The Merced River Gorge

A Brief on Heat Stress

The Problem

While I was enjoying the Merced trail, I also had to deal with heat to 117°. No problem - I had reliable water sources, I paced myself properly, and I always had the cool river for a back-up. All in a day's work. But it occurred to me I was able to deal with dangerous heat thanks to knowledge and experience most people don't have.

Most people who sicken and die from heat never see it coming because they don't recognize the signs and symptoms, or understand the mechanisms. By the time they realize they're in trouble, it's too late.

A few years ago, 2 men near Las Vegas, Nevada, hiked down to a hot spring in a canyon by the Colorado River. After soaking a while, they began the hike back up. The air temperature was only about 106, chicken-feed in these parts, but both men heat-stroked and died on the trail. What happened?

The human body mainly depends on the evaporation of perspiration in air temperatures above 92 degrees. Blood travels from the core to the skin to be cooled, like engine coolant going through a radiator. The system works so well, people can endure the worst heat any desert on earth can throw at them. In fact, back in the 18th Century, early scientists spent up to 12 minutes in a special room heated to 236° Fahrenheit to see just how much heat humans could take. A dog in a basket was exposed for over half an hour. They put a steak in the room, and "the beef-ƒteak was rather over-done in 33 minutes." The girls who tended the stove stood 280° for 10 minutes.(Blagden, 1774 and '75, "Philosophical Transactions" of the Royal Society)

But our cooling system demands adequate hydration - not only to produce sweat, but to maintain the blood volume that carries off the heat to the skin surface, while attending to all the other demands on the circulatory system. Hydration becomes more critical as temperatures rise.

When dehydration begins, blood volume decreases, and the body becomes less efficient at moving core heat to the surface. The body temperature rises - more sweat is produced to compensate - dehydration accelerates.

The early symptoms are headache, dark urine, constipation, and fatigue. The kidneys and colon try to keep as much fluid as possible.

If people ignore the warnings and persist, dehydration advances further. The digestive system will stop, progressive muscle cramps begin. Trying to stay cool with diminished blood volume puts a strain on the circulatory system. Body temperature rises, sweating increases, dehydration accelerates.

If a person is sweating like he's standing under a garden hose, the end is nigh.

Persist further, and sweating may stop. At that point the body has lost its ability to cool itself. Without external cooling the body temperature will rise until the terminal spiral of seizures, coma and death. They say the body's upper limit is 108°.

It sounds like only a fool could ignore all these warnings, but in fact it can all happen very quickly, and the point of no return is like falling off a cliff. No rope, you die.

Dehydration is expressed as a percentage of body weight. A quart weighs 2 pounds - I weigh 160 - therefore 8 quarts is 16 pounds, or 10% of my body weight. In triple-digit heat, 10% dehydration is in the danger zone - 12% is very likely to be fatal. Even at 5% people show signs of heat illness like fatigue and dizziness.

On the other hand, in temps around 70, dehydration up to 20% may be survived.

10 quarts sounds like a lot to lose, but a human can easily sweat 2 quarts an hour, simply walking on a desert trail, while losing additional water through respiration and digestion. You may not even notice it in dry desert air, where it evaporates instantly - or while soaking in a hot spring.

To top it off, many people are chronically a few percent low to begin with, from common diuretics like caffeine and alcohol, and simply not drinking enough water.

During WW II, too many American soldiers fighting in the North African desert succumbed to the heat. Scientists were assigned to solve the problem of maintaining active men in hot deserts. They conducted extensive experiments in the Mojave Desert, which can experience triple-digit heat from May through October, and often hits the 120s in July.

Men were weighed, worked, and weighed again, through various degrees of heat and exertion. The result, for our purposes, was "Physiology of Man in the Desert", published in 1947. It's out of print - fetching around $200 a copy on Amazon today - but it's usually available via inter-library loan, and being government work, it's in the public domain, so you can have it copied. I've offered a simplified summary here, but it's worth reading for more thorough familiarity.

It has one weakness - they never worked their subjects at more than a walk. When runners' body temps rose 4.5° F, they backed off.

But they did their job. It turned out the biggest problem in the desert was officers insisting men could be "hardened" to get by with less water. Some thought a quart a day should be adequate for real men. The Desert Unit studies put an end to that.

The Solution

Obviously the simplest and surest solution is to start off 0% dehydrated and stay that way. Drink plenty of water - easy on the caffeine and alcohol - stay out of the heat - put the AC on - draw the curtains - hide under the bed. But obviously that's not good enough for some people, and if you're on this page, you're one of the difficult ones.

First thing is to understand and acurately predict the degree of heat stress, and be adequately prepared. Here are a few tips and tricks, gathered over 14 summers in the Mojave, a paramedic course and several associated classes, and countless hours of additional study.

Acclimation definitely helps. People acclimated to heat can work harder and longer. Their perspiration is more dilute, losing fewer minerals per unit. I suspect part of acclimation is an increase in blood volume. I've suffered mild symptoms after swimming in cold water, or sleeping out in a blanket on a cold night, and then doing work in heat. When I didn't need the extra blood volume, the kidneys excreted it. When I needed it, it wasn't there.

But full acclimation might take 5 to 14 days of exposure, and it does not reduce the need for water. A certain amount of excess heat still requires the same amount of evaporation to carry it off. In fact more water is probably evaporated by an acclimated person doing more work.

Physical fitness helps. Cooling in high heat puts a strain on the heart and circulatory system. I can remember a hot day's night when it was still 110 at midnight and I was laying in the desert with my heart pounding full stroke over 150 beats per minute and my guts so locked up I couldn't drink. Any heart or lung disease probably would've made that experience fatal. In the event, I passed out, and woke up before sunrise - alive.

Carrying enough water seems obvious, but it's not that simple. You could lose 30 quarts on a long hot summer day outdoors scrambling in the mountains. Water's 8.38 pounds per gallon - you'd have to carry 63 pounds of water to stay hydrated at that extreme. Even 20 pounds - 10 quarts - is a serious burden.

The environment at 110° costs about 1 pint an hour at rest. Basic metabolism costs another 5 ounces. Urine and other losses amount to 2 quarts a day. Add up the hourly losses from exertion, and the standard "1 gallon per person per day" sounds fit only for the most sedentary people.

What's obvious now is you have to minimize your exposure and exertion. Even if you do everything right, you'll find the heat stress is a physical load. It reduces the amount of work you can do, and it requires additional rest and recovery.

As far as I can tell, it's impossible to maintain hydration over days working hard in high heat. You just can't drink enough. The deficit accumulates until it requires extra time off to rest and re-hydrate. Plan for it. If your schedule requires you to work when you need R&R, you're in trouble. Build in some slack.

Direct sun adds about 10° to your heat load, and costs about 8 ounces of water an hour all by itself. Plan your routes so you travel in shadow when the shadows are long. Find shade and a breeze to lay up in the peak heat of the day.

Know how to find and use any coolant that may be available. Anything wet can be used for evaporative cooling, if it's not toxic through the skin. Scummy green water, poison to drink, can still be used for cooling, and conserve potable water.

In a desert wash draining a large basin, clean, filtered water often flows underground long after it's disappeared from the surface.

When you find water, pig out and drink your fill. I'm told some stomachs can hold 2 quarts. I remember one dog of mine who drank so much at a waterhole, when we moved on I could hear him sloshing like a keg.

Forced drinking doesn't work for hydration, and it may be dangerous by upsetting the blood chemistry. "Pigging out" is a way to carry extra water away from a water source, not a way to stay hydrated. If the gut water isn't used in 2 or 3 hours, the excess'll just be excreted.

Rationing doesn't work either - it just creates a deficit. The body doesn't hoard water, or waste it, it just tries to maintain the correct balance as conditions require. Trying to force it to absorb more or use less is worse than useless.

If you drink as much as you can hold and you're still thirsty, you need salt. Half a teaspoonful in a 12-ounce mug works for me, repeated as often as necessary until I feel the effect. People who put down salt tablets just haven't had to move extreme amounts of water through their bodies, but plain table salt does work better, for me. It's probably a lot easier to absorb.

Salty snacks work. I like corn chips for the energy boost. But by all means avoid eating a large meal in high heat. Digesting it will take a lot of body fluid you can't afford, no matter how much water you have handy. It's extremely difficult to drink enough water to stay hydrated in high heat. A working digestive system also takes blood away from other systems - including the cooling system. More than one group of farmworkers has died, to a man, after a big Sunday feast on their day off, camping by the fields in heat over 110.

No doubt alcohol contributed. Too much inhibits the hormone that controls diuresis. Away the water goes, and the dehydration level goes up. Too much coffee does the same. Your body's in an athletic contest with the heat - don't abuse it. If things go wrong, and you don't have an immediate rescue in place, you will die.

Be aware that in high heat, diarrhea can be fatal. It's a failure of the irritated colon to re-absorb water used in digestion, and a lot of fluid is lost. If you're already on the edge, over you go.

That's the crux of this page. When you're operating in high heat, you're on the edge, with a margin for error that's small and hard to judge accurately.

In every case history I've seen over the years, the victims essentially painted themselves into a lethal corner, because they didn't think and plan ahead. Sometimes it's common ignorance, like sweating in a hot spring. Sometimes it's just plain stupidity - like getting stuck miles out in the desert with no water on a hot day. Betting your life on water that may not be there is equally dumb. My experience tells me there's no such thing as a sure-fire waterhole, unless it's the size of Lake Mead - and that one's vanishing, too, now that I think of it.

If you've read this far, you're no longer ignorant, and probably weren't to start with. My last on this will be on back-up theory.

If life's at stake, one back-up won't do. You need 2, primary and secondary. Always have a secondary back-up. A venture in the heat has to be planned like a military operation.

Oh, those guys in the spring. You probably realized they were sweating heavily while they soaked, and didn't realize it or give it a thought. I'm guessing the soak also increased their body temperature while it was reducing their blood volume, so when they started hiking back up, they were already on the edge, and unable to unload the metabolic heat of hiking, which quickly added to the heat load they had. They only got a mile from the spring.


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