Are electric fans safe to use in hot rooms?

August 3, 2011 3:05 PM

Is it safe to run an electric fan in a hot room? I always assumed it was, but a Houston Chronicle blog post claims that doing so will accelerate the body's overheating. That sounds extremely dubious to me, and I wonder if it's simply a variation of the superstitious claim that sleeping near a fan can cause death. In preparation for Hurricane season, I gave battery powered fans to everyone in my family, including my ninety seven year old grandfather, so I'm hoping to clear this up, before any of us actually need the fans. It it weren't for the fact that I'd given these fans to family, I'd probably just laugh it off as sloppy journalism, but I want to be sure that they're safe to use during a power outage. Opinions would be appreciated, obviously.
posted by Beholder to health & fitness (44 answers total)

There's this on the Straight Dope message board--makes sense to me.

Essentially, I think the concern is that the fan turns a hot room into a convection oven. Doesn't sound too off the wall to me.
posted by Admiral Haddock at 3:10 PM on August 3, 2011 [1 favorite]

Can you provide a link to the blog post? I can't find it with a cursory search of chron.com.

The fan motor is generating heat. If you run a fan in a closed room for a length of time, the room will get hotter, and it's possible that it could lead to overheating? But I'm not going to do the math.

If you leave a window open and run a fan (or set up a cross-breeze between two windows), then this won't be a problem at all.
posted by muddgirl at 3:10 PM on August 3, 2011

I think this only makes a difference if the window is closed, right? Presumably once the hurricane is over they'll be able to open the windows. Or is this for use in some sort of closed shelter?
posted by chesty_a_arthur at 3:12 PM on August 3, 2011

I run fans (multiple ones!) pretty much all summer long in my unairconditioned apartment. I have not died yet! Not even once! Of course, I always have a window opened, even when there's a torrential downpour and a tornado watch on, so YMMV.
posted by phunniemee at 3:14 PM on August 3, 2011

Here's the Houston Chronicle link.

Electric fans should only be used in conjunction with an air conditioner. A fan can’t change the temperature of a room; it can only accelerate air movement, and will accelerate the body’s overheating.

Pretty much the convection point I was making above.
posted by Admiral Haddock at 3:16 PM on August 3, 2011

(Which is not to say it's correct!)
posted by Admiral Haddock at 3:16 PM on August 3, 2011

Interesting that a Houston-based site is repeating a common Korean folk myth.
posted by winna at 3:19 PM on August 3, 2011 [9 favorites]

The theory behind this claim is that moving air will more rapidly transfer its heat into your body. However, I would think this only applies when a) the air temperature is greater than body temperature, and b) it is very humid so that evaporative cooling (sweat) does not work.

If those conditions are both true, I can't see how your body would be able to do anything to regulate its temperature, and with or without a fan it is a dangerous environment. The fan would indeed make it worse, but either way you wouldn't want to stay in those conditions for very long.
posted by aubilenon at 3:21 PM on August 3, 2011 [8 favorites]

Here's my take on it.

In the absence of sweat, when the temperature of the room is above your body temperature, more air speed would result in more convection, which would result you feeling hotter than you would in still air.

However, since humans do sweat, as air speed increases, a certain maximum heat removal rate can be reached via sweat evaporation. Once that maximum heat removal rate has been reached, additional air speed will indeed make you hotter.

Basically, this is the equation for thermal homeostasis when the air temperature is above body temperature:

Rate of heat production in body = Rate of heat dissipation due to convection (inversely related to air speed) + Rate of heat dissipation due to evaporation (directly related to air speed)

I'll think about it to see if I can estimate what that optimal air speed is.
posted by Salvor Hardin at 3:23 PM on August 3, 2011 [1 favorite]

a) the air temperature is greater than body temperature, and b) it is very humid so that evaporative cooling (sweat) does not work.

So basically, Texas in the summer? At what humidity does evaporative cooling become ineffective?

but either way you wouldn't want to stay in those conditions for very long

Yeah, the main point is that people should seek out air conditioning during the hottest days, rather than sit under a fan. During a hurricane-caused power outage (when finding air conditioning is unlikely), it's also unlikely that temperatures will be over 100°

The problem with "Korean fan death" isn't the concept, it's their explanations (hypothermia? Seriously?)
posted by muddgirl at 3:24 PM on August 3, 2011

Looks like it came from here:

http://www.houstontx.gov/health/NewsReleases/heatplan.html

Contacts: Kathy Barton
832-393-5045

Porfirio Villarreal
832-393-5041
posted by cashman at 3:25 PM on August 3, 2011

I think you can actually feel this happening. I've been in broiling situations where it was obvious that a fan blowing hot air on me was only making me hotter. 99% of the time a fan makes me feel cooler. You could warn people that if a fan blowing on them makes them feel even hotter to then listen to their body and turn it off...
posted by zeek321 at 3:28 PM on August 3, 2011

Keep in mind that the elderly don't sweat as efficiently as younger people, which is, in part, why the elderly are more prone to heatstroke. Often, these heat warnings are written with the elderly in mind.
posted by Admiral Haddock at 3:30 PM on August 3, 2011

So basically, Texas in the summer? At what humidity does evaporative cooling become ineffective?

Well, it trails off as the dew point approaches body temperature. The record highest dew point though is 94 degrees on the Ethiopian coast. If the dew point were higher than your body, you would have water condensing on you, which would be imparting you with heat more quicky than just the air would! So avoid that.

I'm sure there's a point at which the heat from the air and the cooling from evaporation cancel out, but I don't know what it is. Below that point the more wind the better. Above that point, wind does more harm than good and you need to find some other way to cool yourself. It's a function of both humidity/dew point and air temperature.
posted by aubilenon at 3:34 PM on August 3, 2011

Thinking of the fans blowing at full speed on PATH stations in the dead of summer, I feel there might be something to this. They never seem to accomplish anything but project a steady stream of hot air onto the body. Don't think it's lethal though, just not very effective.
posted by monospace at 3:50 PM on August 3, 2011

Skin temperature doesn't equal body temperature. 98.6F is normal core temperature, but the temperature at your extremities or at your skin's surface can be lower. So it's possible that in extreme heat/humidity, that the dew point could be above skin-surface temperature, and in that case the fan might make the situation worse. (I don't have mine around, but you could point an IR thermometer at yourself and figure out what your skin temperature is. I'd bet it's probably around 95-98 on your face/neck/torso in a hot room when you're flushed anyway.)

But in most situations (dew point below skin temperature, you are sweating) the fan is going to make you feel cooler by aiding evaporation.

Of course, in a totally sealed room any electrical device is going to make it warmer. If a fan takes 5W from a battery or the wall outlet, it's going to be putting 5W into the room.
posted by Kadin2048 at 4:00 PM on August 3, 2011

According to weather underground, the current conditions and Port of Houston are 102°F, 45% humidity, dewpoint 77°F, heat index 120°F. That sounds pretty dangerous with or without a fan.

Again, temperatures are generally (but not always) much lower during or after a hurricane.
posted by muddgirl at 4:03 PM on August 3, 2011

Oh for heaven's sake.... I've got to call bull on this. Look, if you're a) hot, and b) sitting in front of a fan, do you feel hotter? Or do you feel cooler? It's as simple as that.
posted by easily confused at 4:07 PM on August 3, 2011 [7 favorites]

Assuming the exceptionally hot air in Texas these days is as dry as the rainfall stats would suggest (from muddgirl's stats), telling your kinfolk to open the windows, and use the fans to direct air flow at them, in conjunction with wet terry cloth body coverings (like wet bath towels), seems a pretty reasonable way of improving their immediate emergency situations over just sitting in an arbitrarily hot, unvented room, doing nothing.
posted by paulsc at 4:08 PM on August 3, 2011 [1 favorite]

Sweating can cool you, even when the temperature outside your body is higher. The thermal energy in the sweat, keeping it at the temperature of your body, is carried away when the sweat evaporates. The act of evaporation doesn't cool the surface of your body when the temp outside is way hot but your core temp can be reduced. As long as your sweat can evaporate, a fan will help.

Some quotes from wikpedia:
There are four avenues of heat loss: convection, conduction, radiation, and evaporation. If skin temperature is greater than that of the surroundings, the body can lose heat by radiation and conduction. But if the temperature of the surroundings is greater than that of the skin, the body actually gains heat by radiation and conduction. In such conditions, the only means by which the body can rid itself of heat is by evaporation. So when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise.[7] During sports activities, evaporation becomes the main avenue of heat loss.[10] Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.[11] reference

The process of sweating decreases core temperature, whereas the process of evaporation decreases surface temperature.reference

Someone please call bullshit if my knowledge of sweating is wrong.
posted by Foam Pants at 4:23 PM on August 3, 2011

Ultimately it's just crap reporting, but I think the point is to please put the shotgun away, papaw, and let the nice firemen take you to the local high school cafeteria because you don't have A/C and it's 108/70% humidity* and no, having a fan is not a good reason to decline.

*Note to those who've never been: Air in Houston and environs is what you might call swampy. Doesn't matter if it never rains, there's a big old gulf right there to provide the disgusting, refinery-scented moisture.

You're not going to kill your relatives with fans. It's rarely that hot during hurricane season, and they're probably not going to refuse better help just because they have a fan.
posted by Lyn Never at 4:26 PM on August 3, 2011 [1 favorite]

Where else are you supposed to run a fan, if not in a hot room? I've never heard that claim before, and it sounds kind of ridiculous. The motor might heat up the air a little, but certainly not enough to be dangerous.

Seriously. That's what fans are for, hot rooms.
posted by Koko at 4:29 PM on August 3, 2011

I was in Death Valley with an air temperature of 115F and very low humidity. I was certainly sweating, and my sweat was evaporating. I can attest to the 'convection oven' effect when the wind blew. It was very weird and felt a lot hotter.

Evaporation of sweat produces lower temperature on your skin surface and sets up a thermal gradient across the boundary layer of still air next to your skin (maybe half and inch or less). This is like an insulation layer keeping cooler air next to the skin. Moving the air with a fan (or wind) disturbs this boundary layer and allows the hot room air to be right next to the skin.

So I would agree that in very extreme heat using a fan could accelerate overheating. The pros are saying that a heat index of 108-110 qualifies as extreme.
posted by huckit at 4:55 PM on August 3, 2011 [1 favorite]

Basically they took what should have been a specifically targeted message (poor elderly who feel vulnerable leaving windows open and too poor to use a/c during an extreme heat event) and applied it to the general population where it's not so relevant.
posted by mandymanwasregistered at 4:55 PM on August 3, 2011

The only time I've seen it discussed is in the context of multiple days of 100+F heat.
posted by mandymanwasregistered at 4:56 PM on August 3, 2011 [1 favorite]

It's as simple as that.

So your claim is that convection ovens cook food slower at higher temperatures than traditional ovens?
posted by muddgirl at 5:02 PM on August 3, 2011

The blog "Ask a Korean" covers this folk tale in depth.
posted by desjardins at 5:06 PM on August 3, 2011 [1 favorite]

Is there any evidence of a single heat injury or death caused by using a fan? Nup. It's safe.

Technically yes in a completely sealed room, running a fan will increase the TOTAL temperature in the room - but that doesn't mean that a human body will not get cooler. Evaporation is assisted by airflow, and the fan is causing airflow. This effect will be greater than the heating effect of the fan motor itself.

I can believe that in certain humidity/temperature conditions, a fan will not be enough to cool some people.
posted by trialex at 5:27 PM on August 3, 2011 [1 favorite]

I think there are several truisms being brought together and "solved" by saying not to use a fan in a hot room.

Day time temperatures aren't the deadly bit of a heat wave, high night time temperatures are. (this was backed up by research done by a student at Monash Uni that I heard during a lecture on heat waves, but I don't have the citation handy.)
They conjectured that elderly people, concerned about home security, don't open up their windows at night and so don't take advantage of the natural cooling and letting their homes cool down (pensioners are less likely to have air-con) They mapped heat wave related deaths to suburbs around Melbourne, and found that the poorer (outdoors labourers) and 'grey' suburbs (high elderly population) had more deaths than the affluent suburbs with their aircon. Pretty obvious but interesting to see it scientifically mapped.

It is true that fans don't cool a room by themselves, just move air around.

You can also use a fan to suck hot air out of a room or bring in cooler air from outside (night time).

++ the wet bath towels idea + fan.
posted by titanium_geek at 5:28 PM on August 3, 2011 [1 favorite]

Agreeing with Huckit above - have experienced the same thing in the extreme Melbourne summers.

My understanding of it is that your clothes and to some extent the tiny hairs on your body trap a layer of air against your skin.

In the usual case, your body temperature is higher than the ambient air temperature. Therefore, this layer of air trapped against your skin is usually somewhere in between that.

A breeze blowing against you will disrupt that layer of air, bringing the colder ambient air in contact with your skin. That is why wind usually feels "cold".

In the unusual cases where the ambient air temperatures are higher than your body temperature, this layer of air trapped against your skin will again be somewhere in between - warmer than your body temperature, but cooler than the ambient air. A breeze blowing against you will disrupt that layer, bringing hot air in contact with your skin, and thus the breeze feels "hot".

I can see the logic of where this is going, warning that if someone was silly enough to think that fans always have a cooling effect, and when the temperature gets too hot (significantly above body temperature) and they try in vain to cool themselves by turning up the fans... and by doing so, only serve to accelerate the overheating of their bodies.

There was a 113 degree day in Melbourne (coupled with 40mph winds and complete cloud overcast) that I weathered by closing up the entire house at dawn and just sitting quietly reading a book. All the fans in the world wouldn't have helped me if that heat got indoors.
posted by xdvesper at 5:28 PM on August 3, 2011

In the unusual cases where the ambient air temperatures are higher than your body temperature, this layer of air trapped against your skin will again be somewhere in between - warmer than your body temperature, but cooler than the ambient air. A breeze blowing against you will disrupt that layer, bringing hot air in contact with your skin, and thus the breeze feels "hot".

I'm sorry, but I gotta see a cite for that; it completely discounts the affects of evaporation through perspiration, which cools us down. It's the whole reason for sweating. Dew point is what it's all about - maybe that's a concern at some times in Houston - but in dry old Melbourne, the dew point has never and will never reach that high in conjunction with high temperatures.

Fan away, my friend, fan away. If it's so hot peeps start dying; the heat is the culprit, not a fan.
posted by smoke at 5:51 PM on August 3, 2011

I think huckit is almost right, but actually wrong.

Humans stay cool through the miracle of evaporative cooling. And this can create a boundary layer near your skin. But the boundary layer is not what makes you cool, and in fact, the boundary layer can inhibit evaporation. Blowing that boundary layer away with a fan should promote more efficient evaporation, and better cooling.

Aside: Robert Krulwich had an interesting bit on NPR recently about the hottest temperature a human being can tolerate. Apparently some daft members of the Royal Society in the 1700s built an oven-room and subjected themselves to insanely high temperatures. They got it up to 240°F and still endured for 8-minute spells.

Just got home from working out in 105°F heat. Close enough to an oven for me.
posted by adamrice at 6:29 PM on August 3, 2011 [1 favorite]

This is one of those combinations of "hypothetical scenarios" and "splitting hairs" that the internet revels in.

Elderly people die every year from heat stroke. If the conditions are that hot and dangerous, and cool beverages, cool water, or transportation to an air conditioned community shelter are not possible (as in a hurricane situation), then the CDC recommends donning wet towels and sitting near a fan to increase evaporative cooling.
posted by ErikaB at 7:06 PM on August 3, 2011

I took a quick survey of the four portable fans in my house. Power consumption varied between 30 and 60 watts. Or 25-50% of what an additional person would add. Battery powered fans are going to be much less unless they use some deep cycle car battery or something. So the heat added to a room from running your fans is negligible.

Moving air above body temperature still cools by evaporating sweat as long as the dewpoint is below ambient temperature. It gets less efficient the smaller the delta between those two values but it still works. And increasing air movement in going to increase the rate of cooling at all temperatures if the air is approaching saturation. It's how cooling towers work.

muddgirl writes "So your claim is that convection ovens cook food slower at higher temperatures than traditional ovens?"

Food doesn't sweat.
posted by Mitheral at 7:23 PM on August 3, 2011 [1 favorite]

Sounds pretty ridiculous.

That being said when I lived in Texas turning on a fan in 110 degree weather would definitely make you 'feel' hotter. I always assumed it was because the hot air was being propelled at me. We always placed the fan in the window facing outwards instead to suck out the hot air.
posted by trishthedish at 7:46 PM on August 3, 2011

If a room isn't dangerously hot there's no way a fan is going to pose any kind of danger. The "convection effect" versus better evaporative cooling issue... I dunno, man. A hot room with a fan in it is not all that much like a convection oven, and the human body is not much like a dead turkey. On the other hand we definitely cool off by sweating and circulating air definitely accelerates evaporation.

I would be very surprised if there is any real science behind this claim. It is probably good advice from the "be safe in a heat wave" perspective because there are limits to the kind of heat a body (especially a vulnerable one) can tolerate, and you shouldn't rely on a fan to keep you safe in that context.
posted by nanojath at 10:16 PM on August 3, 2011

Coming a bit late, I know, but I read this at breakfast in Doha (with the temperature outside at 95°F and a humidity of 71%, "feels like 118°F").

It ought to be possible to test this quite easily. Long-ago school physics reminds me that there is or used to be a thing called a wet-bulb thermometer--basically an ordinary thermometer with a bit of damp cloth wrapped around the bulb. So get a thermometer, wrap a bit of damp cloth around the bulb and point a fan at it. If the temp goes down, the fan is cooling the bulb. Repeat multiple times in different ambient temperatures and relative humidities. I'm not going to try to predict the result.

However, don't forget that even if the fan cools you in 100°F and 90%, that's not quite the end of the story. it cools you by evaporating sweat, and eventually you get thirsty. If you don't drink you get dehydrated. That in itself is quite capable of killing you.
posted by Logophiliac at 10:26 PM on August 3, 2011 [1 favorite]

Sorry, that's still not quite the end of the story. I was reminded by Wikipedia (sorry, at work and it won't let me link--look for "wet bulb thermometer"): "At relative humidities below 100 percent, water evaporates from the bulb which cools the bulb below ambient temperature. ". At high enough ambient temperature and humidity, the result might not be enough to cool your personal wet bulb temperature below body temperature. In this case the fan won't help, although it shouldn't make things worse (except by increasing the evaporation of sweat, possibly leading to dehydration, as noted above).
posted by Logophiliac at 12:02 AM on August 4, 2011

Put a pan of water under the fan. Evaporation cools the air and keeps it moist. You can actually buy this kind of set-up -- a rack on wheels that holds the pan and the fan -- but home-built is fine. I worked a very manual job in a very hot building (third floor) for a while and this works real well.
posted by CCBC at 12:19 AM on August 4, 2011

Food doesn't sweat.

Will an elderly person in this set-up sweat indefinitely?
posted by muddgirl at 6:36 AM on August 4, 2011

Suggested FTF houstontx.gov:

A fan can't change the temperature of a room; it can only accelerate air movement, and will accelerate the body's overheating dehydration.
posted by Tuesday After Lunch at 10:03 AM on August 4, 2011

People are right that there are two competing effects due to a fan: faster evaporation cooling you, and disruption of a cool boundary layer of air allowing faster conduction of heat from the air to your body.

If your skin is moist, that means all your sweat is not able to evaporate, and the fan will help with that (barring 100% humidity) and make you cooler.

But in Death Valley, my skin was not moist. Sweat was being produced and immediately evaporating carrying away lots of heat. Then the wind blew and the boundary layer effect became evident because the evaporation effect was already maxed out. A person who is only able to produce a certain amount of sweat, say an elderly person, might find herself in this situation.

tl;dr: It depends.
posted by huckit at 1:19 PM on August 4, 2011

One confounding factor not mentioned in the Death Valley Wind Scenarios is that, in general, these blasts of hot wind are super-heated.

{Here's a neat field trip tip for those visiting such places: Sweat evaporates so rapidly in these conditions that it seems like you're not sweating. But if you lick your skin, you'll taste the remaining salt from your sweat.}
posted by Tuesday After Lunch at 5:01 PM on August 4, 2011

That "Ask a Korean" article is full of ridiculous pseudoscientific babble. That guy doesn't know what he's talking about, he just thinks he does.
posted by Jupiter Jones at 11:20 AM on August 5, 2011