and sometimes unconsciousness.

Extreme heat is only blamed for an average of 688 deaths each year in the U.S., according to the Centers for Disease Control and Prevention (CDC). But when sustained heat waves hit a region, the other health ramifications can be serious, including sunstroke and even major organ damage due to heat.

The Chicago heat wave in the summer of 1995 killed an estimated 692 people and sent at least 3,300 people to the emergency room. An observational study of some of those patients revealed that 28 percent who were diagnosed at the time with severe heat stroke had died within a year of being admitted to the hospital, and most who initially survived the high temperatures had "permanent loss of independent function," according to a 1998 study of the heat wave, published in Archives of Internal Medicine.

As temperatures linger above our bodies' own healthy internal temperature for longer periods of time, will we humans be able to take the heat? We spoke with Mike McGeehin, director of the CDC's Environmental Hazards and Health Effects Program, to find out just why—and how—a warm, sunny summer day can do us in.

[An edited transcript of the interview follows.]

How do humans cope with hot, hot weather?
The two ways we cope with heat are by perspiring and breathing.

So is it the heat or humidity that is the real killer?
The humidity is a huge factor. If you have tremendously high temperatures and high humidity, a person will be sweating but the sweat won't be drying on the skin. That’s why it's not just heat but the combination of heat and humidity that matters. That combination results in a number called the apparent temperature or "how it feels".

Obviously there are thresholds for both temperature and humidity above which we see an increase in death, and it's going to be a different temperature in Phoenix than it's going to be in Chicago.

The other major factor in terms of temperature that causes both mortality and morbidity is the temperature that it falls to in the evening. If the temperature remains elevated overnight, that's when we see the increase in deaths. The body becomes overwhelmed because it doesn't get the respite that it needs.

What kind of impact does extreme, sustained heat have on the human body?
The systems in the human body that enable it to adapt to heat become overwhelmed. When a person is exposed to heat for a very long time, the first thing that shuts down is the ability to sweat. We know that when perspiration is dried by the air there is a cooling effect on the body. Once a person stops perspiring, in very short order a person can move from heat exhaustion to heat stroke.

What happens in the transition from heat exhaustion to heat stroke?
It begins with perspiring profusely, and when that shuts down, the body becomes very hot. Eventually that begins to affect the brain, and that's when people begin to get confused and can lose consciousness.

The analogy we use is if you're driving a car and you notice that the temperature light comes on, what's happening is the cooling system of the car is becoming overwhelmed. If you turn off the car and let it cool eventually you can start driving again. But if you continue to drive the car, the problem goes beyond the cooling system to affect the engine, and eventually the car will stop.

What other areas of the body does this extreme overheating affect?
As the body temperature increases very rapidly, the central nervous system and circulatory system are impacted.

In places where there have been prolonged heat exposures, there is probably a broad impact on many organ systems. From heat waves that have been studied, like in Chicago, there are increases in emergency department visits and hospital stays for medical crises that are not normally associated with heat, such as kidney problems.

But it really hasn't been studied very much. One of the reasons for that is the main focus of the studies has been on mortality from heat waves, and there hasn't been that much focus on morbidity. That would take looking at people who are hospitalized from heat exhaustion or heat stroke and following them into the future.

14 Comments

1. 1. tichead 11:44 PM 7/23/10

   I read something a few years back about a 'heat gene'. Sorry, I can't provide a link but I'm pretty sure it was in Sciam. Turn the gene on and H. sapiens can tolerate about 3 degrees F greater core temperature than average without damage.
   
   I live in the subtropics (95 F and 70% RH are normal from mid April to mid October) and work outside, usually in full sun, and drink about a gallon of distilled water each day. I wear light value cotton long sleeve shirts and long pants (T-shirts and shorts are for sunburnt bug bitten tourists). Cotton wicks out sweat and provides excellent evaporative cooling without the need for mosquito repellant and sunblock (icky, sticky chemicals on the skin). I live in an 80 year old house with no air conditioning, except for a window unit in the bedroom, (I like to sleep cold), and am quite comfortable. Five generations of ancestors have, for the most part, lived this way. Conversely, when the dry season temps get down into the forties and sometimes thirties, my body does not want to move.
   
   I didn't grow up in this climate, I moved around alot, born into and living mostly in northern climates with blizzards and freezing winters, and only got back to my 'roots' as an adult.
   
   Has my heat gene been epigenetically 'turned on' or, have I simply adapted as an individual to this climate? If I have simply adapted to this climate, then can anyone adapt to any climate? If so, then how long does it take for H. sapiens to acclimate to any given climate? Lastly, is there such a thing as a 'heat gene'?

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2. 2. Mark B 03:50 PM 7/24/10

   I find Mr McGeehin's comments about fans misleading. I used to work at sea, in a ship's engin-room (hot) without air conditioning, in the tropics with high humidity. With my skin and often my clothes wet with sweat, standing under an air blower could feel positively cold - at well over 100deg. All the while you are sweating, fans help cool you because the airflow increases evaporation. However, if you stop sweating and air temperature is above body heat, it will increase body heat. So, wet clothes and a fan will help cool you in high heat... Water and moving air is all you need, but you need them both.

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3. 3. tichead 10:56 AM 7/25/10

   Mark B: I had similar experience in a ships engine room. The coolest place was between the mains under the fresh air blower. Only problem was that the air volume was so great and moving so fast it would dry one off in a few minutes, clothes and all, so it was back to making rounds in the sweaty spaces then back to the blower.
   
   I am still curious about the 'heat gene'. Have any other readers of Sciam or similar mags heard of this or has the heat caused me to suffer dilusions?

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4. 4. doug l 12:58 PM 7/25/10

   I would guess that a warmer planet would be advantageous for our bodies as we are, according to the fossil record, evolved from tropical primates. Warmer planets have more moisture available and that, along with a longer growing season will create more food, and longer, cooler, climates are not all that good for us. The geological record shows that cooler periods coincide with the expansion of arid regions.

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5. 5. frgough 09:58 AM 7/26/10

   Ya gotta love how SA breathlessly reports June was the FOURTH hottest month on record.

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6. 6. andreanis 05:13 PM 7/26/10

   Yes in fact my friends this is an issue more research should be devoted to..let's remember that for the human body slight increases in temperature do mean a lot.And warmer summers seem to be more and more common:
   in my view research should aim at 1) study in depth the physiologic effects of heat on humans 2) elaborate new innovative technology in order to defend the population (if this phenomenon as it seems,keeps progressing.3) Size the opportunity:higher temperatures also mean more natural energy around.More research on how to convert this new opportunity (eg solar/photovoltaic/heat pumps or else should be done..) into useful civilian uses is something no to be missed..
   
   Join my discussion forum:
   
   http://health.groups.yahoo.com/group/HyperthermiaACLS/
   
   Andy

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7. 7. bongobimbo 05:25 PM 7/26/10

   There are some historical references to the mass deaths of Irish rebels sent to the West Indies to work in the cane fields in the 1700s. I never checked the reports (it's not my historical era) but they could be accurate. As a pale redhead growing up in Miami I went to the hospital twice for something the doctor called "heat prostration"--which seemed to be a combination of sunburn, windburn, 90%+ humidity and high temperatures. My symptoms were nausea, feeling faint (although I don't recalling ever passing out) and lethargy. My dad had many skin cancers down there and mom (naturally swarthy) got her first stroke after working outside in the garden. I agree there's something in the DNA that affects health in various climates. There are some climates, like south Florida's, that are hostile to such human beings. Now the humid heat is creeping north.
   
   I do use fans a lot--my great aunts' advice not to open any windows on very hot days, but run the fans since it was bound to be cooler inside than out (both lived healthily to 96 in hot southern Iowa). To that I add running the A/C and fans together for a couple of hours, then using only fans for as long as tolerable and drinking lots of water and iced tea. But what about sodium chloride salt? Normally I avoid it and use potassium chloride instead. NaCl tends to give me edema of the ankles in extreme heat. Yet--I've known people hospitalized with "salt deficiency" on hot days. NaCl? Or KCl? Or both? I wish that the writer had discussed the role of salt in heat-related illnesses.

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8. 8. ecoligist 06:20 PM 7/26/10

   I've heard that excessive heat disrupts the sodium/potassium balance and maybe of other ions that are critical to nerve transmission - cramps, and distorted thinking. Of course, the body can only stand so much of this disruption before the lethal threshold is reached.
   
   This would account for the efficacy of diuretics which affect the Na/K balance.

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9. 9. marcellesilbert 12:55 AM 7/27/10

   In Perth, Australia we have heat waves in the 40's (Celcius) 100's (Farenheit) and it gets bad. We use a fan because it causes evaporation of sweat and makes you feel cooler. If we followed your advice to not use fans there'd be a lot more dead people. Has Katherine Harmon ever tried using a fan in hot weather?

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10. 10. andreanis 04:12 AM 7/27/10

    Good day Bongobingo..actually I think you are bringing up here an interesting point ie "what can be done to work more efficiently at higher temperatures" ..I did a small internet search and I found this intersting link http://www.cdc.gov/niosh/hotenvt.html
    regards
    Andy

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11. 11. Jim Lacey 03:36 PM 7/28/10

    At Fort Sill in 1957 in the desert heat we fledgling lieutenants were allowed to bring jugs of ice water into the field--not standard issue. Also, we were instructed to take salt tablets with meals. The one benefit of this dehydrating heat was that beer never again tasted as good at the first one after coming in from the field!

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12. 12. Quinn the Eskimo 03:58 AM 7/29/10

    Too much algore can kill anyone. Ask Tipper.

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13. 13. andreanis 03:07 PM 7/30/10

    no doubt electrolytes are messed up by eat ..just think of the lost sweat

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14. 14. AvaMaru in reply to doug l 08:53 PM 7/15/12

    I think growing temps on the planet are horrid for our survival. I live in a little podunk town in northern illinois, summers have been about 115 degrees F. Higher temperatures were the leading cause for giant reptiles which rely on heat to grow. scientists have found bones of a 48 ft long snake, and table sized turtle shells. This was back when they thought the leading cause was climates of temps in the 90s. If that's the case, that little anaconda in your yard can become a human-eating monster...
    also, i don't think we could adapt. The climates change so quickly in the course of 10 years our summers could be from 83F average to 110F average, and our insides cannot handle that kind of change :(

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