how much heat radiates from a person
December 23, 2009 12:51 PM Subscribe
How much heat (preferably in W or KW) radiates from the average person. (that is if they were fully clothed on an average day)
Very roughly 50W (converting from what's given here)
posted by fearfulsymmetry at 1:08 PM on December 23, 2009
posted by fearfulsymmetry at 1:08 PM on December 23, 2009
Really, really roughly, about the same as a lightbulb. More than 40W, less than 100W. 50 is probably a good estimate.
posted by GuyZero at 1:10 PM on December 23, 2009
posted by GuyZero at 1:10 PM on December 23, 2009
The link that fearfulsymmetry gives sort of demonstrates the problem with this question as phrased. Given that there are naturally more children than there are adults in the world, you can't infer the average person from the average adult.
So: are we talking about the average adult's energy?
posted by dfriedman at 1:44 PM on December 23, 2009
So: are we talking about the average adult's energy?
posted by dfriedman at 1:44 PM on December 23, 2009
It's going to be the amount of energy taken in as food minus what is spent in metabolism, respiration, and movement. I'd guestimate those minuses add up to about half of intake and that the average adult takes in about 2000 kcal per day. So convert 1000 kcal/day into Watts.
Humans, like all other objects, radiate throughout most of the electromagnetic spectrum from the near ultraviolet down through radio frequencies, with the peak being in the infrared. Your radio emissions are readily detected with a small feedhorn and a simple receiver. I got to play with such a device at NRAO in Green Bank a few years ago; the feedhorn had an aperture of about 4 inches.
posted by neuron at 1:51 PM on December 23, 2009
Humans, like all other objects, radiate throughout most of the electromagnetic spectrum from the near ultraviolet down through radio frequencies, with the peak being in the infrared. Your radio emissions are readily detected with a small feedhorn and a simple receiver. I got to play with such a device at NRAO in Green Bank a few years ago; the feedhorn had an aperture of about 4 inches.
posted by neuron at 1:51 PM on December 23, 2009
Wait...Everyone's focusing on the total amount of heat generated. Isn't some of that heat radiated and some of it conducted away through the air?
posted by miyabo at 1:56 PM on December 23, 2009
posted by miyabo at 1:56 PM on December 23, 2009
I would actually not approach this as a biology problem but as a thermodynamics one, especially if we are talking strictly about radiation. Body temperature is pretty well nailed down. Conduction to air is pretty low, especially clothed. That leaves convection (also low, I'm guessing) and radiation.
I don't really know how much of body heat loss is to each of those factors, but we can at least determine a lower bound by sticking to radiation. In addition to body temp, you'll need exposed body area and the external "radiative temperature" (there's probably a term for that) using a formula like this.
posted by DU at 1:59 PM on December 23, 2009
I don't really know how much of body heat loss is to each of those factors, but we can at least determine a lower bound by sticking to radiation. In addition to body temp, you'll need exposed body area and the external "radiative temperature" (there's probably a term for that) using a formula like this.
posted by DU at 1:59 PM on December 23, 2009
My copy of Boron & Boulpaep's Medical Physiology says that `the body's rate of heat production can vary fom approximately 80 kcal/h [93 Watts] at Rest to 600 kcal/h [697 Watts] during jogging'.
And `radiation of heat from the body accounts for approximately 60% of heat lost when the body is at rest in a neutral thermal indoor environment. A neutral thermal environment is a set of conditions . . . in which the temperature of the naked body does not change when the subject is at rest".
So I would estimate it as somewhere between 56W and 420W (a very high upper limit - you could not sustain the level of physicla activity necessary for this much heat production for very long at all). So GuyZero is probably about right.
IIRC, there is a number quoted in Guyton & Hall, but I don't have a copy at home.
posted by James Scott-Brown at 2:01 PM on December 23, 2009 [1 favorite]
And `radiation of heat from the body accounts for approximately 60% of heat lost when the body is at rest in a neutral thermal indoor environment. A neutral thermal environment is a set of conditions . . . in which the temperature of the naked body does not change when the subject is at rest".
So I would estimate it as somewhere between 56W and 420W (a very high upper limit - you could not sustain the level of physicla activity necessary for this much heat production for very long at all). So GuyZero is probably about right.
IIRC, there is a number quoted in Guyton & Hall, but I don't have a copy at home.
posted by James Scott-Brown at 2:01 PM on December 23, 2009 [1 favorite]
I was also going to point you to an article about black-body radiation; fearfulsymmetry's link should tell you all you need to know.
To the level of precision you are talking about here, note that you could calculate this in a couple different ways. One is to use the fact that humans should radiate more or less as "black bodies," which tells you that their rate of energy output (i.e. their luminosity) should be proportional to their surface area times their temperature to the fourth power. (The wikipedia entry on black-body radiation calculates the net emission from a person in a room-temperature bath -- which is dependent on the difference between the person's temperature and the ambient temperature, again to the fourth power. That's how you calculate a number in the 50-100 Watt range -- if you just used the absolute temperature, you'd come up with a much higher number.) Alternatively, you could guess at the same number by figuring that the average person takes in a couple thousand food calories per day, or about 8 million joules; they'd better not radiate way way more than that in the course of the 86400 seconds in that day. That method, also, would give you a net luminosity of around 100 joules/second -- i.e. 100 Watts.
Most of that energy is radiated in the infrared -- which is why everyone around you doesn't seem as bright as a 100 Watt lightbulb (unless you have infrared eyes).
posted by chalkbored at 2:03 PM on December 23, 2009 [2 favorites]
To the level of precision you are talking about here, note that you could calculate this in a couple different ways. One is to use the fact that humans should radiate more or less as "black bodies," which tells you that their rate of energy output (i.e. their luminosity) should be proportional to their surface area times their temperature to the fourth power. (The wikipedia entry on black-body radiation calculates the net emission from a person in a room-temperature bath -- which is dependent on the difference between the person's temperature and the ambient temperature, again to the fourth power. That's how you calculate a number in the 50-100 Watt range -- if you just used the absolute temperature, you'd come up with a much higher number.) Alternatively, you could guess at the same number by figuring that the average person takes in a couple thousand food calories per day, or about 8 million joules; they'd better not radiate way way more than that in the course of the 86400 seconds in that day. That method, also, would give you a net luminosity of around 100 joules/second -- i.e. 100 Watts.
Most of that energy is radiated in the infrared -- which is why everyone around you doesn't seem as bright as a 100 Watt lightbulb (unless you have infrared eyes).
posted by chalkbored at 2:03 PM on December 23, 2009 [2 favorites]
DU, for a naked man, about 22% of heat loss is due to evaporation, and 15% due to conduction to air: together, they're quite significant. Obviously, clothes will reduce these proportions.
neuron, 1000 kcal/day is about 48 Watts
My comment about Guyton & Hall was incorrect; I've found a copy, and had mis-remembered a section (it simply says that 60% of heat loss via radiation, without giving a corresponding power). Frustratingly, neither Amazing Numbers in Biology, nor BioNumbers have an entry for typical radiative heat losses from a human.
posted by James Scott-Brown at 2:15 PM on December 23, 2009
neuron, 1000 kcal/day is about 48 Watts
My comment about Guyton & Hall was incorrect; I've found a copy, and had mis-remembered a section (it simply says that 60% of heat loss via radiation, without giving a corresponding power). Frustratingly, neither Amazing Numbers in Biology, nor BioNumbers have an entry for typical radiative heat losses from a human.
posted by James Scott-Brown at 2:15 PM on December 23, 2009
Cooling of the human body. Based on a lot of assumptions about what constitutes an "average" person (although with neat calculators so you can alter those assumptions), and notably looking at an unclothed person, does indicate that radiation far outweighs other methods of cooling at/around "room temperature" and gives a figure of 133W.
posted by DevilsAdvocate at 2:17 PM on December 23, 2009
posted by DevilsAdvocate at 2:17 PM on December 23, 2009
Response by poster: To clear up the assumptions that you have all made, saying that there are more children than adults; thus making the average person in their teens or such, is avoiding the question completely. For this purpose, the average human in my eyes is someone between 5'7' - 5'11'. and weighs about 120 lbs
I will rephrase the question then... That is, how much heat would be radiated as thermal heat energy (not infra red).
posted by sockpim at 2:38 PM on December 23, 2009
I will rephrase the question then... That is, how much heat would be radiated as thermal heat energy (not infra red).
posted by sockpim at 2:38 PM on December 23, 2009
Radiated thermal heat energy is infrared. (Mainly infrared, for humans or other objects at or around human body temperatures.)
posted by DevilsAdvocate at 2:42 PM on December 23, 2009
posted by DevilsAdvocate at 2:42 PM on December 23, 2009
Response by poster: Fair enough then, that's just my lack of terminology.. apologies.
On another update with the information, I have been cold all evening, and have just been standing in a room with four adults and am already warm.. How could this be if the average person only emits 50 watts..?
Cheers
posted by sockpim at 2:46 PM on December 23, 2009
On another update with the information, I have been cold all evening, and have just been standing in a room with four adults and am already warm.. How could this be if the average person only emits 50 watts..?
Cheers
posted by sockpim at 2:46 PM on December 23, 2009
Here's some possibilities; it's probably a combination of these plus other effects I haven't considered.
1. How long were the other people in the room before you got there? Wattage is actually a measure of power, or energy per unit time (and heat is a form of energy); a 50W-bulb generates 60 times as much heat in an hour as it does in a minute.
2. Your net radiative heat loss is significantly reduced because you're absorbing some of the radiative heat from the four other people there. And if it seems like one human generates more heat than a bulb in the 50-100W range, it may be because you're closer to them; unless you're the sort of wallflower seen mainly in 1940s cartoons, you stand closer to other people than you do to the lamp in the corner.
3. The energy generated by incandescent bulbs isn't entirely in (or eventually converted to, at least within the room you're in) the infrared; in fact, since their primary purpose is to light, not to heat, quite a bit of the energy is put out as visible light, and any visible light that escapes through windows, doors, etc., does not end up heating the room.
4. You're underestimating just how much of an effect those bulbs would have, especially if humans tend towards the upper end of the range suggested here; I think a ordinary living-space-sized room would be significantly warmed by 4 100-watt incandescent bulbs.
5. Humans are very sensitive to temperature, particularly near their comfort zone, so the perceived difference between "warm" and "cold" is disproportionate to the heat lost. Put me in a long-sleeved, button-down shirt in a 68°F room, and I want to put on a sweater; in a 72°F room I want to switch to short sleeves. When a parent puts a hand on the forehead of a sick child, the difference between "well, at least you don't have a fever" and "you're burning up!" is 3 or 4°F.
6. The room might have been warmer than wherever you were before for reasons independent of the people in it.
posted by DevilsAdvocate at 3:53 PM on December 23, 2009
1. How long were the other people in the room before you got there? Wattage is actually a measure of power, or energy per unit time (and heat is a form of energy); a 50W-bulb generates 60 times as much heat in an hour as it does in a minute.
2. Your net radiative heat loss is significantly reduced because you're absorbing some of the radiative heat from the four other people there. And if it seems like one human generates more heat than a bulb in the 50-100W range, it may be because you're closer to them; unless you're the sort of wallflower seen mainly in 1940s cartoons, you stand closer to other people than you do to the lamp in the corner.
3. The energy generated by incandescent bulbs isn't entirely in (or eventually converted to, at least within the room you're in) the infrared; in fact, since their primary purpose is to light, not to heat, quite a bit of the energy is put out as visible light, and any visible light that escapes through windows, doors, etc., does not end up heating the room.
4. You're underestimating just how much of an effect those bulbs would have, especially if humans tend towards the upper end of the range suggested here; I think a ordinary living-space-sized room would be significantly warmed by 4 100-watt incandescent bulbs.
5. Humans are very sensitive to temperature, particularly near their comfort zone, so the perceived difference between "warm" and "cold" is disproportionate to the heat lost. Put me in a long-sleeved, button-down shirt in a 68°F room, and I want to put on a sweater; in a 72°F room I want to switch to short sleeves. When a parent puts a hand on the forehead of a sick child, the difference between "well, at least you don't have a fever" and "you're burning up!" is 3 or 4°F.
6. The room might have been warmer than wherever you were before for reasons independent of the people in it.
posted by DevilsAdvocate at 3:53 PM on December 23, 2009
DevilsAdvocate, regarding point 3, despite the fact that the primary purpose of incandescent bulbs is to light, not heat, only a tiny fraction of their output is in the visible spectrum. A traditional tungsten bulb might emit 2.5% of its consumed energy as visible light. A good quality quartz halogen might do 50% better. The rest is heat, in one form or another.
I agree with your other points.
I also want to amplify the notion that the hourly heat output of the average adult is in the area of 100W - 150W, based on an average daily intake of 2000-3000 Calories (kCal). The amount of stored potential energy from "work" done through the day is likely small.
One thing I'm not sure about is the variation in metabolic rate throughout the day, but I'd guess for someone just sitting around, it might be 1/2 the average.
posted by Good Brain at 4:41 PM on December 23, 2009
I agree with your other points.
I also want to amplify the notion that the hourly heat output of the average adult is in the area of 100W - 150W, based on an average daily intake of 2000-3000 Calories (kCal). The amount of stored potential energy from "work" done through the day is likely small.
One thing I'm not sure about is the variation in metabolic rate throughout the day, but I'd guess for someone just sitting around, it might be 1/2 the average.
posted by Good Brain at 4:41 PM on December 23, 2009
To view this from an entirely different discipline, when calculating how much air conditioning a room will need, you can figure 500 BTU/hour per person, or about 150 watts.
posted by exphysicist345 at 5:09 PM on December 23, 2009
posted by exphysicist345 at 5:09 PM on December 23, 2009
Best answer: In architectural planning for solar heating systems we figure about 100W per at rest person. Jives with exphysicist345 as you want to over compensate on the high side for A/C and under compensate on the low side for heating.
posted by Mitheral at 6:18 PM on December 23, 2009 [1 favorite]
posted by Mitheral at 6:18 PM on December 23, 2009 [1 favorite]
If you just do the straight unit conversion, 2000 kilocalories a day is 100 watts. I once had an air conditioning salesman try to tell me to allow five hundred watts per person. The air conditioner we bought was great: it had way too much capacity and easily kept the room super cold.
It's a mistake to try to separate out the fraction that's radiated from the fraction that's carried off by convection or that goes into generating motion. If you are in a room with other people and their presence heats the room up, their convective heat also heats up the room.
Four 100 watt light bulbs will certainly heat up a living room. Try it.
posted by fantabulous timewaster at 6:20 AM on December 24, 2009
It's a mistake to try to separate out the fraction that's radiated from the fraction that's carried off by convection or that goes into generating motion. If you are in a room with other people and their presence heats the room up, their convective heat also heats up the room.
Four 100 watt light bulbs will certainly heat up a living room. Try it.
posted by fantabulous timewaster at 6:20 AM on December 24, 2009
For event planning, we use the same figure as Mitheral - people in a room generate a lot of heat but the venue always wants to heat up the "cold" empty room, which is a mistake. The math becomes interesting at wine fairs and trade shows, where red wine needs to be "room temperature" but the temperature of the room is hugely impacted by the number of people in it.
posted by DarlingBri at 6:30 PM on December 24, 2009
posted by DarlingBri at 6:30 PM on December 24, 2009
This thread is closed to new comments.
posted by mr. remy at 1:06 PM on December 23, 2009