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It is not the heat that will kill you, it’s the humidity.

While working on a post about how to store and dry life support climbing equipment I realized from the many comments that protecting our gear from humidity and moisture goes much further than that.

Below is the post on my FaceBook page,

“Summertime thoughts.

A human cannot survive 95 degree heat and 100% humidity for very long but can survive being in 95 degree water for an extended time.

and moist air is lighter than dry air.

Have a good day, if you want.”


I have always found it interesting that in nature many things are counter intuitive.  For example, that clouds holding millions of tons of water are lighter than the air that surrounds them.  This is why we most often see clouds billowing up into the sky and at times going so high and getting so large that they cannot continue to hold the moisture turning to rain.

I also found it very interesting that humans cannot survive in 95°F temperatures with 100% humidity.  Evidently, it is also not generally known, as many of the comments ignored the humidity and stated things like, “that’s nothing, I can do that all day long”.  Fortunately, these exact conditions are extremely rare but there are many times we may be working in the extreme danger zone and not even know it.

First of all, most of us are very aware of the temperature, we have thermometers everywhere, hanging on the porch, mounted on the window and even on our vehicles instrument display but seldom is the humidity noticed.

What is humidity and why is it important to us?  Humidity is the amount of moisture that air can hold.  It is measured in relative humidity, relative to the maximum amount that air can hold at a given temperature.  As air is warmed it can hold more moisture and conversely, as it cools it can hold less.  This is why a rising cloud that cools as it goes up may lose the ability to hold the amount of vapor or moisture that it has, and it starts to rain.  So, humans use this concept for cooling, we basically have an evaporator type cooling system.  Our car engines just use a heat transfer method, air flowing over a hot radiator and taking the heat with it as it goes.  It is important to understand as well, when liquid changes state, vapor to liquid or solid etc., there is an energy transfer.  In simple terms, when water evaporates, turning from a liquid to a vapor, it takes heat with it.  This is what happens when we sweat.  Our body perspires putting a film of water on our skin that then evaporates and removes heat from our body.

Seems simple enough and I think most of us understand that.  What I think gets missed is how important these numbers are, how often they are overlooked and to what extent they can cause us problems.

In my example above the humidity is 100%, the air cannot hold another molecule of water.  As we work or even as we sleep our body is generating heat.  That heat can be lost by simple heat transfer, like the car radiator, if there is an adequate temperature difference.  This is why a room temperature of about 72° is suitable for our 98° bodies.  But what if we are exposed to a 95°F temperature and 100% humidity?  Our body must release heat, even while simply resting. But now the evaporator air conditioning, that comes standard at birth, is broken and cannot work.  We are left with no way to cool our body temperature and as it warms it stops running.  The technicians ‘code reader’ says, “heat exhaustion followed by heat stroke and death”.

So what can we do to prevent having heat related health issues.  Pay attention and learn how to figure out what the heat index is for a given day.  The, “feels like” number may not tell the whole story.  There are many heat index charts available. 

This is one from the NATIONAL WEATHER SERVICE.


Next, know the humidity otherwise just the temperature may deceive you.

This is an inexpensive hygrometer/thermometer I purchased on-line, approximately 10.00 USD, has Bluetooth, works with Govee app and has other features.  I use them for my 3D printing filament putting one in the Ziplock bag giving me constant humidity readouts for the filament.  Note: 3D filament is extremely sensitive to absorption of moisture causing printing problems.  I use them for my climbing gear storage container (excess moisture also leads to mildew, bacteria growth, corrosion, and stinky gear), beehive, birdhouse, and basement etc.
 Have one with you at the jobsite and a copy of the heat index.

In conclusion, many stories are available of a heathy young hard-working individual that has scummed to heat stroke. Don’t be fooled by only knowing the temperature.  Realize that even if you are young or in great shape, your bodies ‘evaporator air conditioning system’ may be disabled and cannot control your body temperature as it should and damage to your ‘vehicle’ may occur.  Reference and know the numbers and understand the risks. 

We are dealing with properties of physics, NOT physical properties.

Climb and Work Safe