Why Is Comfortable Air Temperature So Much Lower Than Body Temperature?

Jared U. asks: Why isn’t normal body temperature the temperature that humans are most comfortable in?

cooling-off-in-sprinklerEach summer as temperatures approach 98° Fahrenheit (about 37° Celsius), we hear news stories about people suffering (and sometimes even dying) from overheating. Yet these temperatures are essentially the same as normal body heat. So why isn’t this a comfortable temperature for us?

Our bodies constantly generate heat through the moving of muscles in the arms, legs, diaphragm and heart, by the work of tiny little “ion pumps” that send electrical messages through the nerves and, of course, in the process of digestion, among other ways.

While the body must maintain a certain temperature (about 98.6° F / 37° C  give or take a degree or so depending on a variety of factors), too much heat, or too little, can be fatal for a variety of reasons including many necessary enzymatic functions not working properly outside of certain temperature bounds. So, to maintain the proper balance, your body uses several processes including exhaling warm air, sweating and having warm blood circulate close to the skin’s surface to cool off. Conversely, it might burn extra calories, constrict certain blood vessels (to limit blood flow to the skin), and shiver to warm up. (And, note: drinking alcohol makes you feel warm when you’re cold primarily because it’s dilating your blood vessels, overcoming one of your body’s natural defenses against the cold. Thus, drinking alcohol to stay warm actually is a sure-fire way to rapidly cool yourself off, often without you being aware of it because your skin feels warm, which is particularly dangerous in certain circumstances.)

Beyond ambient temperature, humidity also impacts these processes on both ends. In cold, damp weather, the humidity acts in a similar manner as sweat – causing evaporative cooling. However, on hot and humid days, that same humidity interferes with the evaporative process.

Of course, those of us who are “always” cold, as well as those of you who are always hot, know there must be more to it than just air temperature and humidity. And, in fact, there are actually six general thermal comfort factors that influence whether you’ll feel hot or cold in a given ambient environment.

These include the presence or absence of wind or other air movement. For instance, according to the National Weather Service, a daytime temperature of 25° F (-4° C) and a wind of 20 MPH would produce a wind chill of 11° F (-12° C).  Another factor that affects your thermal comfort is whether you are receiving heat from a radiant source, like the sun or a fire. The amount and type of clothing worn is also on the list.

The final general factor is metabolism- essentially the rate at which a body can transform chemical energy into heat and work. Generally speaking, the more active a body is and the more muscle mass you have, the higher the metabolic rate (and amount of heat produced). For reference, according to the standard table, the rate for sleeping is 0.7 met, for sitting quietly is 1.0 met and for walking and generally moving around 2.0 met. Of course, metabolic rates can vary widely between people based on a variety of factors, which helps explain why you might be freezing cold (all other thermal factors being relatively equal), while your partner sitting next to you is “soooo” hot.

So, the bottom line is that for (most) people in the majority of circumstances, ambient temperatures that are around human body temperature are simply going to be too hot for your body’s cooling mechanisms to overcome the sweltering heat combined with the natural heat it generates while doing what’s necessary to keep you alive. However, your mileage may vary depending on a variety of factors like what kind of clothes you’re wearing and if it’s windy or if you’re dehydrated, how much you’re exerting yourself, etc. For instance, I personally know a very elderly woman who feels cold unless the ambient temperature hits at least in the low 90s. Getting heat stroke while going on a road trip with her has a non-zero probability of occurring when she gets to set the car temperature to her comfort level and you made the mistake of wearing a sweatshirt for a top because it was 10° F (-12° C) outside the car. (True story.)

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Bonus Fact:

  • Carl Bergmann, a German biologist, observed in 1847 that larger animals of a species would live further away from the equator than smaller individuals, which led to the realization that larger animals produce more heat. This is due to a couple of factors, including that bigger beings simply have more cells generating heat, as well as the fact that their greater volume to surface area ratios (volume increases much faster than surface area) mean the larger individuals lose less heat relative to their size when compared with smaller individuals of the same species. People studying this phenomenon (called Bergmann’s Rule) have found that it can be applied to humans as well as animals. In the 1950s, researchers looked at 100 different human populations and discovered that people who lived in high-temperature climates usually had low body masses, while those who lived in colder temperatures had higher body masses. Of course there are exceptions. Neither Arctic Inuit nor Tibetan Sherpas are known for their great size, and conversely, the Dinka of South Sudan and the Tutsi of Rwanda are each renowned for their exceptional height.
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  • An annoyed reader

    I stopped reading this article after it started measuring temperature in °F on multiple occasions. This is only useful for the less scientifically developed North Americans. And as a European science student… this article sucks. You might as well write any number next to the F since any non-american has no references to your values. If you want to be a scientific page please stick to scientific standards.

    • Daven Hiskey

      @An annoyed reader: “This is only useful for the less scientifically developed North Americans.” Beyond the absurdity of a statement like “less scientifically developed North Americans,” we do Fahrenheit first in these articles, despite that most of the authors here are British, because approximately 88% of the readership of this website are people from the United States. If most of the readers were from Europe, we’d put Celsius first. Either way, these are just handy measurement scales; no need to get religious about their usage. 🙂

      “You might as well write any number next to the F since any non-american has no references to your values.” You mean like where we included the degrees in Celsius in parenthesis to the right of the “F” like we always do in articles so everyone has a reference point and can understand what we’re saying? Did your scientifically superior brain miss that part? 😉 Or do you really just get that offended by the usage anywhere of the Fahrenheit scale? I mean, whatever floats your boat, I guess, but there are bigger problems in the world than one group using one measurement scale in everyday life and another using a different one. 🙂

      Incidentally for any who are curious, see Who Invented the Fahrenheit and Celsius Temperature Scales and What Zero Degrees Fahrenheit Signifies: http://www.todayifoundout.com/index.php/2014/12/fahrenheit-scale-isnt-arbitrary-seems/

    • If you’re so superior to us Fahrenheit users, then it should not be an issue to easily convert the number with a simple equation.

  • Well said Daven. My thoughts exactly. If one wants to get upset over something posted online, there are certainly any number of more important issues.

  • If you’re so superior to us Fahrenheit users, then it should not be an issue to easily convert the number with a simple equation.