When sweat dries on the skin while using a portable fan, it becomes difficult to lower body temperature. That results in the surface temperature of the skin rising when it is exposed to air warmer than the body temperature.

This doesn't make sense to me. So if people are sweaty and they get onto an air conditioned train and the sweat dries is that also a risk? How about when the sweat dries naturally on the skin, is there a difference? I don't have any of these portable fans, but se many people using them, and if they feel it is giving them relief then I don't see the harm!

Isn't sweat supposed to remove heat? I'm calling nonsense.

So then this person is saying to not use a portable fan? What else, that people should stay away from the wind as well? Maybe seek a spots of stagnant air instead?

Many think sweat cools the body, but its actually the evaporation of that sweat which creates cooling. And using a fan moves air over the sweat, which then increases evaporation, which in turn increases cooling.

So the act of not using a fan would raise the risk of heatstroke considerably more.

I would question his suitability for the post he occupies if this is the level of his competence.

I think there is some misunderstanding here. The quote is correct. Sweat, as it evaporates, helps cool the skin. Blood vessels feeding the skin also dilate, which allows warm blood to flow to the skin surface. This helps remove heat from the body core.

The fans blow sweat dry before it has a chance to evaporate and cool the body, thus causing the body to stay hot or get hotter rather than cool.

I've never understood the obsession of "sweat is bad" here. My son had a coach who would have them constatntly towel off sweat and I see people constantly wiping themselves down. They are making things worse for themselves. Sweat is icky, but it has an important purpose.

Since sweat also contains oils, salts, etc., if they dry on the skin, that could reduce the ability to secrete more sweat. It also depends on what part of the human body that Miura is referring to as sweat rates and the capacity of evaporation dare different for a person's head and chest regions relative to their arms/legs.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7806205/

The main thing is to wipe your skin with a towel, preferably wet, ocassionally and it'll make it easier for your skin to breathe and cool down. Nothing to do with portable electric fans.

So... when we sweat and there's a breeze, my sweat dries and I'm more exposed towards a heatstroke. Impressive... Good things we've got experts out there like this guy; he just saved my life...

Isn't sweat supposed to remove heat?

Yes, and as you sweat your skin cools down, and the sweat on your skin also cools down. But when the air is higher than body temp, you might eliminate this.

When the air temperature is higher than body temp, fans can make things worse.

So then this person is saying to not use a portable fan? What else, that people should stay away from the wind as well? Maybe seek a spots of stagnant air instead?

The person is saying that there are situations where a portable fan may not help (air too hot for ventilation to be effective) so other measures should be put in order, like bringing a spray bottle with water to hidrate the skin or going to a cooler place, it is never recommended to just stay in the heat without the fan.

I would question his suitability for the post he occupies if this is the level of his competence.

So when you don't understand what an expert is saying your first reaction is to think that expert must be wrong? it is simply more likely you did not get the point as in this case.

When the air temperature is higher than body temp, fans can make things worse.

Using a fan with a cooling element might be helpful, though I've never used them so I don't know how well they work...

When air is warmer than body temp using a portable fan is a little like using a hair dryer on your skin

Those junk fans are landfill waiting to happen.

I want a physicist to chime in: seems like a pretty badly designed system if it didn't work in 38 degree wind.

virusrexToday 11:27 am JST

So when you don't understand what an expert is saying your first reaction is to think that expert must be wrong? it is simply more likely you did not get the point as in this case.

No, he's correct. This "expert" doesn't understand the simplest principles of science.

When sweat dries, evaporation, it consumes a rather large amount of heat converting a liquid into a gas.

When sweat dries, evaporation, it consumes a rather large amount of heat converting a liquid into a gas.

Yeah, but once the skin dries, you're just blowing hot air against dry skin. As ian wrote: "like using a hair dryer on your skin."

Without a fan, your skin stays wet and cools as it is meant to.

I've never used these fans, but on a very hot day when I bicycled home around noon it felt like I was in an oven because of the hot air hitting my body; but simply walking was fine (hot but not so uncomfortable).

No, he's correct. This "expert" doesn't understand the simplest principles of science.

On the contrary, the expert is correct in the assessment that air that is hot enough can surpass the effect of sweat and heat up the skin. The skin does not have an infinite capacity for sweating, so saying that a fan directing hot air to the skin (when this capacity was surpassed) heats more than refreshes is completely correct.

When sweat dries, evaporation, it consumes a rather large amount of heat converting a liquid into a gas.

But once it is dry the excess of heat acts on the skin as expected. This becomes clearer in the original article from which the quote is taken:

When using portable fans in intense heat, Miura suggested dampening the body with a spray or something similar.

@RawBeer,@virusrex to understand this you'll need to study convection, conduction, and radiation.

Also, know that cooling occurs long before you see any sweat on the body. It's the visible liquid that indicates your body is trying to cool faster than the environment that your in will accept.

This is the reason humid weather feels so much hotter. It's much harder to cool down when sweat evaporates slower.

I don't use a portable fan but we use fans at home. I spray my face with a mint water solution.

@RawBeer,@virusrex to understand this you'll need to study convection, conduction, and radiation.

Again, your point completely depends on the skin having infinite capacity to cool you with sweat which obviously does not happen. There is no convection, conduction nor radiation mediated by sweat when its capacity has been surpassed and you only have hot air directed to the skin without any evaporation happening.

Once again, this is much clearer with the other quote from the original article, where spraying the body with water help ameliorating the problem of using fans during intense heat.

A useful reference to support the warning of the quote:

https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(24)00030-5/fulltext

we found that although fan use improves sweat evaporation, these benefits are of insufficient magnitude to exert meaningful reductions in body core temperature in air temperatures exceeding 35°C. Health agencies should continue to advise against fan use in air temperatures higher than 35°C

Although a fan directed at the body improves convective heat dissipation in cool environments by increasing airflow above the skin surface, this same mechanism means that fans exacerbate heat gain in hot environments 

For this reason, many health agencies, including the WHO and United States Centers for Disease Control and Prevention, have historically cautioned against the use of fans in high air temperatures

Thinking that health agencies don't understand the topic is not a rational position to take, oversimplification of the situation is what is making you reach mistaken conclusions.

...when its capacity has been surpassed and you only have hot air directed to the skin without any evaporation happening.

At this point you would be dead, completely dehydrated like one of those crypt keepers from the movies. Maybe a mummy?

The mechanics of sweating for cooling will be complex, and I'm not sure he's describing the full picture here. Lots of sports clothing is sold on the premise of quickly wicking sweat away from the body. It would not be designed to do this if having having sweat at the skin were desirable for cooling. This includes actual sportswear worn by pros throwing money at winning and not yoga pants for fashion victims.

I have some very fancy pants cycling gear that does this, and it does work. Thankfully it also works if you buy your fancy pants cycling gear for pennies on the dollar second hand like I do.

Burning Bush

I spray my face with a mint water solution.

Lol.

You shouldn’t mock what you haven’t tried. Distilled water with a small amount of mint essential oil in a small spray bottle kept in your pocket or bag. Refreshes every time.

A two-sentence quote on a scientific issue is probably not very helpful. There is a longer article at the link below.

https://www.asahi.com/ajw/articles/15379579

I suspect the issue is when air temperature is higher than body temperature. But I'm no expert.

My son had a coach who would have them constatntly towel off sweat and I see people constantly wiping themselves down. They are making things worse for themselves.

I think when sweat accumulates on the skin, it evaporates less quickly and not directly from the skin surface. Wiping yourself with a towel means the next bit of sweat is more likely to evaporate directly off your skin and cool you down. There's probably a reason those sumo wrestlers constantly wipe themselves with towels.

At this point you would be dead, completely dehydrated like one of those crypt keepers from the movies. Maybe a mummy?

The scientific reference provided clearly, unequivocally prove this is not the case, do you have any argument against the professional opinion of the experts that say this is a danger that have to be taken into account? because if you don't they have the science on their side to disprove your personal misunderstanding about the problem.

Sweating mechanism cools the body by using excess body heat to evaporate the sweat.

Wiping the sweat will make you sweat more if body temp is still higher than normal

If the air around is hotter than the body you will be using the heat from the environment also to evaporate the sweat. When there is no more sweat you will then be heating up the skin if you keep on directing the hot air to your skin using the fan.

That's how convection cooking works

virusrexToday 03:41 pm JST

do you have any argument against the professional opinion of the experts that say this is a danger that have to be taken into account?

You've presented no professional opinion of experts that say,

...you only have hot air directed to the skin without any evaporation happening.

As you have said.

If you'll work on your understanding of convection, conduction, and radiation you'll get this. Conduction of heat from the surrounding air exceeding body temp and surpassing evaporative cooling has nothing at all to do with sweat drying on the skin while using a portable fan.

That would still occur even if you were soaking with sweat.

When sweat dries on the skin while using a portable fan, it lowers body temperature. As for me oversimplifying this issue, well you are having trouble understanding it and that probably has a lot to do with how Kunihisa Miura states this. He seems to be trying to make himself sound smarter than he is.

Anyway, don't give up. Physics is fun.

You've presented no professional opinion of experts that say,

Yes I did, even quoted.

we found that although fan use improves sweat evaporation, these benefits are of insufficient magnitude to exert meaningful reductions in body core temperature in air temperatures exceeding 35°C

When the evaporation is no longer producing any decrease of temperature then keeping the flow of hot air is making the problem worse, completely the opposite of your baseless claim that this would never happen,

If you'll work on your understanding of convection, conduction, and radiation you'll get this.

Again, completely false, the reference clearly explain how your oversimplistic understanding is wrong, and even the figure 1 is enough to explain (to anybody that is still rational) how your understanding is the one that is not congruent with reality.

You just repeating that the experts around the world and a scientific report explicitly disproving your claim are wrong without making any actual argument against their conclusions is not a rational position to take.

The expert quoted here is just repeating the consensus of science, either the scientists of the world (and their clear data to prove their conclusions) are wrong, or a nameless person on the internet that does not properly understand the topic is, it should be obvious who is mistaken here.

well you are having trouble understanding it and that probably has a lot to do with how Kunihisa Miura states this

You have yet to address the report that disproves your claim and warns about using fans in extremely hot weather, pretending they don't understand the topic is beyond irrational, you have not found any flaw about what Miura have stated, he is still correct

Anyway, don't give up. Physics is fun.

Unfortunately you have to properly understand it first.

Maybe if you bring a scientific report proving that the quote is false? can't find any? that is because the quote is still completely correct.

The use of LOL is an insult, not a joke.