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Extreme heat by historical standards is in the news, and most likely in our future. So this question is how to know when you are headed for being seriously affected by the heat, and what to do about it. To clarify, you want to notice early danger signs, and apply remedies before the person is seriously affected.

This is not a question about how to avoid adverse effects from heat, although avoidance is the best policy. That must have been answered elsewhere on TGO, and includes e.g., getting well hydrated before the trip, staying hydrated during the trip, salt, resting in the shade (if any) as necessary, putting a wet cloth on your head and around your neck, stuffing your cap with snow (if available) and hiking in the early morning and napping during the heat of the day.

The question assumes that you have not been sensible, although you know better, and a drink and a short rest are no longer enough to restore you. What are the first warning signs of danger, and what should you do?

Assume that you are on a long hike or a backpackpacking trip, and are hours from the trailhead. Walking out to airconditioning is not an option. You have a PLB, but rescue will take many hours. Assume two scenarios: (1a) plentiful water that is nearby and potable; (1b) plentiful water that is nearby but possibly or probably not potable; (2) Scarce water -- i.e., you your companions (which number only several) have several quarts apiece left. You were expecting to resupply in several hours.

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    The question assumes that there has been no prevention and that no timely treatment is available, so the answer is that there is great risk and not much you can do. You can do common-sense stuff like moving the person to shade and giving them water, but those are not really the effective treatment that you want when someone has serious heat injury.
    – user2169
    Jul 24 at 16:55
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    @Ben Crowell Agreed, but the question was how to catch a person headed for serious heat injury before he gets there. Will edit to make this clearer..
    – ab2
    Jul 24 at 22:45
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    Well, the early warning sign is they stop drinking. Water is fuel when you're in the hot, but some people think "that doesn't apply to them". If you don't have enough "fuel" to complete your journey as planned, then abort and return to base. If you're far beyond the point of no return, then recognize that you are "in over your head", and pull the ELT now. The rescuers would much rather find you with 30 minutes of water left and carrying your own ruck onto the jeep, than be med/evac'd out on a stretcher. Jul 24 at 23:38
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    A third scenario might be plenty of water that you're not confident about drinking, but that can still be used for cooling
    – Chris H
    Jul 27 at 13:16
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    @Chris H Added your scenario.
    – ab2
    Jul 27 at 18:58
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Serious heat related illness in wilderness medicine is broken up into two categories, heat exhaustion and heat stroke.

This blog post from NOLS has a pretty good overview of the two (including signs & symptoms [S/Sx], and treatment). The information is similar to what would be covered in a WFA/WFR course regarding heat illness. That being said, I would not consider looking over a few online resources to be equivalent to a good hands-on WFA/WFR course.

The following is covering some of the main points, but should not be taken as comprehensive!

First of all, the hypothetical you propose where water, salts (also an important part of prevention you left out), and rest can't restore the patient you probably have a patient closer to heat stroke than heat exhaustion. For heat stroke there is basically a singular treatment plan. Immediate, rapid cooling followed by evacuation (What that evacuation plan is will be highly depending on situation. It may very well involve using a PLB). This cooling process often must involve water, but the water need not be potable to cool. One of the most effective treatments is full body submersion in a body of water. If the only water available is drinking water for the rest of the group use as much as you safely can without jeopardizing the health and safety of others (again, this quantity is highly dependent on the situation. But remember, one patient is always better than two). In terms of warning signs of heat stroke, one key indicator is mental status. Mental status is generally classified by the AVPU scale (Alert, Verbal, Pain, Unresponsive). If A-, or below, is reached you should be immediately concerned.

Personally, in your hypothetical situations (both 1 and 2), if a patient has heat stroke I am pressing the PLB with no hesitation. Heat stroke is a very serious life threatening medical issue. To reiterate, this is for heat stroke, not for heat exhaustion.

Heat exhaustion will be your early warning sign for heat stroke. For heat exhaustion, the signs and symptoms you are looking for will be focused on vitals (Heart Rate [HR], Respiratory Rate [RR], Skin) and a few key patient complaints (Headache, nausea, weakness, tiredness). Treatment is going to involve rest and hydration (including salts as necessary to prevent Hyponatremia). Your early warning signs for heat exhaustion will be based on the S/Sx. You are looking for a pattern to emerge in the vitals and complaints. If you begin to see any one thing emerging you shouldn't panic, but you should consider if actions need to be taken (rest in shade hydration, snacks, etc) to prevent the condition from worsening. In practice, in a hiking/backpacking scenario, this will often be a simple as asking your friends how they are feeling. Or if you see a friend breathing really heavily/not drinking water/not having any sort of snacks/etc checking in on them. Heat exhaustion is very preventable with proper planning.

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    This is helpful, and I upvoted it. But it does not make clear what the first warning sign(s) is (are). For example, I think I came close to mild heat exhaustion after 3.5 hours with my yard guys on a humid 97 F day. My clue was not feeling less tired after going inside and sitting down for 10 minutes. I called off the work.
    – ab2
    Jul 24 at 0:58
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    Maybe I am misunderstanding your question, but I gave S/Sx for heat exhaustion/stroke. For exhaustion it is vitals/patient complaints in the last paragraph. S/Sx are the closest thing you get to early warning signs. Heat illness, and most of wilderness medicine in general, is about recognizing patterns. If I notice an elevated RR in someone hiking with me I might check in on them and see if they have any complaints. Or similarly if they complain they are tired I might check in on their vitals (particularly RR/Skin which is checked from a distance without them knowing, making it a good start).
    – noah
    Jul 24 at 16:19
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    please spell out the abbreviations. S/Sx -- I will be alert for it, but first I have to know what it is! Ditto elevated RR.
    – ab2
    Jul 25 at 0:00
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    Sorry. S/Sx is Signs and symptoms. RR is respiratory rate. HR (in answer) is heart rate. AVPU (also in answer) is Alert, Verbal, Pain, Unresponsive. I think that is all the abbreviations I used.
    – noah
    Jul 25 at 0:50
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    I've added better descriptions of abbreviations in my answer. I've also tried to adjust my answer a bit in regards to the slight change in question wording.
    – noah
    Jul 25 at 1:08
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Speaking from a few occasions, I can describe this list: (Anectodal being what it is, YMMV).

Generally, I was aware of feeling hot and discomforted by heat

  1. A very alarming generalsense of unease. Something is not as it should be.
  2. Light-headedness; like when you stand up quickly sometimes, but persistent
  3. Slight nausea, though likely connected to the first two.
  4. Fatigue and weakness, very significant. Coordination problematic.
  5. Breathing seems laboured, quick and short.

And then fortunately I have been able to get to (self) medical assistance by figuring out what was happening. Copious amounts of water, both external and drinking and removal of clothing helped rather swiftly. I started sweating profusely after drinking and removing insulation though, a healthy sign of a body trying to fix the problem.

I have to say that it was hard understanding what was happening to me, so some mental impairment is probably number 0.5 on the list.

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I wanted to add some details that might compliment the other answers here by looking at specific signs and symptoms that help evaluate dehydration. My experience as an EMT weighs in here, and links are below for learning more details. However, dehydration does not affect everyone in the same way. Each person will have a different tolerance level to dehydration and may be able to survive without water for longer or shorter periods than someone else.

  1. Frequency of urination. When did this individual urinate? What was the color and quantity of the urine? Evaluation of urine is one of the easiest and earliest ways to assess hydration levels.

  2. Check the skin for turgor. If you pinch and lift up a small fold of skin, how does a return to a resting state? Does it "tent up"?

  3. Check cognitive status. Speech, level of consciousness. balance, confusion, irritability are all indications. However, individuals can "lose 3 to 4 percent of your body weight through dehydration without feeling any real symptoms, says Alp Arkun, MD, the chief of service for emergency medicine.... Yet, once you have lost 5 to 6 percent, you’ll start to feel the symptoms of mild dehydration"

  4. Remember studies have shown thirst is not a reliable indication of hydration levels in hot environments.

  5. If you have a thermometer in your gear, you can take a rectal temperature. Heat exhaustion is a body temperature of 101°F (38.3°C) to 104°F (40°C). Temperatures above 103 F - 104 F are more serious. They can indicate heat stroke and as internal temperatures increase a corresponding risk of damage.

  6. Increased tiredness, headaches, nausea, and paresthesias are experienced at about 5% to 6% water loss.

  7. With 10% to 15% fluid loss, one may experience symptoms of muscle cramping, dry and wrinkly skin, beginning of delirium, painful and/or decreased urine output, and decline in eyesight, sunken eyes. lack of sweating, low blood pressure, increased heart rate.

https://www.physio-pedia.com/Dehydration https://onlinelibrary.wiley.com/doi/full/10.1111/j.1753-4887.2012.00527.x https://www.medicalnewstoday.com/articles/325174 https://www.medicalnewstoday.com/articles/153363#complications https://www.everydayhealth.com/news/unusual-signs-of-dehydration/ https://www.hopkinsmedicine.org/health/conditions-and-diseases/exerciserelated-heat-exhaustion%3famp=true

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  • I think it is worth noting that dehydration is not equivalent to heat illness (although dehydration can often coincide with/lead to it). One can still be hydrated and experience heat illness. Still a good answer with valid points nonetheless. To improve your answer consider providing context for the links. Or if they are meant to act as references for you answer provide them inline with [1], [2], etc.
    – noah
    Aug 23 at 8:09

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