In the Houston Chronicles heat advisory, there was this bit of information.
“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.”
I agree that it won’t change the temperature, but won’t it facilitate the evaporation of perspiration leading to cooling of the body? How would a fan accelerate overheating?
As always, thanks.
If the air is so hot that you overheat simply by sitting still, a fan will make it worse. If the air is cooler than that, a fan will help. I don’t know what the critical temperature is, though. I presume it’s somewhere below 98.6 degrees F.
As long as the temp is below body temp, the fan still helps. It probably helps well above that too as it allows faster evaporation from the skin. You need a ton of water input to support the sweat. Heat stroke is on the way soon.
Fans definitely cool you off without A/C. That’s entire reason why we sweat in hot weather: so the wind can cause evaporative cooling. People managed just fine with just fans and no A/C.
If the air is highly humid, it slows evaporation, but as long as it’s less than 100% humidity, the fan will be cooling you.
Even if it hits 100, a fan will cool. The temperature of the air is not as important as its ability to absorb moisture.
It depends on the dew point, i.e. the temperature at which water will condense. The dew point is always less than or equal to the ambient temperature. When they are equal, we call that 100% humidity. If the dew point is less than body temperature, there will be net evaporation and you will be cooled. If the dew point exceeds body temperature, you are in trouble, since there is no way for you to maintain your temperature.
But why would a fan make it worse? Does it make the sweat evaporate quicker than the body can produce it?
If the sweat can’t evaporate at all, it provides no cooling to the body. In that situation, all a fan would do would be to pump hot air over your body, which will warm you up due to being over body temperature and which will not wick any sweat away, and thus warm you up faster than no fan.
The fan motor also generates heat.
Very high heat and near 100% humidity sustaining itself without resulting in daily rainstorms is a fairly specialized set of circumstances not seem in the vast majority of the US.
Well, it was the Houston Chronicle, from a city known for very high heat and humidity. But as to what’s reported in the OP, there could be situations where what they wrote is correct, but most of the time it is incorrect. I’d have to see the original context.
The heat AND humidity would have to be pretty damn high for the fan to not do any good.
There is a measurement that I believe is called the wet bulb temperature. You measure the temp with thermometer. Then, you take a thermometer that has a wick on the bulb, get it wet, then spin it around in the air on a string for a bit. It will be cooler than the dry bulb. IIRC you can use those two measurements to figure out the humidity. There are probably charts on the net you can find.
My guess is that when the wet bulb temp get close to or exceeds your body temp the fan and sweating arent going to do any good. You could use that in combination with the chart to figure out just how humid and hot its gotta be before it won’t work.
Without further context, I’d vote for this as most likeliest reason, esp. if it’s a small closed room.
The second runner-up would probably be complications from loosing too much water through sweat.
I just watched a lay-science TV report about how to deal with high heat*. They referenced the infamous case of AC breaking down in several ICE trains** in the hot summer of last year, where the temp.s inside the trains went to over 40 C and passengers had to be treated en masse for heat shock and heat exhaustion.*** They compared a family with two children and a group of teens, and also put three volunteers - an overweight mid-thirty guy, a young and an old woman - into an artificial climate chamber and raised the temp. to 50 C with a doctor explaining which strategies are best.
Turns out that keeping your skin covered with clothing is better than the bikini look, because it conserves water (sweat) with a layer (and of course the Arab Djellaba and similar long flowing robes/ dresses bring lots of cool air to the skin. (The other tips were drinking a lot, which the teens hadn’t prepared for; and laying down and resting instead of running around).
*obviously made in advance and broadcast by optimists, since so far June and July have been rainy and overcast, and August isn’t starting out sunny, either.
** As it turned out, the important AC - because due to the high speed of up to 300 km/h, the cabins are pressure-sealed, so windows can’t be opened to get air inside, instead, AC is built in - could handle all temps. fine, unless it got hot: they were designed for temps. up to 32 C, which might have been hot enough in the 80s, but with climate change bringing hotter summers; and with the sun shining on metal heating the trains up far beyond air temp., the Bahn did realize belatedly that maybe it would be a good idea to put in ACs capable of working with higher temps., too. If they get around to it, because well, lots of trains, little money yada yada usual excuse for incompetence and lack of giving a damn about their passengers…
*** The technical problems were compounded by the usual incompetence of the Bahn: the local ticket collector on the train doesn’t dare make his own decisions because management has come down hard on them in the past; and despite being in contact by radio with stations and presumably headquarters from there, nobody decided to stop the train at the next station to let things cool off, to distribute water from the bistro in the train for free or do any other things they could have done to minimize health damage to their passengers.
Courts decided they have to pay injury money to passengers who had to go to hospital from being overheated, which hopefully will cause them to think and act different next time.
Sorry…I’m a bit dim; but it still makes no sense.
Here is the original article:
http://blog.chron.com/newswatch/2011/08/houston-activates-heat-emergency-plan/
If it was because of the heat of the fan motor, that would mean the advisory should say “don’t use a fan without ac or opening windows.” It doesn’t even mention spraying yourself sometimes.
I live in Galveston and currently have everything open. It is 90F inside, there is a 9mph breeze outside, 73% humidity, and I’m operating purely on fans and the dogs and are are comfortable. Rooms that the breeze doesn’t come in are warmer.
I’m starting to think it is folklore in the medical industry like the full moon. Just something that has been passed down with no basis in reality.
The article as-written is over simplistic to the point of being incorrect to state as they have done.
Doesn’t the rate of revolutions of the fan blade and the actual size of the fan play into this? I have a HUGE industrial-sized fan in the equipment room where I work, and while the room is a concrete floor and ceiling with cinder block walls with no other ventilation at all, the fan still helps to cool me off when I need to work on something back there…but its gigantic, too.
When my a/c in my house failed in late May and we had a heatwave (around Memorial Day) I brought that fucker home and placed it in front of my bed with it on high…it was the only way we could sleep when it was close to 90 degrees F in the house. I have to believe that the sheer amount of air a very large fan moves makes a difference.
I’m guessing what they meant to say was that the fan might not provide adequate cooling as hot as it’s been. It hit 113 here in Dallas and I’d hate to have to endure that with only my shop fan. But I can’t imagine a fan actually making things worse in any situations you’ll encounter on this planet.
volts x amps = watts x 3.413 = Btus iirc…
The average fan draws less than .5 amps, and I would guess as low as .1.
A 115V fan at .2 amps would generate around 78 btus of heat into the room. Even at .5 amps it will generate 196 Btus.
A 1000 ft² apartment is probably seeing 18,000 Btus from infiltration alone.
I say, don’t worry about the fan.
eta I think your heart is generating around 400 Btus at rest. Keep the fan on. Just stop your heart.
Convective heat transfer is dependent on the flow velocity and the surface area under effect. Fan RPM helps with the former, and fan size helps with the latter (whole body is under airflow vs. just parts of it)