Sweat in Humid vs. Dry Air

I think you sweat less in dry weather but you lose more water too.

Wait you mean to tell me, in order to be initiated to Let's Run forum you have to be called an' idiot' or a 'moron' at least once?... ;)

My intuition is that, all things being equal, you lose more sweat in the more humid conditions. My rational is that the sweating mechanism is trigged mainly by skin temperature and, since the sweating mechanism becomes less efficient as humidity goes up, your body's reaction is to increase the sweat rate.

There are a number of confounding factors that complicate this simple view including at least the following:

1) In very dry environments one loses a lot of water via respiration even when not involved in physical activity. This fluid is significant and has to be replaced but it isn't really sweating per se. During heavy exercise repiratory evaporation effectively cools the body with (presumably about) the same efficiency as sweating so it isn't a plus delta to the net fluid loss to maintain steady state body temperature.

2) We tend to find very low humidity at altitude or in hot climates. At altitude the lower air pressure contributes to respiratory fluid loss as noted above. In very hot climates one is going to sweat regardless, but in the very hot humid climate one is not able to sustain the same activity levels without overheating and being forced to stop, that is the performance level hasn't been maintained constant.

I think I may try to test this out. For the next two weeks I will try to keep track of my water loss(weight after minues weight before running) for all my easy runs that are about the same distance. I'll make a note of the temp and humidity for each run

As someone pointed out, there is water loss due to respiration and not necessarily sweat but I can't think of any simple ways to weed out that factor.

Former altitude runner wrote:

mari wrote:

If this were true then swimmers would never sweat while in the pool.

Exactly, and swimmers definitely do not lose as much sweat while working out as a runner, because the sweat isn't evaporating as quickly. So I think you confirmed my thought as opposed to refuting it. I never said you are not sweating at all or losing any fluid in the humidity, it just isn't evaporating as quickly, just like in the pool.

Former altitude runner, you are making incorrect assumptions right and left.

"Swimmers do not lose as much sweat while working out as a runner."

True.

"...because the sweat isn't evaporating as quickly."

Um, evaporative cooling does not come into play at all when swimming. Most swimming pools are at 80 degrees or cooler. That is way below body temperature, and is sufficient to cool the swimmer via conductive and convective heat loss. THAT is why swimmers sweat less. Evaporation has nothing to do with it.

Look, I used to live in East Texas (hot, humid) and I've lived in Boulder for 12 years. People, your guesses mean nothing. My guess would mean nothing. If you want to know the answer, look for some actual science.

Former altitude runner wrote:

Exactly, and swimmers definitely do not lose as much sweat while working out as a runner, because the sweat isn't evaporating as quickly. So I think you confirmed my thought as opposed to refuting it. I never said you are not sweating at all or losing any fluid in the humidity, it just isn't evaporating as quickly, just like in the pool.

"Sweating is controlled from a center in the preoptic and anterior regions of the hypothalamus where thermosensitive neurons are located. The heat regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the skin. High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core temperature. Overall, however, the sweating response to a rise in hypothalamic ('core') temperature is much larger than the response to the same increase in average skin temperature. The process of sweating decreases core temperature, whereas the process of evaporation decreases surface temperature." ~ wikipedia

So it is controlled by temperature, not the presence of fluid on the skin. If you would like to know about the physiological aspects of sweating you can click here

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You will lose more water in a dry climate, but just not when you are running (with all other variables being equal). The first response was correct, but you are right that if you are in a dry climate you should drink more water. That is due to evaporation during normal activities, not running. As explained in previous posts altitude will decrease the temperature where evaporation occurs, thereby increasing the rate of evaporation. However, evaporation will cool your skin, making you less sensitive to your core temp and thereby reducing sweating. Its an interesting balance.

Some excellent responses to this thread and I know I have certainly learned something. My responses have been based on my own experience, but not in a scientific way. I knew I wasn't just imagining having to drink a ton more at altitude, but some of the responses are showing why that would be the case, with it not necessarily being that I was losing more sweat than when I was in more humid weather. Thanks for the insight everyone. My wife is an exercise scientist and I really should be talking to her about this as she would have more physiological knowledge of all of this than I would.

Former altitude runner wrote:

My wife is an exercise scientist and I really should be talking to her about this as she would have more physiological knowledge of all of this than I would.

I think that's an excellent idea. I'd be very interested to here what she says.

Interesting thread.

But now I wonder for a heavy sweater like me whether I want a cool-dry or cool-(more)humid day/location for my next marathon. I can't seem to get enough fluids in me so the last 6 miles are always a problem...

Citizen Runner wrote:

2) We tend to find very low humidity at altitude or in hot climates. At altitude the lower air pressure contributes to respiratory fluid loss as noted above. In very hot climates one is going to sweat regardless, but in the very hot humid climate one is not able to sustain the same activity levels without overheating and being forced to stop, that is the performance level hasn't been maintained constant.

I think this should be pointed out again. Many people on this thread are pulling from experience of dry weather vs. hot weather, but most of the dry weather experiences are at significant altitude, which in itself has a dehydration factor. So saying that the dry weather at 8k ft dehydrated you more than Flordia's summers when the temps were similar is probably true due to what Citizen Runner pointed out: that the lower atmospheric pressure is contributing to fluid loss more so than the dry vs. humid aspect of the air.

If you have access to a university library, check the masters theses by Dick Daly and Linda Paumer from the early 1980s. They did a treadmill chamber study in Doc Adams's lab comparing the ability of men and women to train in hot dry and hot humid conditions. I'm sure there's ots of more recent work on this subject, too.

Okay, same 10 mile run. I lost 3.0 pounds @ 58 degrees and 75% humidity, and I lost 4.5 pounds @ 68 degrees and 80% humidity (50% more).

I think that looking at the dew point is more helpful, since it takes the temperature and humidity into account. This morning, the humidity was in the low 80s, but the temperature was about 51 (dew point 45ish). Pretty much anything 55 and above for the dew point, and I sweat a lot. With a dew point under 55, the humidity can be high, but I don't sweat much because the temperature is low (and I'm not overheating).

Everyone is different in his/her heat and cold tolerance. So, there's no exact number when we start to overheat. Know yourself.

Arete wrote:

Everyone is different in his/her heat and cold tolerance. So, there's no exact number when we start to overheat. Know yourself.

Yes, and please let's not forget if you're on medication as well. Anti-biotics or acne medications as well as many others are certain to affect the body.

Another Florida runner here, and I'm dreaming of the point of time somewhere in mid-November when we might get down to a 55F dew point. Our dew points have consistently been over 80 this summer anytime after about 8 in the morning.

The thing with running in high humidity conditions isn't just the sweat rate while you're running. It's that you frequently tend to keep sweating for a long time after you stop.

I concur with the logic about sweating more in humid weather. We do not have galvanometers in our skin to figure out how much we water we have on its surface.

My experience fits with this as I sweat way more since I have moved from Upper Midwest to the Southeast. Of course, one can not rule out physiological changes from aging.

dingle wrote:

I concur with the logic about sweating more in humid weather. We do not have galvanometers in our skin to figure out how much we water we have on its surface.

My experience fits with this as I sweat way more since I have moved from Upper Midwest to the Southeast. Of course, one can not rule out physiological changes from aging.

But doesn't it make sense that if the humid air is not letting the water evaporate from our skin to create the cooling effect, wouldn't we be sweating less, even though we are feeling it more since it is just sitting on our skin? Yes, some of it is dripping off, but a lot of it is staying on the skin, preventing us from cooling. I do understand what everyone is saying, but haven't totally bought into it. A long time ago I remember Benji Durden, former Olympic marathoner, being quoted after a race saying he had hydrated really well before the race but since it was humid and he wasn't losing the fluid as quickly he ended up having to take a bathroom break. This was back in the 80's.

By the way, I did talk to my wife, who is an exercise physiologist, and she didn't really have an answer based on any studies that she had read. She did agree with what a lot of people are saying about dry air at altitude being different than dry air at sea level and a big reason for my dehydration in Colorado was from other factors besides losing fluids from sweating.

Its not the liquid that cools you when you sweat its when it evaporates that the actual cooling happens. Take for instance if you were to wash your hands, when that water is evaporating it is cooling you, and you will notice your hands are much colder after the water is gone. The same effect is true in sweat. Thus, running in humid air will result in more sweat and less cooling. Dry weather is a more efficient climate to run in than humid weather.

Lydiard claims in "Running with Lydiard" that he gets much more dehydrated in hot, dry climates than in humid ones. He also points out that when it's extremely cold (below zero), it's dangerous to run with any moisture in the air. High heat, high humidity; low heat, low humidity. He gives no science to back up his claim besides "... the moisture perspired can remain on the skin surface to help cooling."

Where I live and my experience, I'm not 'exactly' at sea level, more like 1500ft and in the winter it averages about 12% or lower humidity--fairly dry compared to most sea level areas in their cooler months and if someone is not from around here, they will notice the difference. However, I do agree, at higher altitude, you will be MORE dehydrated overall compared to sea level or even at 1500ft. I've noticed that myself when I go up North.

I think the bottom line, high heat with humidity or high dry heat with lower humidity is just not ideal to race.