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Exertional Heat Illness – Signs, Symptoms and Treatment

By Donna Speedie


Exercising in hot and humid environments imposes physical strain on the cardiovascular and thermoregulatory systems of the body (Tatterson et al, 2000). For many, this can be difficult and challenging to manage. In simple terms, this physical strain is known as exertional heat illness (EHI). Being able to recognise the oftentimes-subtle signs that you are suffering from EHI and having the ability to manage your own expectations is critical for runners who live in hot and humid environments.

Running in elevated temperatures and humidity means that you will be moving slower despite an oftentimes tangible increase in exertion and intensity. Furthermore, sweat output, comprising both water and electrolytes (sodium chloride, potassium, calcium and phosphates) can quickly exceed intake. Therefore, it is prudent to have some understanding of basic terminology, causes, signs, symptoms and treatment of EHI.


EHI manifests itself on a spectrum from mild to severe e.g., heat cramps, heat syncope (fainting due to low blood pressure), heat exhaustion and life-threatening heat stroke (Lipman et el, 2019). EHI is far from being an esoteric medical condition. Clarity is important for accurate diagnosis of the different forms and recognition of their severity (Gamage et al, 2020). These forms are defined and summarised as follows;

  • Milder forms comprise heat oedema (swelling), heat rash, heat cramps,

  • Moderate forms comprise heat syncope and heat exhaustion which occurs from increased heat production and impaired dissipation of heat. This is often accompanied by dehydration and/or electrolyte imbalance,

  • Severe Forms comprise heat stroke. Heat Stroke is a life threatening medical emergency and can occur when heat exhaustion is left untreated or has gone unnoticed.


EHI results from exposure to high ambient temperatures or strenuous exercise and is often accompanied by dehydration due to insufficient fluid intake and/or heavy sweating, electrolyte imbalance or a combination of both.

The process in which the body adjusts its temperature is based around a temperature control mechanism known as thermoregulation. This mechanism is important because many processes in our body only work well within a certain range of temperatures. Our body cools itself by sweating, when sweat evaporates it helps lower our body temperature.

In hot and humid conditions, sweat often cannot evaporate quickly enough from our skin to aid the cooling process, robbing our body of one of the most important ways of getting rid of extra heat.

The treatment of EHI follows a critical common theme, i.e., lowering of the body’s core temperature and restoration of water, salts and sugars (Storrie-Lombardi, 2007). Mild and moderate forms of EHI generally resolve themselves by moving into a shaded cooler area, cessation of running and rehydration with a solution that contains approximately the human specific ratios of water, salts and sugars (Storrie-Lombardi, 2007). However, severe forms of EHI are life threatening and require medical attention.


It is important to be able to quickly recognise and understand the symptoms of EHI. Symptoms will vary from person to person and may include:

Symptoms of milder forms

  1. Heat oedema (swelling),

  2. Heat rash,

  3. Heat cramps,

Symptoms of moderate forms

  1. Headache,

  2. Dizziness,

  3. Nausea/vomiting,

  4. Muscle cramps,

  5. Heavy sweating,

  6. Rapid heart rate,

  7. Cold clammy skin, chills.

Symptoms of Severe form - Heat Stroke is a life threatening medical emergency. Call 999 or the emergency services number in your country for medical support.

  1. Confusion/irritability/hallucinations,

  2. Core temp is greater than 40ºC (104ºF),

  3. Dry skin (we stop sweating),

  4. Seizures,

  5. Unconsciousness/Coma/Death.



The best way to prevent dehydration from occurring is to put a strategy in place that will help you to prevent the onset of any of the symptoms listed previously. There is much debate about the volume and concentration of water, salts and sugars respectively, that you should drink prior to, during, and after intense exercise. Truth be told, there does not appear to be a consensus about this issue in the literature. Over the years, the advice has changed from ‘drink as much water as you possibly can’, to ‘drink to thirst’, to ‘it depends on your individual needs’. The bottom line is that there is no set answer or equation, so it often comes down to trial and error.

I have recently listened to some interesting podcast episodes hosted by running coach, Jason Koop, particularly those where he discusses the topics of hydration and electrolyte imbalance in athletes (primarily runners) brought on by high intensity exercise. You may wish to check out episodes 31, 33, 36 and 43. These podcasts do not discuss novel ways to deal with EHI or electrolyte imbalance but will give you some food for thought and help you become more informed. If nothing else, they confirm how difficult it is for runners and athletes to manage hydration and electrolyte imbalances when running in hot and humid conditions.

The following are some questions that you may wish to ask yourself and some things to consider before or during a run:

What is your intended distance and intensity?

This may help you decide how much water you should carry and the concentration of salts and sugars to add to the water. If you plan a shorter low intensity run, do you need a hydration system that can handle a larger volume of water, or can you manage with soft flasks? If you are planning on longer distances at a higher intensity you may wish to consider using a hydration system that holds a larger volume of water. Consequently, you will also need to increase the concentration of salts and sugars. Drink little and often and make a point of finishing your water by the end of your run.

Are there opportunities to get additional water on your chosen route?

If there are no opportunities to get additional water on your chosen route, you may need to filter water from streams or water pipes. Stream water is a great resource and can be used to help cool the body down.

Did I drink enough water on my run?

The colour of your urine is a good indicator of your hydration status. Straw coloured urine suggests that you are adequately hydrated, whereas, dark orange coloured urine suggests that you are dehydrated.


Electrolyte imbalance is one of the contributing factors of heat exhaustion. There are many opinions on how to manage the intake of electrolytes depending on your exercise intensity. Signs of electrolyte imbalance may include muscular fatigue leading to lower blood sugar, cramping and poor thermoregulation that makes heat feel progressively more intolerable.

How do you replace electrolytes during and after your runs?

Some runners opt for plain water, however Storrie-Lombardi (2007) argues that “fluid replacement with mixtures of electrolytes, sugars and water has demonstrated performance improvements for athletes in a wide variety of sports.”

Further to this, there are countless products on the market that promise to keep you hydrated, boost energy and balance electrolyte levels e.g., Tailwind, Gatorade and Nuun. There are many other proprietary products on the market, so you may have to take a trial and error approach to see what product works best for you and determine what works for your gastrointestinal tolerances.


Sun in SE Asia can be very intense. Sunburn i.e., the after-effects of exposure to ultraviolet radiation can cause your body to generate more unwanted heat which can contribute to the moderate and more severe forms of EHI. Before you run, you may wish to consider applying a high factor sunscreen to protect your skin from sunburn. It is worth remembering that on overcast days, we can also get sunburn.

Have you ever come home from a long run with bright red skin or aching sunburn?

Applying sunscreen at least 30 minutes before exposure to the sun will allow time for the product to fully penetrate the skin. This is a preventative measure often missed, especially when we take part in races that start at night and finish the next day when the sun is fully up.


We all want to look good when we run, however, what we wear can have a significant impact on our running performance and can contribute to the onset of EHI.

What should I wear when running in a hot and humid environment?

Opting for lightweight, sweat wicking, quick drying, loose fitting and breathable fabrics will help moisture to evaporate from your skin. Fabric colour is also very important. Objects that are white, reflect all wavelengths of light and therefore absorb the least amount of energy, whereas, dark coloured or black objects, absorb all wavelengths of light and reflect none, increasing the likelihood that you will become hotter.


Choose your running route carefully and be cognisant of the prevailing weather conditions so that you can get the most out of your runs without running the risk of overheating.

Why does my running route and time of day make a difference?

On sunnier and more humid days, you may wish to consider choosing a route that offers some shade and one that intersects streams, rock pools, water pipes, or one that passes a public washroom where you can access cold water.

You may also wish to consider scheduling your runs earlier in the morning or later in the evening when the sun is not directly overhead, and temperatures are lower. Reduce running intensity in the heat and humidity and remember that it’s fine to slow down and take walking breaks.


In summary, there is no doubt that the subject of EHI, its causes, effects on our body and treatment is a complex and nuanced subject. To compound this, individuals respond differently to environmental stressors, meaning that what works for one, may not work for another. As such, I would recommend that you undertake your own research on the subject and couple this with a fair amount of experimentation.

While both heat exhaustion and heat stroke are severe and serious heat illnesses respectively, they can be prevented. Knowing the signs and symptoms of both conditions is the first step in preventing either of these conditions from not only occurring, but also from negatively impacting your running performance. The following are key takeaways;

  1. Preparation is the key to avoiding EHI. This includes proper hydration, nutrient intake Before, during and after running, applying sunscreen, opting for light coloured apparel that readily wicks-away sweat from the body and choosing routes that take the prevailing weather conditions into consideration,

  2. Being able to recognize and initiate a prompt and appropriate response when you start to feel uncomfortable or experience any of the milder forms of EHI mentioned previously is critical. An appropriate response can be anything from slowing down, using streams to lower body temperature and/or rehydrate with an isotonic solution,

  3. The initial treatment of the moderate forms of EHI, and heat exhaustion involves stabilisation in a cool area and the restoration of electrolytes, water and sugar,

  4. Unless the factors leading to heat exhaustion are corrected swiftly this can progress to life threatening heat stroke.

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