Dry heat vs humid heat

[Dymanic]

But high heat low humidity will out right kill you from dehydration faster, much faster, unless you have access to a lot of water.

So the simple and obvious solution is to make sure you have plenty of water. And drink it. What is the simple and obvious solution to dealing with excessive heat combined with high humidity?

 

[luchog]

Well I disagree. In dry heat you loose a significant amount of water through respiration. Enough to make you dehydrated very quickly. It is not a slow process and will continue even after you stop sweating. This is much reduced in humid conditions.

No, it's not a very fast process. Dehydration through respiration is insignificant compared to dehydration through perspiration; and in any case, dehydration is still a much slower death than heat stroke. It's only possible to die from dehydration in a high-heat low-humidity environment because the heat index is low enough that heat stroke is less likely, because the body is still able to cool itself through perspiration. Even in very hot/dry climates, dehydration typically takes days. Most often, heat stroke will kill due to lack of perspiration reducing the body's ability to cool itself.

 

[RussDill]

No, it's not a very fast process. Dehydration through respiration is insignificant compared to dehydration through perspiration; and in any case, dehydration is still a much slower death than heat stroke. It's only possible to die from dehydration in a high-heat low-humidity environment because the heat index is low enough that heat stroke is less likely, because the body is still able to cool itself through perspiration. Even in very hot/dry climates, dehydration typically takes days. Most often, heat stroke will kill due to lack of perspiration reducing the body's ability to cool itself.

I have no idea how fast fluid is lost through respiration, but I wouldn't discount it without a source.


The only source I could find is this (unfortunately, the source they link is no longer available):

http://www.thenakedscientists.com/forum/index.php?topic=15130.0

Fluid loss of a sedentary person such as a pilot would be between 200-300mls per hour. The loss from flight attendants would possibly be 300mls depending on their activity level. Military pilots in full flight suits would conceivably be as high as 600mls per hr.

And that's at cabin temperatures and 20%-30% humidity. I imagine it'd be higher at high temps and 5-10% humidity.

The human body can perspire at a maximum rate of 2-4 liters per hour. The human body contains an average of 20 liters of water. Either the person dries out completely after a short time or they stop sweating.

I don't think the argument is that people die of dehydration, but that they are dehydrated to the point that they stop perspiring. And if you can't perspire, the heat index doesn't really matter. Which is what was meant by "dehydrated".

So which is more miserable, a dry heat, or a humid heat? I'd say a dry heat since it's what I'm used to. But as far as which is more dangerous (assuming similar heat index values), I'd say a dry heat as once your fluid levels drop below the level at which you perspire, the only thing that matters is the absolute temperature and how long your remaining sweat will last.

 

[Dymanic]

The human body can perspire at a maximum rate of 2-4 liters per hour. The human body contains an average of 20 liters of water. Either the person dries out completely after a short time or they stop sweating.

Have we established that a person perspires less in humid heat than in dry heat (given the same heat index)?

So which is more miserable, a dry heat, or a humid heat? I'd say a dry heat since it's what I'm used to. But as far as which is more dangerous (assuming similar heat index values), I'd say a dry heat as once your fluid levels drop below the level at which you perspire, the only thing that matters is the absolute temperature and how long your remaining sweat will last.

And this dry heat you are used to, is it more dangerous for you?

 

[jimbob]

I guess it might be different if you were an amphibian

 

[Dymanic]

I guess it might be different if you were an amphibian

A lot of things would. The sex is the first thing that comes to my mind, for some strange reason. Frog sex just doesn't look like much fun to me, though I suppose a frog might disagree. Things would be different if I were a bird, too. Given a choice, I think I'd have to go with bird.

 

[RussDill]

Have we established that a person perspires less in humid heat than in dry heat (given the same heat index)?

I'd don't know one way or the other, but from what I understand about the heat index, I would assume that the perspiration rates for similar heat indexes are similar.

 

[Travis]

I will say that I find it interesting that people tend to live in hot/humid regions but not hot/dry regions.

 

[GlennB]

I will say that I find it interesting that people tend to live in hot/humid regions but not hot/dry regions.

The original settlements needed the plentiful water, I imagine. Today's major agricultural areas still do. That doesn't mean they're pleasant places to live, just productive.

My area of the world is hot with low humidity, but was probably relatively attractive for settling as there is mountain-fed water all year in places. Terrace the hillsides, build cisterns, dig wells, carry water where necessary ....

 

[luchog]

I don't think the argument is that people die of dehydration, but that they are dehydrated to the point that they stop perspiring. And if you can't perspire, the heat index doesn't really matter. Which is what was meant by "dehydrated".

Wrong. That was exactly the argument. And that's not what "dehydrated" means. You're moving the goalposts.

But as far as which is more dangerous (assuming similar heat index values), I'd say a dry heat as once your fluid levels drop below the level at which you perspire, the only thing that matters is the absolute temperature and how long your remaining sweat will last.

Unfortunately for you, all of the evidence points to heat index being the primary factor, not absolute temperature. Links to the evidence have already been posted.

Perspiration is triggered by body temperature, not by relative humidity. A body that is cooled by evaporation is going to be at a lower temperature and perspire less than a body that is not able to cool itself. A lower absolute temperature with a higher heat index is much more likely to cause dangerous dehydration than a higher absolute temperature with a lower heat index. And that doesn't include the body temperature problem already explained.

 

[luchog]

I will say that I find it interesting that people tend to live in hot/humid regions but not hot/dry regions.

Hot/humid regions are easier to survive in, simply because of water availability. Water to replenish what is lost by perspiration; and water to grow plants necessary for food, both for human and animal populations.

 

[RussDill]

Perspiration is triggered by body temperature, not by relative humidity. A body that is cooled by evaporation is going to be at a lower temperature and perspire less than a body that is not able to cool itself. A lower absolute temperature with a higher heat index is much more likely to cause dangerous dehydration than a higher absolute temperature with a lower heat index. And that doesn't include the body temperature problem already explained.

I'm trying to compare similar heat indexes, not similar absolute temperatures.

 

[Travis]

Hot/humid regions are easier to survive in, simply because of water availability. Water to replenish what is lost by perspiration; and water to grow plants necessary for food, both for human and animal populations.

I didn't say inexplicable just interesting. Obviously having ready access to water would help.

 

[luchog]

I'm trying to compare similar heat indexes, not similar absolute temperatures.

And, as previously noted, similar heat indexes will have similar risk; since the heat index is effectively a measure of the body's ability to cool itself/regulate temperature. Absolute values aren't really important, as there really isn't a significant difference between reactions to different absolute values at the same heat index. All that matters is temperature regulation. The absolute temperature that generates a high heat index at low humidity is going to have to be so high that perspiration isn't going to be able to cool the body effectively enough to avoid heatstroke.

Heat index isn't the last word, however, although it's a good starting point. A more effective measurement is Wet Bulb Globe Temperature, which is used by the military to manage activity in hot climates. WBGT calculation accounts for more variables than the heat index, and is the best current estimator of risk.

 

[deadrose]

I lived for a few years in Kitchener, Ontario, where summer temps generally ranged in the 30s (90s for the Fahrenheit folks) in the summers I was there, with high humidity, like 90-94%. Apparently those summers were hotter than the average, and I had the added bonus of an un-air-conditioned apartment plus being pregnant or nursing every one of those summers. I was actually thankful to go to work, just because it was cool there.

http://en.wikipedia.org/wiki/Kitchener,_Ontario#Climate