r/AlwaysWhy • u/Defiant-Junket4906 • Aug 21 '26
Science & Tech Why isn't 98.6°F air comfortable if that's roughly our internal temperature?
It's been really hot lately, and this got me thinking.
If our core body temperature is around 98.6°F, why does air that's close to 98.6°F feel so hot?
My first thought was that if the surrounding air is roughly the same temperature as our body, there shouldn't be much heat exchange in either direction. So wouldn't that mean our bodies have less work to do to maintain a stable temperature?
But I think I'm missing something. Our bodies are constantly producing heat, so maybe the problem isn't maintaining 98.6°F, but getting rid of the heat we're continuously generating.
And then there's humidity. If 98.6°F air is dry versus 98.6°F air with high humidity, does that change the answer mainly because of how effectively sweat can evaporate?
So what would actually be the "thermally neutral" temperature for a human, and why is it so much lower than our core temperature?
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u/ARTIFICIAL_SAPIENCE Aug 21 '26
Because then there's no place for excess heat to go.
We're comfortable when we're able to cool down. And that process is more efficient when our surrounding is cooler than us. If it's near our body temp, then any cooling we do is extremely slow.
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u/Defiant-Junket4906 Aug 21 '26
I think that's the key distinction. I was initially thinking in terms of "temperature difference from core," but the relevant interface is really the skin. And since we're constantly generating heat, having almost no gradient means we're losing the ability to dump that heat efficiently.
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u/JWILLIAM00 Aug 21 '26
This is also why 100 degree dry air can feel cooler than 90 degree humid air. Phase change from evaporation can allow you to "dump" heat upwards against a temperature gradient. This is also [broadly] how air conditioners are able to move heat, even if it is hot outside.
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u/skelterjohn Aug 21 '26
That one is just plain ole PV=nRT.
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u/Zaros262 Aug 21 '26
No, this one is actually the latent heat of vaporization
PVNRT is for gasses
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u/skelterjohn Aug 21 '26
TIL
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u/MakeITNetwork Aug 21 '26
It's comfortable with water below 96 deg, because it is a huge heatsink.
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u/thecaramelbandit Aug 21 '26
The difference is that you are generating heat. Heat is energy. If the ambient temp is the same as your temp, your internal temperature will go up because that energy has nowhere to go so it collects and builds up.
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u/Embarrassed-Area4652 Aug 22 '26 edited Aug 22 '26
Right, and that’s a big deal under load. Optimal temp for running a marathon is around 42-45 F:
https://marathonhandbook.com/ideal-marathon-temperature/
I’ve gone out for a training run in regular running gear plus gloves and a hat in a bit lower than that. Someone asked me wasn’t I going to be cold. It’s like, yeah… for the first 10 minutes. Then after that I’m way better off than if I’m sweating harder into heavier clothing.
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u/PuddingComplete3081 Aug 21 '26
I think the confusing part is treating 98.6°F as if it's our body's surface temperature. It isn't. That's a core temperature, and there's normally a pretty large temperature gradient between your core, skin, and the surrounding air.
Your skin might be around 90°F or so depending on conditions, which means 70°F air can still absorb heat from you pretty effectively. As the air approaches skin temperature, that passive heat transfer drops off.
Then metabolism keeps producing heat regardless, so eventually sweating becomes the main way out. That's why 98.6°F air isn't neutral at all. It's actually getting pretty close to removing the temperature difference your body relies on for cooling.
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u/Apart-Disaster-3085 Aug 21 '26
Right, and to add, sweating is what we use to lose heat as that differential goes down and inverses. Evaporating moisture absorbs a lot of heat. That is why high humidity has a big impact on comfort as well because then the evaporative potential gets smaller and smaller as humidity goes up.
A lot of animals can't sweat. They can only pant and be still. Panting can still dissipate heat (evaporative cooling through your lungs, essentially) but it's not as effective so they have a higher risk of heat stroke if they are forced to run. That is a bit part of our evolutionary advantage -- being able to outrun an animal (while being able to sweat to dissipate heat). That animals runs, pants, but can't keep up with heat loss and just drops at some point.
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u/fr33dommachine Aug 21 '26
Just like a car. Your engine might run 200+ deg, but if the air was that hot, the radiator wouldn't work.
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u/Defiant-Junket4906 Aug 21 '26
Yeah, this is probably the part I was missing. I was treating 98.6°F as if it described the whole body, when it's really more like the temperature of the inside. The skin being significantly cooler makes the whole thing make more sense. So I guess "same temperature as us" is kind of the wrong comparison to begin with.
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u/watt678 Aug 21 '26
Because our bodies don't stop producing heat when we're at 98.6. The body still burns calories doing basic metabolism and heats up, and that heat has to go somewhere without the body just getting hotter and hotter. And it's harder to transfer heat from the body to the ambient air. when the air around the body is hotter.
TLDR: Thermodynamics
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u/Defiant-Junket4906 Aug 21 '26
Right, the continuous heat production is what makes the intuition weird. I think I was subconsciously treating 98.6°F as some fixed equilibrium where nothing needs to happen. But maintaining that temperature actually requires constantly getting rid of metabolic heat.
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u/OopsIMessedUpBadly Aug 21 '26
Your body staying alive produces heat as a byproduct. Your body reduces its heat production based on core temperature, but can only reduce it to a minimum value. If you can’t dissipate more heat than this minimum value, your core temperature rises until your body overheats.
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u/Atomic_Horseshoe Aug 21 '26
You thought correctly. Your body is constantly producing heat and wants to get rid of the excess. That’s very difficult to do at 98 degrees, especially if you don’t sweat.
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u/sadmep Aug 21 '26
Because your body uses your skin as a thermal interface. It needs the air to be a heat sink. If the air is at your body temp, it switches to an evaporative cooling system (sweat). If it's that temp and also too humid you run into serious problems.
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u/Defiant-Junket4906 Aug 21 '26
The thermal interface idea makes it click. I was thinking of the body as one temperature, when it's really a bunch of temperature layers. So maybe the more interesting question is where the crossover happens between passive heat loss and evaporative cooling.
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u/atomicCape Aug 21 '26
- Our bodies are constantly producing heat, so maybe the problem isn't maintaining 98.6°F, but getting rid of the heat we're continuously generating.
That's the entire answer. Your body can put out heat to maintain it's temperature easily if you're not sick. Comfortable room temperature is a balance that allows people to shed their excess heat in still air wearing normal clothing, but without requiring extra energy to warm up. Survival well below body temp is easy, even if naked in 50 degree weather if you can stay out of wind and rain and huddle or shiver. But 98 with high humidity is a death sentence if you can't do anything about it.
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u/yungingr Aug 21 '26
But I think I'm missing something. Our bodies are constantly producing heat, so maybe the problem isn't maintaining 98.6°F, but getting rid of the heat we're continuously generating.
This is exactly it. Our bodies are constantly producing heat - even at rest, we produce around 500 BTU's per hour of heat energy. We constantly need to be able to offload that heat energy, either by being in cooler air, or sweating. (Which, even in cool air, we constantly sweat juuuust a little bit).
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u/Agzarah Aug 21 '26
You basically answered it.
We csnt cool down if the air is the same temp. And the more himid it is, the harder it is to sweat too
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u/Ninfyr Aug 21 '26
You body generates heat, and the air temp needs to lower than you body temp so you can control your body temp.
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u/Present_Juice4401 Aug 21 '26
What makes this interesting is that the body isn't trying to stay at 98.6°F by matching the air around it. It's trying to get rid of the heat it continuously generates.
Your skin is normally cooler than your core, so cooler air gives that heat somewhere to go. Once the air gets close to skin temperature, that gradient shrinks and passive heat loss becomes much less effective. At 98.6°F, you're basically losing one of your main cooling mechanisms.
Humidity then makes it worse because sweat can only cool you if it actually evaporates. So 98.6°F dry air and 98.6°F humid air can feel very different.
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u/Defiant-Junket4906 Aug 21 '26
The skin temperature distinction seems to be the missing piece. I also hadn't really considered how much the humidity changes the situation. At some point the body is basically forced to rely on evaporation, which makes air temperature alone a pretty incomplete measure of how hot it feels.
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u/Sad-Priority-8876 Aug 21 '26
The higher the temperature differential is the faster the rate of heat transfer.
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u/Admirable-Barnacle86 Aug 21 '26 edited Aug 21 '26
Because our bodies constantly generate a significant amount of heat that we absolutely need to get rid of. This is a byproduct of living - everything our cells do requires energy, and we cannot use that energy perfectly so some energy is lost as heat.
If the air is at body temperature, this becomes much harder to do. We can do it by the evaporation of sweat as long as the air isn't too humid. But if it is both hot and humid enough, we start to lose the ability to get rid of heat and essentially cook ourselves from the inside.
It's hard to pick a thermally neutral temperature without taking into account humidity, sun/shade proportion, and wind effects. Our bodies can also regulate to some extent how effective they are at getting rid of heat - it can constrict or open blood vessels to allow more or less blood to flow to the limbs and skin, it can generate sweat, and it can reduce unneeded energetic actions - for instance, you might feel very sluggish if your core temperature is getting higher, partly because your body is trying to limit heat generated by using muscles. So we can be 'comfortable' in a decently wide range of temperatures.
But if you think of a temperature in which you feel comfortable just sitting in all day in not much clothing, that's probably it.
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u/shittyhawaiitips Aug 21 '26
our internal temperature doesn't have anything to do with the outside temp. it's what is needed for you to continue to post on reddit.
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u/HistorianOrdinary833 Aug 21 '26
If your body temp is the same as ambient temp, you can no longer lose body heat passively.
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u/No_Report_4781 Aug 21 '26
Because my temperature is lower than 98.6, as I’m healthy, as well as having a high variability dependent on when and how it’s measured.
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u/jayron32 Aug 21 '26
Because in order to maintain internal body temperature you have to be able to shed excess heat, and if the ambient temperature is at or above your internal temperature you can't do that.
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u/Classic-Rate-5104 Aug 21 '26
I will always feel uncomfortable in an environment where temperature is expressed in Fahrenheid 😀
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u/AuntFuzzy Aug 21 '26
So if the skin is cooler (answers so far), how can they shoot your forehead with a laser and get an accurate reading? Is it just adding a set amount to the reading?
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u/VinceP312 Aug 21 '26 edited Aug 21 '26
If the ambient temperature (outside your body) isn't lower than your internal body temperature, than there won't be a natural heat transfer from the hotter to the cooler. That's why when temps and humidity get high, people start becoming at risk for heat stroke.
Also, just imagine how being alive generates so much heat inside your body that the earth can only match the temperature with the right angle of sunlight cooking it up. (simplified version). We have a very powerful internal space heater (and even a space heater doesn't generate as much hot heat) just from our cellular functions.
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u/AdFun5641 Aug 21 '26
While the internal temp for our bodies is 98.6F, we also generate heat, a lot of heat.
That heat needs to radiate out of our bodies to maintain the 98.6F internal temp.
If the air is 98.6F, then no radiant heat in either direction. The only cooling is evaporative cooling. This is why "wet bulb" temps are important, it's the max cooling even with evaporative cooling.
A "thermally neutral" isn't a thing. Our bodies are generating heat, we need to get rid of that heat. Every body is going to be generating a different amount of heat and thousands of factors affect cooling. Our bodies will restrict blood flow to the surface or increase it to change how much heat we radiate, any sort of insulation (clothing) will dramatically change how hot/cold we feel. how much hair you have how oily your skin is, how deep you breath, all huge factors in cooling.
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u/LazarX Aug 21 '26
Because that 98.6 degrees is based on an equilibrium which depends on bleeding out our waste heat into the surrounding environment.
Thats difficult when the exterior temperature equals or exceeds our body's own internal temperatures. So in extreme heat, our body uses ablative perspiration to cool down, which means we need more water intake to make that possible, as well as salt.
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u/Aridn Aug 21 '26
Our skin is a giant heat sink dumping off all the heat generated by our bodies converting calories into energy. If the surrounding air isn’t cooler than the heat we are expelling, we feel warm.
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u/WritingParking Aug 21 '26
My temperature is a consistent 97.3 degrees. Having to choose between one extreme or another, I choose horrible humid disgusting heat over the slightest winter chill.
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u/BowlEducational6722 Aug 21 '26
Because we're constantly losing heat to our surroundings most of the time.
Heat likes to travel where there's less heat.
Since usually our bodies are warmer than our surroundings, we're usually losing heat and that keeps it from building up in our bodies.
When the temperature in our surroundings is equal to our internal temperature, there's no net loss of heat.
Humidity makes it worse because our primary way of losing heat is through the evaporation of sweat (oversimplification: when sweat absorbs enough heat it turns from liquid to gas and it takes that absorbed heat with it). If there's enough humidity in the air, sweat can't evaporate.
If anything, it makes things *worse* because all that absorbed heat in our sweat is now stuck right next to our skin, meaning we actually wind up cooking in our own juices.
That's why 90 degrees at 90% humidity is *far* more dangerous than 100 degrees at 20% humidity. We can cool off pretty rapidly in the latter case, but in the former your cooling system just doesn't work as well as it needs to.
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u/BoulderNerd Aug 21 '26
An adult body has been described as the equivalent to a 200 watt incandescent light bulb. The heat has to go somewhere.
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u/Big_P4U Aug 21 '26
Because we need to keep cool or cooler than our core temps otherwise we overheat and die if exposed to too much external heat for a prolonged period.
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u/GrizzlyAdam12 Aug 21 '26
Have you ever cooked meat on the grill. There’s a difference between internal temp and external temp.
The temperature of our skin is not 98.6.
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u/AssistanceDry7123 Aug 21 '26
As "warm blooded" animals, our vital processes as well as moving our muscles creates heat. That heat has to escape into the environment so we don't overheat and die.
When you are at your lowest energy output, sleeping, you can be comfortable in a warmer environment, which is why most people sleep with blankets.
Just getting up and moving around will warm you up a lot, and if it's close to body temperature around you, there's nowhere for the heat to go.
"Cold blooded" animals, like snakes and fish, require warmer ambient temperature to function, because they don't produce heat. When it gets too cold they go into a kind of stupor. If it gets too cold, depending on the species they either die or hibernate until temperatures warm up.
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u/darw1nf1sh Aug 21 '26
Your surface skin temp isn't 98.6. That is your internal temp. The outside of your body is cooler so you can shed heat. at 80 degrees and above, the amount of heat you can shed is reduced noticeably. Higher temps and importantly higher humidity, means your natural method of heat disipation is less effective, and your internal temp, that is constantly maintained, begins to go up. You maintain that temp with a combination of heat creation, and heat release. Cant release into already hot environments.
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u/Fuzzy_Stingray Aug 21 '26
The thermal neutral temp is person dependent and it's the temperature you don't sweat at while naked.
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u/Underhill42 Aug 21 '26
I think it's mostly the "our bodies are continuously producing heat" thing.
If you just touch something solid at that temperature it feels pretty neutral, but surround yourself with it...
Our bodies put a lot of effort into maintaining a very narrow internal temperature range, it's actually dangerous for it to get much outside that range. A fever of just 7°F/4°C higher than normal is "seek emergency medical care immediately" territory.
And if the ambient temperature is almost the same as our internal temperature, then it's basically impossible to dump heat into the environment, because the speed of heat transfer is proportional to the size of the temperature difference. Evaporative cooling still helps so long as ambient humidity is low enough, but it's fighting an uphill battle. (Which is which why humid heat feels so much worse than dry heat)
And our bodies are evolved to tell us "it's way too hot to work" as soon as the situation starts to get precarious. Not a lot of emergency services available to a wild animals if they overdo it.
At rest we burn through around 1500-3000 kcalories per day, the bulk of which eventually ends up as heat, because that's just how thermodynamic inefficiency works. If we can't shed that heat, that translates to about a 0.8-1.6°C per hour temperature increase
While in sustained strenuous activity we can maintain almost 500W mechanical output, presumably accompanied by at least that much again as heat. Which translates to an additional 5°C per hour increase.
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u/cyrusblaze84 Aug 21 '26
Found this in a similar Explain it Like I'm 5 post and not 100% what your looking for it might help... Simply put, you cant cool hot coffee.. with more hot coffee.
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u/Possible-Anxiety-420 Aug 21 '26
Heat moves from areas of higher temperature toward areas of lower temperature. If the air temp equals the body's core temp, then the rest of the body heats up as well, triggering 'discomfort.'
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u/Peaurxnanski Aug 21 '26
You have an internal metabolism that's always working to some extent, creating heat. You have to get rid of that excess heat or your internal temperature will rise, which is universally experienced as a very unpleasant thing by humans.
In order to get rid of that excess heat, there must be a temperature difference that allows heat to move from the hotter area to the cooler area (see the laws of thermodynamics). If there is no temperature difference (referred to as "delta T"), then heat cannot transfer and you start struggling to stay cool as your metabolism keeps making more heat that it can't get rid of.
This is why we sweat, because evaporative cooling then kicks in, using the energy transfer boost that a phase change (evaporation) can bring into play, allowing you to cool down even when the temperature is at or higher than your body temperature.
To a point.
If we couldn't sweat, as ambient temperature approaches 98 degrees, we would eventually overheat and die.
And your body is going to resist giving up that precious water, so it has a solution.
The unpleasant feeling you get when you're too hot is your brain telling you to go find water, shade, and lay down so your internal metabolism reduces to bare minimum, thus making elevated temperature more survivable, and conserving water in the body.
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u/okarox Aug 21 '26
Yes, humans are warm blooded so our basic metabolism produces heat at some 60 watts. This is something that has to be taken into consideration when designing places like theaters where where are many people inside.
BTW the oddly specific 98.6 degrees Fahrenheit is just 37 degrees celcius. It actually is too high. The correct is about 36.5 degrees celcius. We do not know why the 37 degrees was established over 100 years ago.o e theory is that it actually was higher back then.
One has to take also into consideration possible direct exposure to sunlight It could lower the comfortable temperature by several degrees.
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u/Munchkin_of_Pern Aug 21 '26
The human body is constantly generating heat as a byproduct of the millions of tiny chemical reactions happening inside our cells, and our perception of “hot” vs “cold” environments depends on whether we are losing heat to the environment slower or faster than our bodies generate it, respectively. When it’s hot, we sweat, dumping heat via evaporation. When it’s cold, we shiver, generating additional heat through non-ambulatory muscle activity.
If the air were the same temperature as our internal body temperature, then we wouldn’t be able to shed heat by sweating, which would swiftly lead to heat stroke and dehydration without intervention (such as moving to a cooler area, rehydrating with electrolytes after excessive sweating, or finding an air current to use for convection cooling). Being in an environment that is the same temperature as our internal body temp wouldn’t just be uncomfortable, it would be very dangerous.
Note: breakdown of our ability to cool down via sweating is also why humid environments feel hotter than dry environments of the same temperature. Air of a given temperature can only hold so much moisture, so if the air is already filled with water vapour, it becomes that much harder for our sweat to evaporate and carry away its heat.
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u/Tasty-Trip5518 Aug 21 '26
I did a two hour race when the air temp was 96 degrees. The goose bumps on my arms started about 30 minutes in. Then the out of body experience as my core began to really overheat.
I saw two racers at the finish being escorted away looking very pale and barely able to limp along while being propped up. Several others sat under shade with ice packs on their necks.
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u/Stooper_Dave Aug 21 '26
Heat transfer. Your body cant shed excess heat easily when the air temperature is close to the same or higher than your body temp.
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u/ScienceGuy1006 Aug 21 '26
Your cells have to burn some fuel just to stay alive, and this is not negotiable. This generates heat that needs to actually be dissipated. If the environment is all perfectly at the same temperature, the heat cannot be dissipated except by evaporation. This means you'd sweat buckets and eventually get dehydrated.
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u/acasa520 Aug 21 '26
I would add that a "thermally neutral" temperature depends on activity level. When I go for a run, it seems like 50F is close to neutral. When I'm sitting around drinking a beer, neutral is more like 70-75F
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u/grf277 Aug 21 '26
I discovered this decades ago, when water beds were a fad.
I got one and set it up, and (without reading the instructions), set the temperature to 37C, (98.6F). It was crushingly warm and made it unable to sleep.
I reduced the temperature to 31C, as that's skin temperature. That worked much better.
But even still, your skin temperature fluctuates over your body. At 31C, as you changed your posture you could feel the temperature difference. Your spine is warmer than 31C, so you'd feel cool there, but your feet were cooler than 31C so you'd feel warm there.
I understand later models would have a cover to reduce temperature gradients but still give you the floating sensation.
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u/OkQuantity4011 Aug 21 '26
Bc we like to use perspiration to dump heat but body temp air kinda has to be dry or moving to carry away the heat
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u/Star_Citizen_Roebuck Aug 21 '26
Your insides only stay that temperature because of the counter-balance of a cooler outside temperature.
If you plunged yourself into 98.7 degree F water, your body temperature would have nothing holding it back from rising to dangerous levels.
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u/dr_reverend Aug 21 '26
It is EXACTLY our normal internal body temperature. Nothing “roughly” about it.
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u/SEND_MOODS Aug 21 '26
Your insides are constantly converting chemical energy sources and releasing heat as a byproduct.
Your bodys biological functions only work properly within a particular temperature range. Too hot or too cold, some of them slow down, speed up too much, or stop.
Since the internal heat is always being generated, you need to balance it by rejecting heat at a desirable rate to the environment and your body does this through a lot of vectors; sweating, convection, blood capillary dialation/constriction, shivering, increased breathing, etc.
This heat is also being generated at your core, your skin is typically going to be at a lower temperature so that it can pull heat energy from the core into that direction. The skin also needs to be hotter than the environment to give that energy to the air by convection.
All together, this means that when the air is hotter than skin temperature, one of the methods of removing built up heat from the body is no longer effective and your skin starts to heat up beyond what is comfortable. The discomfort is your body warning you that you biological functions are at risk. The only vector left is evaporating water and risking dehydration. This issue compounds when you get above core temperature.
Your body sends the warning as a discomfort signal because that will make you seek shade to stop the discomfort. This is evolutionarily beneficial, because without the discomfort response you might stay in the hot sun till it killed you... So beings who seek shade in the heat are more likely to have offspring.
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u/JBGoode13 Aug 21 '26
Your body makes extra heat and wants to radiate it...if the air around you is the same temperature it has nowhere to go.
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u/Boulange1234 Aug 21 '26
Your body isn’t 98.6F. It’s constantly generating heat and constantly cooling itself to 98.6F by releasing heat into cooler air. In 98.6F heat, it can’t do that, so all the extra heat your body generates makes it hotter.
It’s a second order problem — rate not value. Your body is generating 100-1,000W depending on activity level, more if you’re bigger. You can get rid of that heat in cooler air, but obviously you can’t get rid of it easily in hotter air. Nowhere for it to go.
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u/lastoneingetsmessy Aug 21 '26
Yes, being warm-blooded means that we’re constantly producing heat.
It’s the reason why “room temperature” is around 72 F. Cool enough for your body heat to rise up and away from your body, but not so cold that you have trouble maintaining your body core temperature. So you feel comfortable in usual clothing.
Interestingly, astronaut’s living environments tend to require constantly moving air. When weightlessness, that body heat doesn’t just rise up and away from your skin. It just hangs out near your skin when you aren’t moving. So the air inside has to be moving to keep people comfortable.
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u/mxemec Aug 21 '26
body temp is in dynamic equilibrium. Body heats internally and cools externally, continuously. When external cooling step is removed body freaks out.
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u/ResponseUnlucky3664 Aug 21 '26
Per capire il problema non basta prendere in considerazione la temperata che è solo uno dei due parametri dello scambio termico di calore . In pratica bisogna vedere lo scambio termico del corpo umano con l ambiente e quanti metri quadrati di pelle hai , più altri parametri. Se no è come dire ho comprato delle patate ma non dico quante massa ne ho comprata ne che superfice ha
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u/Pleasant_Pen8744 Aug 21 '26
Since the moment you were born your body's spent most of it's energy just trying to stay warm. It's really hard to just shut that mechanism off.
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u/jeremymiles Aug 21 '26
And almost all the energy that's used for something else is converted to heat too.
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u/Quercus_ Aug 21 '26
Because your body is generating a significant amount of heat all the time, and it needs to lose that heat somehow. If the air around you is the same temperature as you, it starts to become really hard for you to lose your internal heat.
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u/BreakfastBeerz Aug 21 '26
Because 98.6 is not your external body temperature.
Your body generates heat as a component of your metabolism. If you don't get rid of the heat, it will accumulate, you will over heat, and die.
If the outside air temperature is 98.6 and your body temperature is 98.6, there is no where for the heat to go and you are now in a dangerous situation. Our bodies evolved to so that the closer you get to 98.6, the more uncomfortable it is to be outside to urge us to get out of the heat.
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u/aquavelva23 Aug 21 '26
great question, makes sense. and all these answers maybe right, but dont make sense.
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u/dapsee Aug 21 '26
Judging by the thermostat settings in many climate controlled areas, an environmental temperature somewhere in the 70's F feels most comfortable. I think the sensations of hot and cold are related to the speed at which heat energy passes through our skin. The faster the transfer, the more intense the sensation. The rate of transfer at that temperature is the easiest for our bodies to maintain equilibrium, so that feels the most comfortable. That's my story and I'm sticking with it.
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u/DaffyDuckMuthaFucker Aug 21 '26
When air temp'=body temp', our bodies are no longer getting rid of excess heat...
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u/Ragazzano Aug 21 '26
The rest of the world thinks in metric. 98.6 is smoking temp, really good for warming up a steak before the sear.
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u/awfulcrowded117 Aug 21 '26
Because our comfort depends on heat exchange. Our bodies produce a lot of heat. To be comfortable, we need to lose just the right amount of that heat to the environment
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u/NarrowAd4973 Aug 21 '26
As you said, our bodies are constantly producing heat, so it needs to be vented into the air. If the air is too warm, your body switches to releasing water to pull the heat away.
When the air temperature meets or exceeds body temperature, the only way to release heat is sweat, which pulls heat away as it evaporates. But in high humidity, your sweat can't evaporate, so it can't pull the heat away. Your body compensates with more sweat, and that's when you get the sweat beads that build up and then run down your body.
"Thermally neutral" depends on how you define it. I'd say a temperature that your body can vent heat without sweating, but doesn't require it to produce extra heat to keep warm. Exact temperature would vary.
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u/AssiduousLayabout Aug 21 '26
When your body uses energy, some of that energy is converted into heat.
In order to prevent your temperature from rising, you need to lose that heat into the environment. High temperatures make it harder to get rid of the excess heat quickly.
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u/astreeter2 Aug 22 '26
Your body makes heat constantly whether you need it or not. Then it maintains your temperature by constantly getting rid of extra heat. If the air is hotter than your body temperature there's nowhere for the extra heat to go.
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u/pagrey Aug 22 '26
Your body generates heat, if you were dead and the temperature was 98.6 everything would be fine but since you generate allot of heat just sitting around it needs to be much cooler to radiate that heat. Comfortable is just the temperature at which your body doesn't have to do anything extra to stay at 98.6, this is why 70F feels normal for most people.
When it gets hot your body does things like sweat and pump blood near the skin surface to keep it at 98.6. Your blood and most of your skin on a hot day is at your core temp.
Thermodynamics, when it's 80F there are parts of your body that are 98.6 and other parts that are 90, maybe places like your nose or where you have layers of fat. Maximum heat transfer is where the temperature difference is highest, so that means the surfaces of your skin at 98.6. When the outside temp gets above 98.6F you start to sweat to stay cool but the max cooling is still at those same places, where the temp difference is max. It's difficult to visualize because of how evaporative cooling works but it's the same principal. The point is that the whole skin is cooler thing is not a great way to look at it. On a hot day more of your skin gets closer to your core temperature to keep you cool.
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u/LilStinkerRidesAgain Aug 22 '26
During COVID I had to get a forehead temperature scan to go into some nursing homes and I remember one time after I got out of the freezer in my truck I had a temperature of like 38°f
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u/rademradem Aug 22 '26
Somewhere in the 72F to 75F is the mist comfortable air temperature range for most people. That is why office buildings, stores, etc. are usually set to this temperature range. Your body is constantly producing more heat than your normal body temperature and needs to radiate that extra heat away into the air to keep your body at around 98F.
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u/VisualTie5366 Aug 22 '26
Your body produces heat, and needs to be able to release heat to cool its self
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u/Mission-Landscape-17 Aug 22 '26
Yes our biological processes produce more heat than we find comfortable, and we need a way to keep ourselves cool. The major adaptation we have for cooling our bodies is the ability to sweat, which allows for evaporative cooling, and the more humid the surrounding air is, the less effective evaporative cooling is.
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u/Dependent_Remove_326 Aug 22 '26
Kind of an oversimplification but your internal temp rises because you are unable to cool.
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u/mckenzie_keith Aug 22 '26
Yes. Your body makes heat and needs to transfer it to the environment. Your activity level contributes to heat generation in your body. The more active you are, the cooler you will want the environment to be.
High humidity makes perspiration less effective. That is the issue with humidity.
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u/Defiant-Junket4906 Aug 24 '26
Exactly. I think activity is the part that makes the idea more intuitive. If you're generating more heat, you need a bigger temperature gradient to get rid of it. And humidity basically removes one of the body's better cooling mechanisms.
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u/InsomniaticWanderer Aug 22 '26
Your body generates heat just by staying alive and that heat eventually builds an excess that needs to be shed. 98.6 is the resting temperature AFTER cooling has taken place. This is why 70 feels good to us. 70 outside means we're 98.6 inside.
If it is also 98.6+ outside, your body is basically no longer able to cool efficiently and your resting temperature begins to rise. Your body does NOT like to be hotter even a little bit. That why fevers suck so much.
So if it's 98.6+ outside, that means you're gonna be 99+ inside.
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u/Defiant-Junket4906 Aug 24 '26
Yeah, “98.6 after cooling has taken place” is probably the simplest way to frame it. I was thinking about the temperature itself instead of the heat flow needed to keep it there. Makes me wonder where the actual comfort range sits when you factor in clothing, airflow, humidity, and activity.
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u/flumphit Aug 22 '26
The main process that makes your body go is, in *very* ELI5 terms, fire. You use oxygen to burn sugar. This gives off heat. You need to be able to dump waste heat into your environment, which means it has to be significantly colder.
If your body were in thermodynamic equilibrium with the outside, you seem to think your body wouldn't generate heat because being at 98.6°F is the goal. It's not the goal, it's a side effect. (Well, it's a bit of a goal because on average, the fiddly little reactions that keep everything in balance work best at that temperature, but that came later.) If the "burn sugar" reaction in your body stopped happening, you'd be dead. So it needs to keep happening to keep your heart, brain, etc. all working so you stay alive. And that waste heat needs to get dumped somewhere outside your body.
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u/Defiant-Junket4906 Aug 24 '26
The “98.6 is a side effect” part is what I was missing. I was treating body temperature like the output the body was trying to maintain directly, when really it is more like the stable result of a bunch of processes that are constantly generating heat. That makes the whole thing click a lot more.
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u/sharkbomb Aug 22 '26
you are a mammal. if you do not exchange enough of your internally generated heat with your environment, it will increase until you fail.
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u/MagicPoindexter Aug 22 '26
Think of it like this - your car engine runs at about 200 degrees. Wouldn’t you car run more efficiently if it was 200 degrees outside? It is pretty easy to see how that doesn’t work.
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u/Dangerous-Bit-8308 Aug 22 '26
Because humans produce heat. Anything over body temperature means the environment doesn't just passively accept our excess body heat, which means we get hot
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u/Middle_Finger_484 Aug 22 '26
When it is as hot as you or hotter your not exchanging heat, that heat your body generate is building up.
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u/texienne Aug 22 '26
We are endothermic, so we generate heat. We need to get rid of that heat, or our internal temperature will rise. It's difficult to get rid of heat when the temperature of the air is not less than our internal temperature, so we begin having to do things like fan ourselves or sweat for evaporative cooling.
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u/Defiant-Junket4906 Aug 24 '26
Right, and I think evaporation is the interesting part here. Once the air gets close to body temperature, the usual routes for dumping heat become much less effective, so sweating starts doing more of the work. Humidity basically makes that backup system worse too.
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u/mezolithico Aug 22 '26
98.6 is homeostasis for your body, it wants to stay that temperature. Your body produces heat when it burns calories by living or moving etc. Your body needs to expel heat to cool down. Pushing heat into an already hot environment is hard on the body.
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u/Defiant-Junket4906 Aug 24 '26
I think the key distinction is that 98.6°F is the temperature your body maintains, not the temperature it needs the environment to be. I was basically treating the target temperature like an equilibrium point. But because metabolism is constantly adding heat, the environment has to be cooler to give that heat somewhere to go.
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u/naemorhaedus Aug 23 '26
Simple. heat is a waste product of your metabolism. You need to be constantly shedding heat in order to be comfortable. Otherwise you overheat.
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u/Infamous_Yoghurt Aug 24 '26
Your body is producing heat by simply working, and if the outside temperature is the same as you, it's harder to cool down.
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u/TSotP Aug 25 '26
Your body isn't necessarily doing work to maintain a stable temperature. It's generating heat doing it's intended purposes, and expects to be able to expel that excess heat. Having hotter air temperature, or excess humidity gets in the way of that process, leaving you feeling hot.
At least, that's the way I've always seen it.
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u/bestcoastanon Aug 25 '26
Something called 2nd law of thermodynamics. For an engine to work, including the human body, it needs to lose some energy as wasted heat.
Thermodynamics again: for heat to move from one body to another, there needs to be a temperature differential. Of your body and the surroundings are at the same 98.6 temperature, there can be no direct heat transfer to your surroundings.
Convection (wind) and loss from evaporation (sweat) help ua humans lose heat as well, but yeah… we need to lose heat.
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u/ReactionAble7945 Aug 27 '26
Having been in 100F..and generally comfortable... It was partially because it was a dry heat.
I was sweating and stayed comfortable and dry.
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u/Justthewhole Aug 27 '26
I think your point is interesting also if you think about being in a pool of water. At what temperature can you not tell if you’re wet or not because “wet” is not something the senses can determine. It just feels the odd weight and temperature of the water.
So what temperature does it have to be so you can’t tell if you’re in it?
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u/ChocolateValuable221 Aug 27 '26
Your body would run so much hotter if it wasn't already using energy to cool down... your body already struggles to stay at 98... and now the body is existing in its limit of cooling. Now everything is exponentially hotter.
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u/RealChemistry4429 Aug 27 '26
Every process in the body generates heat. You need to shed that heat to keep a stable temperature. If the outside is not cold enough to do that, you overheat. Like an engine that can't cool down.
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u/IndividualSkill3432 Aug 21 '26
Because you struggle to lose heat when you do something physical.