Cholla cactus in 107F, Mojave desert south of Yuma, Arizona. Infrared slide.
After a long winter or a wet spring, it can be hard to remember how to manage the first summer days of excessive heat. But you can welcome to the dog days of summer, the annual hot, dry, stagnant weather period that, for most of North America, falls between early July and late September. It's a committing season. In winter, you can layer on the down and Gore-Tex, but in summer, you can only strip so naked, and survival becomes more about smarts than gear.
If you're going to be outside exercising, and especially backpacking and camping where you're out 24/7, you need to understand what heat does to your body, and how to control its effect.
Exercise increases your metabolic rate, which increases you body temperature - especially when combined with hot air temperatures and solar heating. The human body does not like running too hot, above say 102 degrees Fahrenheit, because proteins start changing--along with cellular reactions and brain synapses. In order to prevent organ damage from high body temperatures, your brain triggers arteries and veins to expand, shunting blood flow to the skin like a car's radiator. This puts less blood, and considerably more stress, through your heart and lungs.
To adapt, the body increases its sweat rate, becoming more efficient at cooling. On a cellular level, your body also develops 'heat shock proteins' which allow cellular functions to continue at somewhat higher temperatures. Adaptation speed and total heat adaptation are limited, even with training. You can only push so hard in hot weather.
In healthy situations, your blood vessel expansion and evaporative cooling from perspiration can adjust to the heat. But if you push too hard, or too long, or it's just too damn hot (as in air temperatures above 100 degrees F), the body's cooling mechanisms get overwhelmed, leading to hyperthermia - meaning dangerously high body core temperatures.
High humidity increases hyperthermia risk, since sweat won't evaporate into saturated air. Add strong sunlight to high humidity, and the heat index (how hot your body feels) can easily climb 20F or more above actual air temperature. Bookmark this daily heat index Map to the U.S. In dry sunny heat, sweat evaporates better, lessening the heat index, but sun makes your skin temperature hotter, sunburn decreases the skin's ability to sweat and cool, and you dehydrate more rapidly when breathing low-humidity air.
Exercising in heat is just like exercising at high altitude; You need to acclimatize; You can't work out in an air-conditioned gym and expect to stroll through the Grand Canyon in July. Failure to understand heat acclimatization has led to numerous deaths by hikers in the Grand Canyon, football players in summer training, and kids taking hoods in the woods wilderness therapy programs. Citizen athletes who are used to pushing through pain can easily force themselves into fatal hyperthermia. While training helps, toughness itself is just another predisposing factor.
For serious hot weather project like a desert thru-hike or marathon, you need to gradually increase your exercise temperature and exertion, pushing each training episode hard enough to create profuse sweating. Even one to five days of increased hot weather training can reduce your exercise heart rate by 15-25%. You can seriously reduce your potential for heat exhaustion with eight days of heat training. For maximum effect, gradually increase the length and intensity of your hot weather exercise for at least 14 to 18 days. The more the merrier, assuming you never cross the threshold to heat injury. The bad news is that heat acclimatization disappears quickly, and air conditioning greatly inhibits heat acclimatization. Athletes looking to maximize their abilities should avoid air conditioning altogether.
Heat Injury Checklist
Symptoms for the conditions and predisposing factors of heat injury:
The earliest symptoms of dehydration are dark or deep-yellow urine, and thirst. Both occur by the time you've lost 2% of normal water volume. Symptoms progress to headaches, upset stomach, visual artifacts like 'snow' or zig-zag pattern in your vision, dizziness, and fainting. Exercise-dehydrated athletes can suffer performance losses of 30%, with flushed red skin, extreme weakness and rapid heart rate. Heat cramps (below) are common. Severe dehydration leads to seizures, hallucinations, delirium, unconsciousness. Death can occur with water volume losses of 15%. Exercising hard in hot weather, you'll need to drink of 8 to 10 ounces every 10-15 minutes - nearly two liters an hour. The old guideline of 'a gallon of water per person per day' is grossly inadequate in hot weather.
Typically cramping in the abdomen, arms and calves caused by dehydration and loss of electrolytes due to prolonged heavy sweating, particularly if sweat is replaced with water lacking in the ionized electrolytes sodium or potassium. Sodium (N-)and potassium (K+) are important because they allow electrical charges to pass through the nervous system, and the "sodium-potassium pump" regulates the passage of water through cellular walls. Sodium makes cells retain water; potassium makes cells excrete water. Both electrolytes are expelled in sweat and must be replaced (along with calcium).
To remedy heat cramps: Rest and cool down. Drink an electrolyte sports drink. Nibble on salty snacks. Stretch and massage the cramps.
Heat cramps often accompany 'hyponatremia,' meaning clinically low sodium levels. Hyponatremia is rare, except in the Grand Canyon and along other oven-hot desert trails where people pound water under conditions too hot to sustain their appetite for food. Since sodium is excreted in the sweat and not replaced, electrolytes get far enough out of balance that nerve and cellular functions go south. Your basic trail hyponatremia is characterized at first by extreme weakness. Patients often sit numbly, seemingly unable to even raise their hand. Serious cases my vomite anything ingested, including water. Rest, shade, salty snacks and electrolyte drinks often bring hyponatremia cases around within 30 to 60 minutes.....unless they're seriously debilitated. Sucking on salted pretzel or potato chips can often do the trick. Solve this problem by recognizing it. Weakness + fatigue = rest & salt intake. Victims can recover if caught in time.
Heat exhaustion and heat stroke
Once the body's cooling mechanisms become overwhelmed, core body temperature climbs above the normal metabolic 'set point' of approximately 98.6 degrees Fahrenheit. Core body temperatures above 102 F are dangerous. Such high temperatures affect chemical reactions in the central nervous system, disabling most of the body's life-sustaining functions. Brain death begins at 106F. By 113F, death is virtually certain.
Heat exhaustion and heat stroke are merely different severities of the same problem, namely hyperthermia, or dangerously elevated core temperature. The symptoms usually progress from merely 'sun-slammed' heat exhaustion, to potentially fatal heat stroke. Serious dehydration and sudden lack of sweating often triggers the transition.
With heat exhaustion you become flushed red, sweaty, faint, dizzy, weak and trembly. You might get a severe headache, nausea, and a rapid, weak pulse; or alternate between feeling hot and chilled. As heat exhaustion progresses into heat stroke, your sweating stops, your body temperature begins to soar, and you'll get vomiting, loss of coordination, hallucinations, and eventually convulsions, unconsciousness and death.
Avoidance On the Trail
 Pace yourself. If you feel hot, flushed, weak, take a break in the shade. Drink plenty of water — a cup ever 10 or 15 minutes, even if you don't feel thirsty. For multi-hour workouts, drink sports drinks to replace the sodium and potassium. Dress in loose clothing and light colors. Wear a hat and sunscreen, sunburn decreases your body's ability to cool itself. In desert hot weather, cotton clothes saturated with water act like air conditioning. Avoid midday heat and sun. Travel morning and evening. Find shade and rest during hot, midafternoon hours (currently, in Utah, this is 3-6p.m.).
 Don't get bullheaded. Any citizen athlete who's good at suffering can push themselves into serious heat illness. This is not rocket science. Smarten up there, caveman.
 If you suspect a heat-related illness, stop exercising and get out of the heat. Drink water, and wet and fan your skin. If you don't feel better within 30 minutes, get out and get checked out. If you become faint or confused, get cool, bail out, and seek immediate medical help.
 If you are overweight, taking prescription meds, age 60 or over, or have existing medical problems, you should really talk with a doctor before attempting hot-climate adventures.
If you're feeling like hell on a hot weather hike, treatment is simple: Stop exercising, get out of the sun and heat. Drink some water. Saturate a shirt and bandana with cool water and let the evaporation cool you. Drink water to restore good circulation and sweating ability. Of course, most trekkers get cooked because they didn't have any of these necessities with them. But there are instances where reasonably well-prepared people hiked out into Death Valley temperatures of 120F and fried in a half hour....because being well-prepared in such conditions means a full-on space suit.
Cool 'em. Hydrate 'em. If the victim has lost consciousness, they need to be placed on their side, their airway maintained open, and CPR might be necessary if they go into cardiac arrest. Get the patient evacuated. If you've got a beacon, hit that button. This is serious, life-threatening stuff. Aside from your determined cooling and hydration efforts, anyone who becomes seriously debilitated by heat injury needs emergency medical support, a climate-controlled environment, and examination by a physician. Delayed reactions are common. Brain, nerve and metabolic functions may be permanently affected. Self-evacuation might be honorable, but it is not a smart call once someone gets seriously cooked.
Like all backcountry emergencies, heat injuries are much easier to avoid than to remedy. Stay cool out there. And sometimes, that's not so easy. - Steve Howe