What is the ideal ambient room temperature for a human being?

Question

I know there is an ideal room temperature where a human being will be most comfortable - it was mentioned in an episode of Star Trek Next Generation (sad I know) but I cannot remember what it is - over to you guys...

Answer 1

An interesting subject is regarding how efficiently a human operates as if it were a machine! We know that automobiles of the 1970s generally operated at around 15% overall thermal efficiency (which has now been increased to around 21%). We know that electric power plants and the grid distribution system is around 13% overall thermal efficiency, regarding the energy that was in the coal or uranium and the electricity that actually gets to us. We know that photosynthesis in plants (such as corn and wheat crops) is only around 1% to 2% efficient regarding the energy received as sunlight. So what about us?
We generally have a decent idea of the energy that we take in as food. A 2,000 calorie daily diet regimen means that the person takes in food that includes 2,000 (kilo-)calories of thermochemical energy in it. No problem there. By the way, 641.2 calories equals one horsepower-hour of energy.

Early in the 20th century, factory owners did many studies regarding just how much actual work they could get out of their employees! It was generally found that a healthy 35-year-old (European) man could use up a total of 0.49 horsepower for an 8-hour shift. If you do the math, you can see that accounted for around 2,500 calories used up during that work shift! People working in such factories HAD to eat enough food each day to account for that!

Unfortunately for those factory owners, only around 0.1 horsepower of that was actually available as useful work. That's about 64 calories per hour or 75 watts. The other 0.39 horsepower (about 256 calories per hour or 1,000 Btu/hr) was used up in basal metabolism and maintaining body temperature. Bummer! This data indicates that such human factory workers were capable of around 20% overall thermal efficiency during their work shift. More recent research has suggested that an optimal efficiency for a human is probably around 25%, but that most existing mechanisms are not able to deal with the herky-jerky way we tend to create such work!

(Younger 20-year-old men could produce 15% more and 60-year-old men could produce 20% less than these values.) The basal metabolism requirements depend on health and environmental conditions, particularly the air temperature. In an extremely cold environment the body must expend even more energy in maintaining body temperature.

When a person is not fully exerting oneself like in those factories, the basal metabolism rate drops somewhat. A sedentary or desk-person needs far less than that 0.39 horsepower rate, actually around 0.16 hp (or according to the ASHRAE Handbook charts, 390 to 450 Btu/hr or around 100 calories per hour). Relatively few Americans are now in factory jobs that are as demanding as those harsh tests considered. Therefore, the necessary daily dietary intake does not need to be as high as it was a hundred years ago.



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The body is interesting in that it chooses to allocate that 0.1 hp of available work as it finds necessary. Digestion is a process that requires quite a bit of energy. If there is no immediate requirement for physical activity or for mental exertion, the body will allocate nearly all of the available work toward digesting a meal. This greatly explains why we are often in a mood to take a nap after a Thanksgiving feast, as the body recognizes that it has a lot of food to digest! Creative thinking also seems harder right after a giant meal!
If there is NOT a large amount of food to digest, the body will allocate most or all of that available work to either mental (thinking) activities or to physical activities or both.

IF you take in more calories in a day than you use up, the extra energy is turned into sugars and fats to be stored away for a possible future need. Equally, if you use up more calories in a day than you take in, some of that fat and sugars is converted back into forms that can become energy.


Where the Energy Goes
There are a LOT of processes that go on in the human body, nearly all of which use up energy! The heart pumps blood, the lungs pump air, and countless chemical reactions occur to accomplish various functions. Virtually all of them result in HEAT being created as a result. In some circumstances, such heat is considered waste. But in the body of any warm-blooded animal, that heat is very important, in maintaining a constant warm body temperature. Many of those chemical reactions cannot happen very well in cold environments, and so the heat that seems wasted is actually accomplishing a very important task. Cold-blooded animals have less efficient chemical processes and so they tend to have less efficient brains and muscles.
In any case, nearly all of that wasted heat eventually gets to the skin to be either radiated or convected away. We can do some rough calculations regarding these things. For heat radiated away, there is a standard equation that describes this so-called Black Body Radiation. The Stefan-Boltzmann Law is that the amount of radiation is equal to a constant (called the Stefan-Boltzmann constant!) times the area of surface times the FOURTH power of the absolute temperature. For a situation where a radiating object is within a room which is at a lower temperature, it becomes = (sigma) * A * (T14 - T24). (We are leaving out here the constants regarding the emissivities of the surfaces).

If all of a human's skin were at the same temperature, this could be easy! We have around 20 square feet of surface area (A) and the constant is 0.1713 * 10-8 Btu/sf/hr/R4. The head is maintained at a fairly high temperature, to ensure clear thinking in case of emergency! The arms and legs tend to be much cooler. The way the body does this is by restricting or permitting blood to flow freely to different areas. (When someone falls in very cold water, the body attempts to nearly completely shut down blood flow to the limbs, to try to conserve body heat for the brain and torso where it is urgently needed.)

For our estimate, let's say that the body attempts to keep the AVERAGE skin temperature to be 5°F above the ambient room temperature. In that case, we would have 20 sf * 0.1713 * 10-8 Btu/sf/hr/K4 * ((78+459)4 - (72+459)4). This is about 104 Btu/hr, which is around 26 calories per hour of radiated heat.

For convective heat losses, we are going to simplify by assuming that there is no wind. Therefore we can use formulas for Natural Convection. A very simplified version gives h = 0.2 * (T1 - T2)1/3, for our situation above, h = 3.42 The convective heat loss is then that number times the surface area (of 20 sf) times the temp difference (5°F) or 340 Btu/hr. (around 85 calories per hour).

This then gives a total of around 110 calories per hour of heat energy sent away from the body. During the night, with covers on the heat loss is generally minimized, so for a ballpark number we could consider a day of losing 110 calories for 16 hours, or 1760 calories per day as being credible.

This amount does not include the "productive" work output which could be as much as 1/4 of that (as noted above). This then accounts for around 2,000 calories of input energy, in rough agreement with what dieticians say.

The numbers above are generally meant to apply to adult men of around 200 pounds weight. Women generally have smaller total surface area and therefore they need to use less energy to maintain their core body temperature, so they tend to need to eat less, lower daily dietary intake. But the reasoning is still completely valid.

Answer 2

I think "room temperature" in most of my chem/bio experiments in college was given at 25 degrees Celsius. That was the assumed experimental temperature unless other information was given in a lab.

The ideal room temperature for a human being will vary with the human being. You are lucky if you can get three or four people in a room together to agree on the thermostat setting. I have a co-worker who wears a polar fleece sweater and uses a space heater under her desk year round. Temperature where I am in July ? 95-105 F!

I would think that the most comfortable temperature would somehow be related to the surface temperature of the individual's skin (slightly higher or lower than that temperature). After all we are really feeling the difference from our own temperature rather than measuring degrees when we perceive the ambient temperature as warm/cool/just right.

Answer 3

In the UK there is a legal lower limit for workers of 13 deg C and the Trades Unions are pushing for an upper limit (which is currently undefined) of 30 deg C. I believe a recommended comfortable working/living temperature is between 18 and 20 deg C.

Answer 4

I'd say 22 Degrees C, but its a matter of preference, isn't it?

Answer 5

human being must adopt himself to any conditiom .

Answer 6

**** man i dont know like 73 degrees ? thats about how i like it

Answer 7

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