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Managing heat stress

Photo credit: Adrian Benn.

Managing heat stress


Last updated: 19-Oct-18

By Adrian Benn

Adrian Benn is a serving army officer and a keen ultra runner and endurance athlete. He contacted RunUltra to share his experience of both working and running under extreme heat stress. He has some suggestions on how to manage the potentially lethal trio that is humidity, air temperature and solar (sun) light.

I have served with the Army for 24 years and I love endurance races: ironman, ultras or just very long, isolated trekking. This means I have run, cycled and, on occasion, crawled through some very hot places, and I have done it without falling over through heat stress. The foundation of that success comes from my father’s profession. He was an Environmental Health Officer in the Army and I was one of the few kids who went to work with a man who added cockroach clearing or hornet nest splatting to my homework.  
It would be fair to ask what on earth that has to do with summer running. Part of his duties was to advise people on the risk the environment creates for activity levels. Early on, I understood the link between humidity, solar radiation and air temperature as well as the parts per million of chlorine in a garrison swimming pool. As I said, an unusual childhood.
The point I want to explore here is that three-way link between humidity, air temperature and solar (sun) light that, if not managed well, may lead to a poor race day, or even a fatality. It’s something I work with daily in civilian fitness, mountain leadership and in the military. 
As part of my work, I have been able to access a device that is widely used in assessing the risk to individuals in certain working conditions. It is called a Wet Globe Bulb Thermometer (WBGT) which used to cost thousands but is now available for a few hundred pounds. Don’t worry, I am not suggesting you go out and buy one but I am going to use its findings to help you understand and manage your own heat stress. 

A WBGT machine is made up of three separate devices. Each relates to how the body builds up or sheds heat. In effect, it is a handheld version of you. Just as the sun, wind and humidity affect you so they affect the three sensors on the apparatus:   

  1. Globe. A black ball thermometer placed in the sun to measure solar direct heating. It’s round to ensure that reflected radiation from walls, water or snow is accounted for.  
  2. Dry Thermometer. This measures the air temperature, generally the figure you get on the weather report. Think of it as the equivalent of the heat you get when you stand in the shade.  
  3. Wet Bulb. A thermometer wrapped in a wet gauze which allows it to be cooled by evaporation – the equivalent of human sweat.

What is revealing, and critical is that not all three readings are given the same weight. Roughly it is 10% for the Dry Thermometer, 20% for the Globe but a whopping 70% for the Wet Bulb.

As you can see, by far the most weight is given to the effects of humidity. This is because high humidity reduces the effectiveness of sweating and wind-assisted evaporation.

For runners, understanding this allows you to efficiently manage heat even if you don’t have the device available. Hot, sticky days impede your sweating which has 70% of the impact. A hot, but dry night, will reduce the 20% globe and 70% wet bulb effect even if the 10% air temp is high.  

If you understand the above relationship, it empowers you. It allows you to view your running in a different way. You can take advantage of what you can change and manage what you cannot. 

In the military, one big battle for soldiers is “on the man” equipment. Body armour, helmets and packs all increase the inefficiency of the body’s sweating as well as adding weight. Runners, generally, are not worried about gunfire, I hope, but by choosing the right clothing that allows your skin to sweat and then evaporate that moisture makes the best use of the 70% humidity factor. If you add barriers to that process, and the relative humidity is high, you are making the whole situation worse.

You can reduce the effect of sun (globe) by choosing a wise route. I have seen runners taking the shortest route in the sun, when the other side of the path is in the shade. Choose the shade as it will reduce the thermal load. Running closer to the down slope side of a path will allow more updraft breeze to cool you. Don’t stop in the sun to read a map, when 10ft away is a tree they can cool under, while doing the same. It’s about choices, if heat is killing you then that must be the key consideration, not the fastest line.

Think flat surfaces. Your car bonnet (perpendicular to the sun) heats more than the sides (parallel to it). You can protect the body’s flat areas i.e. top of head and shoulders, as they will heat up the most. Such precautions help reduce the addition of heat to the body.

There are a few points where you can actually assist cooling. Where blood passes closest to the skin’s surface you can cool more efficiently than where it is covered by blocky muscle or the unmentionable wobbly stuff. You notice this effect in reverse during winter. Covering the throat (Carotid Arteries) and wrists (Ulnar and Radial Arteries) plus associated veins makes you warmer. So, a bandana loosely tied over wrists, throat and even ankles, that is wetted regularly, will have a cooling effect.

Remembering to use shade, cool what you can and cover what you must. If you start over heating think about stopping in shade until you recover. Even SLOW DOWN and let your internal combustion engine rev down a bit. Maintain hydration to assist sweating. 

We are all individuals and I have deliberately left out climate adaptation, disease and fitness here, that’s perhaps for another day. What I would advise you to do is run the WGBT elements through your head and adjust accordingly. A sticky, humid day is different to a clear, dry, sunny day. I’d rather run in a desert like Iraq in 50C+ than in a 30C jungle. Change your race plan to match the environment. A few minutes slower per mile, may mean the difference between a finish or a DNF.

For more on Adrian:

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07:32 09-08-17

Interesting, though my favourite races have been of the 35+ degrees jungle variety. I find that keeping running as much as possible works better for me and it's when I slow to walk that the heat seems to increase. Running seems to create its own cooling breeze. Or is that just psychological?