This post is an adjunct to the challenge for 6-10-25 Heat Experience. It is actually dangerous. It is offered because not having this knowledge or experience is arguably more dangerous
In my years of Doing Stuff™ I’ve been subject to and witnessed heat sneaking up on people and biting them hard. For trail running & OCR it is a special class of bonk. Heat bonk sucks. Heat saps your work capacity, can annihilate it entirely, and if you do not manage it correctly and on time you will at best have a multiple days long recovery, often weeks, worst you die. This topic is no joke and people massively underestimate what ambient temperatures will to do them and how they must compensate. This is not a toughness thing. It’s a physiology thing.
Let’s use some preparation, knowledge, and experience to reduce the impact heat will have on your work capacity so you can remain useful when the world is trying to burn you into a crisp.
Why external temperature a problem: Your body temperature is 98.6F, or close to it. At any given time there are a certain number of watts you need to reject into the environment so that temperature does not rise. The more activity your metabolism undertakes the more heat energy you need to reject in order to maintain that body temperature. The larger the temperature gradient you have with the environment the more watts you can reject easily. Energy naturally flows from you out into the world. Running at 50F is pleasant, maybe even a little cold (what you’re losing to the environment is greater than your rejection requirement). Now at 95-100F you have no gradient (or you’re on the wrong side of it!). You must mitigate this by reducing your pace/effort such you are not working as hard so you are not making as much heat. Yes, you may lose a minute a mile, maybe more, a heat bonk will cause you to lose much more off your average pace. You need to slow down, and likely you need to slow down further than that.
It’s not that bad though because you’re human so you have a cheat via evaporation and convection.
Evaporation & Convection (Sweating) Humans are special in our ability to use evaporative convection to reject extra heat into the environment. We secrete water onto our skin, radiate heat into that water via our capillaries, the most energetic of those water molecules shift to gas phase, and leave… taking a bunch of heat energy with them. We can do this until we’re out of water. If only it was that simple… we also secrete other molecules to make this work better. Unfortunately some of those are electrolytes used by our nerves & muscles, sodium and potassium in particular. This means you cannot just take in water, you must also be taking in potassium, sodium, magnesium, & calcium ion in sufficient amounts! In the case of sodium it could be north of 500-1000 mg per liter consumed depending on your genetics. Can be more. You know you're doing badly on electrolytes & fluid when your sweating slows or stops and/or your muscles start cramping or you see a significant power drop. You will often also experience a light headedness or mild headache. It may feel like your eyes can only work in a kind of zone-out mode. Final complication, sodium is on a co-transport with glucose/dextrose, you ***MUST*** ingest sugar concurrently with your salts for effective absorption!. High humidity and high dew points sabotage all of this as sweat does not evaporate as effectively when the air is already full of water.
Sunburn. Solar Radiation. The Sun Sunscreen helps you stay cool by keeping your skin from being damaged to the point its ability to sweat becomes deranged. A very light sunburn is enough to do this. Less than an hour outside in direct sun. Wear your sunscreen, not just to prevent cancer, but to help you now. Particularly on your head. If your head starts getting hot or burnt up everything will go sideways super fast. Even better are the new UPF long sleeve hooded shirts that let infrared radiation out, but reflects infrared from the sun. Then you don’t even absorb as much of what the Sun is beaming at you, thus less heat you need to reject. It seems like they’re hotter but they’re not, not at all. You can also use the hood & sleeves to augment your sweating system by dousing them in water. There’s a reason you see ultra-runners in hot environments in these kinds of shirts. The right attire, and/or sunscreen, puts the balance of radiation in-out more on your side.
Conduction, Utilizing Environmental Heat Sinks The third way to reject heat is conduction. You touch something colder than you and dump a whole bunch of heat into it. It gets hotter you get colder. Shade would fall into this category. Rivers and streams absolutely (or the dunk wall!). Ice in your water pack or ice from support that you can stuff in your shirt (forearms, elbow crook, inside your arm between bicep & tricep is super effective). This can be something as simple as running on the side of the trail that is shaded, running between shady islands, slowing down a bit when you get into shade so you spend more time there, taking advantage of any flowing water you find to cool off. You’re not above heat bonking. Take advantage of what’s there.
Acclimation You absolutely can acclimate to heat. To a point. It takes a couple of weeks and you need to diligently be out in the heat and actually working while practicing the above (sunburn and lack of electrolytes will really get you). Garmin says it can happen in a minimum of 4 days, I’m not sure if I’ve ever seen that, or at least not going from 70F air conditioned office, rainy 50F Montana, to muggy humid valley at 95F in the direct sun. If you haven’t purposefully acclimated to the heat, or do not work an outdoor job, you are probably not as acclimated as you think. People get nauseous working in the heat then refuse to eat, drink, or take in salts. Exactly the opposite of what they should be doing. Guaranteed heat bonk. Ultra-runners will do daily saunas to acclimate, especially if their course is in a location hotter than where they live or soon enough they won’t have time. You can’t do anything about being on the wrong side of a temperature gradient when its 100F outside without a cloud in the sky. There are biochemical adjustments that acclimation can do to help keep your heat rejection systems working, but they can’t violate the laws of physics in regards to thermal gradients. Personal knowledge of your work capacity when your heat rejection is blunted and how an oncoming heat bonk feels so you can mitigate early can make all the difference. Mitigating a heat bonk is an emergency and must be treated as such. Even if you do everything right you might still bonk, but maybe remain operational and making good decisions.
Stupid Things Your Body Does
- Already covered is your skin becoming damaged due to sunburn, makes it so you can’t sweat (or as effectively) so you overheat further which makes the sunburn worse in a nice vicious cycle. Protect your skin from the Sun
- You’re sweating out the very atoms needed to run your nerves and muscles… also not great. And, when you run low you stop sweating too, so can’t cool off, and are cramping with a headache, awesome. You need more electrolytes than you think. Sheer numbers of atoms matter. Thousands of milligrams, not low hundreds.
- If your core temperature starts to rise too much your body will pull your blood out of the surface capillaries that are radiating heat away thus making you clammy and cold to the touch - this is a dangerous condition and it’s time to find shade and stop for as long as is necessary. It’s also the opposite of what your body should do! Dehydration and being really low on electrolytes will do the same. Note this looks a lot like hypovolemic shock, and in some ways it is. Don't think you can ride it out. Take care of heat bonk early and preventatively. You can't turn it around once it's there.
- There’s a limit to how much water you can absorb per a given time. The harder you’re working the worse you are at absorbing water because blood has been directed away from your gut and towards your muscles.Drink small amounts, regularly from the very start. Time your water consumption if necessary. Your Garmin has a drink alarm for activities.
- Dealing with heat, even effectively, can add 10-15 beats per minute to your heart rate. So, your zones and lactate thresholds aren’t where you think they are. Thus why it’s important to feel what things are like when you’re doing things right (and wrong) by the numbers. Your max heart rate does not change. Everything feels harder because it is. You cannot work as hard for the same effort so don't. This isn’t a toughness thing.
You start making REALLY stupid decisions in the middle of a heat bonk. Counter productive, dangerous, decisions. In an effort to cool down and make things easier you may decide to:
- Sit in the sun because right now it’s easier than getting to shade. Try to drop your water and leave it behind because it’s too heavy. Refuse to take on salt, or sufficient salt, because you want to puke.
- Decide your water is too hot so you shouldn’t drink it.
- Take off the clothing that is shielding you from the sun.
- Increase your pace because at least you’re not in the heat as long.
- Forgo jumping in that stream because it’ll make your clothes wet and swampy, thus hotter. It’s the opposite, if it’s hot enough you have a problem those will be dry completely in less than 10-20 minutes. Or, not willing to dump water on your skin or utilize the cold sink of a stream in fear the sheen of water will make you sunburn.